ALL of Edexcel IGCSE Biology 9-1 | PAPER 2 | IGCSE Biology Revision | SCIENCE WITH HAZEL

Hi, guys, and welcome to my all-in-one IGCSE Edexcel Biology video (9-1). This much-requested video is finally here. I'm going to take you through, hopefully, every single spec point so that by time you've watched it, you can feel really confident about getting that grade 9. Don't forget my revision guides. I sell these online. They are my perfect answer revision guides. I've spent hundreds if not thousands of hours compiling these, with their perfect questions, perfect answers. And go check out my website:, where you can see previews and buy them.

Let's get started, however, on this humongous video. We're going to start by looking at features that all living organisms have in common, and lots of people call that Mrs Nerg or Mrs Gren. So remember, if they say, "Give some features that all living organisms share," you're going to say: movement, respiration, sensitivity; you're going to say nutrition, excretion, reproduction, and growth – and that just means getting bigger. So if it's nonliving, like a virus, you can easily say: it does not move, it does not respire, it does not excrete. So just list any of the Mrs Nerg factors, and you will get the marks. Now we're going to look at the plant and animal cell – very basic biology here.

First of all, let's start by listing the organelles that both animal and plant cells share. So remember, they both have cell membranes, cytoplasm, nuclei or nucleus. They have ribosomes, mitochondria. Now, in terms of the plant cell, there's a few extra organelles you need to list. That's the cell wall, the vacuole, and also chloroplasts. We now need to look at the role of each of those organelles in turns because it's so key that you get these really basic questions right in your exam. Because if they say, "What does the nucleus do?" you need to be able to write that.

So, what does the nucleus do? It controls the activities of the cell. What does the cytoplasm do? It's where chemical reactions take place. What is the role of the cell membrane? It controls what enters and leaves the cell. What is the role of ribosomes? And this is new for this specification. It's where protein synthesis takes place, i.e. it's where proteins are made. Looking more closely at the plant cell now, what is the role of the cell wall? First of all, state that the cell wall is made out of cellulose and that it protects and supports the cell. The vacuole. Remember, that's filled with cell sap, which also helps to maintain structure of the cell. Lastly, chloroplasts. Remember, they're full of a green pigment called chlorophyll. It gives the leaves the green colour, and that's where photosynthesis takes place.

A couple of tricky words you may have come across is eukaryotes and prokaryotes. Don't worry. It's just a very posh way of describing the type of cell we're talking about. Eukaryotes are all animal cells as we know it, and that's because they contain membrane bound organelles, such as nuclei, mitochondria, et cetera. So an animal cell, like I said, is an example of a eukaryotic cell. Prokaryotes – we're talking about viruses and bacteria here because they contain no membrane bound organelles, so they contain no nuclei, which we know because they contain strands of DNA or RNA instead. So, we now need to take lots of different types of cell in turn and know quite a lot of information about them. So I'm going to start with a bacterial cell. We can see from the diagram that a bacterial cell has a cell wall. Sometimes it has a slime capsule around the edge. Sometimes it has a flagellum, which is a tail that helps the bacteria to move. As I've already said, it doesn't have a distinct nucleus. Instead, it has a circular chromosome, which we call a nucleoid.

It has other small rings of genetic material; we call these plasmids. And that, remember, is important when we talk about genetic engineering. Then you find more typical things, such as cytoplasm and the cell membranes. In terms of things like whether they're pathogenic or non-pathogenic, remember that they can be both. So, a pathogen is a microorganism that causes disease. It makes sense, therefore, that some bacteria are pathogenic. Such examples include pneumococcus, which is responsible for pneumonia; tuberculosis – remember, that gives people TB, where they cough up blood – very horrible disease. However, some bacteria are very useful, like those used in yoghurt making. The example here is lactobacillus bulgaricus. Remember, lastly, that bacteria are unicellular, which means that they're made of one cell only. Looking at viruses now because that leads on quite nicely from bacteria, these are very, very small things; they're much smaller than bacteria. They're far more simple because they're simply made out of a protein coat, which surrounds either DNA or RNA. They don't have any typical organelles you would find in other types of cell.

Crucial thing here, as I've already said, is that they're nonliving. They do not excrete. They do not respire. They do not grow. They're always pathogenic. There's no such thing as a good virus; they're always out to hurt you. And examples here include the flu virus; the cold virus; HIV, which is very famous because it causes AIDS. A new virus you need to know about is tobacco mosaic virus, which causes discolouration in plant leaves, and that's due to the fact it prevents chloroplast formation. Next up, we're looking at the protoctists. This is known as the dustbin kingdom. Lots of various organisms which don't fit into the other categories fit into protoctists.

Some of them have animal cell properties. Some of them have plant cell properties. Starting with algae and also chorella: these both have chloroplasts, which means they're more plant-like. Things like amoeba are more animal-like. You'll see that they don't have chloroplasts. They don't have a cell wall as such. And they all use diffusion in order to obtain their nutrients and get their oxygen. One key one you need to know about is plasmodium. This is pathogenic because it causes the disease malaria. And the plasmodium is the small protoctist that lives in female mosquitoes' bodies, and that's what she injects when she bites you, so that's actually what gives you malaria. Do note that they can either be unicellular or multicellular, so made up of one cell or many cells.

Funghi now. Now, funghi, they're quite easy because you can draw, literally, a plant cell but just make it slightly more circular. So it has the same organelles you would find in a plant cell apart from the fact it obviously doesn't have chloroplasts. But it does have a cell wall; this is made out of chitin. You do need to know that. It has a cell membrane, cytoplasm. It has a vacuole. Now, there are lots of different examples of funghi, including muchor and mushrooms. One thing you do need to just be able to mention – this is a case where you just shove in some keywords and it doesn't even matter if they don't even make sense. They'll give you a mark as long as you mention it. They have things called hyphae, which are thread-like structures which form a network called mycelium. Do notice that funghi carry out saprotrophic nutrition, and that means that they extracellularly secrete enzymes onto dead matter, which it breaks down and then absorbs as its food.

Crucial words here are extracellularly; secreting enzymes, which break down dead matter, and that's how they actually obtain their food. There are some useful examples of fungi, including yeast. Now, remember, that's used in beer and bread making. Why? Because when the yeast undergoes anaerobic respiration – that means respiration without oxygen – it breaks down glucose into ethanol, which is clearly used in beer making, and also carbon dioxide. And it's those bubbles of carbon dioxide that actually help that bread to rise. If we're going to use the correct nomenclature when we're talking about naming things, we can talk about the five kingdoms, and that consists of plants, animals, protoctists, bacteria, and fungi. One small thing to notice which can always catch you out is how carbohydrates are stored. So, in animals, it's stored as glycogen; in plants, it tends to be stored as starch. Things like potatoes are very starch-heavy. And then in fungi, it's stored as glycogen also. Now we're going to look at organisation within organisms, so we're looking at key definitions. The crucial thing here is to just learn one definition and then just use it as a template for all your other definitions.

So, I'll explain what I'm talking about right now. So, if we start, we look at a cell. Now, remember, cells are full of organelles, which I've just listed: nucleus, cell membrane, et cetera. So, what is it cell? Well, it's a group of organelles working together to perform the same function. We're going up a step further, so we're now looking at tissue. What is a tissue? It's a group of cells working together to perform the same function. Then we're going to look at organs. So what is an organ? It's a group of tissues working together to perform the same function.

And lastly, what is an organ system? It's a group of organs working together to perform the same function. And lastly, what is an organism? Well, it's a group of organ systems working together to perform the same function. Now I need to list all the various organ systems within the body, which I've always been really bad about remembering all of them. So, I'm going to try. There's the digestive system. There's the endocrine system, reproductive system, circulatory system, respiratory system, nervous system, and excretory system. But if we focus in on the digestive system, for example, well, you could, first of all, take the fact that the digestive system is obviously a system.

So what organs make up that digestive system, that organ system? Well, it's things like the stomach, esophagus, pancreas, small intestine, large intestine. Then we can look at the tissues. So, for example, in the stomach, you've got glandular tissue, which secretes hydrochloric acid. You've got muscular tissue, which help to churn the food. Don't worry too much about this detail, by the way. I just want you to get the full picture so you're not thrown off if a question's slightly strange in the exam. And then you can obviously look at the cellular level within the stomach and the organelles, so it's a big hierarchy, really.

So, we're now going to look at a zygote. So what is a zygote? Now, remember that when the sperm and egg meet at fertilization, then you're going to form one cell. That cell is called a zygote. That zygote has to divide in order to eventually form the embryo. So, remember, it divides by mitosis – which is a type of cell division – firstly into 2 cells, then 4, then 8, then 16, and so on. Now, new for this year, we need to know about stem cells and differentiation, so let's start by looking at some key definitions. So, differentiation is the process whereby cells become specialised, for example, nerve cells. Now, a stem cell is a cell that has the potential to divide many times without being differentiated. There are two types of stem cell you need to be aware of: the embryonic stem cell and the adult stem cell.

And just to look at the various advantages and disadvantages of both, the amazing thing about embryonic stem cells is you can basically get them to differentiate – to specialise – into any type of cell. So if you've got a damaged liver, you could get these embryonic stem cells to make you a new liver; damaged brain, the same. Now, the thing about an adult stem cell is it does not have the ability to differentiate – or specialise – into any type of cell; it's stuck making only one type of cell, which is why it has quite a limited use.

But an example here is bone marrow, which is used to make red blood cells and white blood cells. Let's look more closely at the use of stem cells. So, in leukemia, for example – now, leukemia is a serious cancer which affects the blood, and specifically, it destroys white blood cells. And as you know, white blood cells are all to do with your immune system. They're very vital. Now, in order to treat leukemia, we use chemotherapy. Chemotherapy uses very strong chemicals to destroy cancer cells. But the issue here is it can also affect healthy cells because it doesn't know which ones are healthy and which ones are cancerous. So, you can be left with lots of damaged healthy cells, basically, because they're not going to be able to perform their function.

So the great thing about stem cell therapy is you can use stem cells in order to generate brand-new cells to replace those damaged ones. A tiny thing we do need to mention with the embryonic stem cell use is obviously, there are great advantages with the fact that they can differentiate into any type of cell, so that means that many diseases may be treated successfully in this way, such as leukemia. The big disadvantage is the ethics involved with using embryonic stem cells because the problem is, they come from aborted fetuses or fetuses which have miscarried, and clearly many parents do not want their unborn fetuses being used in this way. So there's a big ethical issue here. And just be aware of that. (no audio) An enzyme is a biological catalyst.

This means that it speeds up the rate of a chemical reaction without being used up. Now, an enzyme has a very special part on it called an active site, and that's the biologically active part of the molecule. And what happens is the substrate molecule binds to the active site. It forms an enzyme substrate complex, which then splits up to form the useful product that we're after. And we're going to talk now about digestive enzymes because I think it makes sense for us to look at the various products and substrates involved in digestion. So there are various enzymes you need to be aware of. Firstly, amylase. And notice that enzymes tend to end in -ase, so ase. Amylase is made in the salivary glands, in your small intestine, and your pancreas. And what amylase does is it catalyses the breakdown of starch into glucose.

So, in this case, starch is the substrate. It's the thing entering the enzyme, which is amylase, and then the product here is glucose. So what you can see is a very large sugar is broken down using amylase into a much smaller, simpler sugar called glucose. Now we need to look at protease. This is more straightforward because, as the name suggests, it breaks down proteins. So protein is the substrate. And it breaks down proteins into amino acids; these are the products. And protease is found in your stomach, in your small intestine, and your pancreas. The last enzyme we need to be aware of is lipase. Lipase breaks down lipids – or fats is their more colloquial name. And lipids are broken down into fatty acids and glycerol. So do try and remember; that's the most complicated one. Now we need to just touch on enzyme activity. So, the two things you can alter is both the temperature and the pH. So, if you look at this graph, you can see at low temperatures, the enzyme activity is low.

The reason for this is all to do with chemistry; it's to do with collision theory. Because at low temperatures, enzymes have very little kinetic energy, as do the substrate molecules, so it means that the enzymes and the substrate aren't coming into contact very often. That means they obviously can't bind at the active site and so the reaction can't be catalysed. Catalysed? Catalysed. As we increase the temperature, you can see the enzyme activity increases.

That's because those molecules are coming together more often, and at 37°, as is the case with most animals, you will find that enzyme activity has reached its peak; it is at its optimum temperature – the best temperature – and that means the enzymes and the substrates are coming together very frequently. After this temperature and above this temperature, we see a massive decrease in enzyme activity, and that's because the enzyme has become denatured. Never say the word killed. They're not living; they can't be killed. You need to say they're denatured. What that means is that the enzyme's active site has changed shape, meaning that the substrate can no longer fit. If we take a look at pH now, you can see a very distinctive graph shape here, and that's because enzymes have different optimum pHs, and if the pH is either too high or too low, around that optimum pH, you'll find that the enzyme denatures, which is why you have that cone shape.

Because let's look at the enzyme which has an optimum pH of 7 … If you go to 6.5 or 7.5, the enzyme will denature. And this is true for all enzymes. Some enzymes prefer different pHs to other ones. So, protease is in the stomach, for example, because they're surrounded by hydrochloric acid. They'll have an optimum pH of approximately 3, whereas throughout the rest of the digestive system, we find a slightly alkaline optimum pH, so around 8, which is why you can't take stomach protease and put it in the small intestine and expect it to be okay. Now we're moving on to transport, so let's first of all touch on the three types of transport. So we're looking at diffusion, osmosis, and active transport. So you need to know the definitions of these terms in great amount of detail. Now, remember, with diffusion, it's the net movement of particles from an area of high concentration to an area of low concentration. So that's the reason why you spray perfume in one side of the room, where there's a high concentration of perfume particles, and it moves across to the other side of the room, where you can smell it.

That is by diffusion. It is a passive process. It does not require energy. Osmosis is very similar to diffusion; however, it involves the movement of water, which is why our definition this time is – osmosis is the net movement of water from an area of high water potential to an area of low water potential across a partially permeable membrane. Now, potential is just a really posh way of saying concentration – so somewhere where there's lots of water to somewhere where there's little water. And do add that it's across a partially permeable membrane. If there's no partially permeable membrane, it's not osmosis; it's just diffusion. So notice when water leaves the stomata from that leaf, that is by diffusion because stomata is a hole, not a partially permeable membrane. Lastly, active transport. As the name suggests, it's an active process. This means it requires energy.

The reason being is because it's the net movement from an area of low concentration to an area of high concentration – so against the concentration gradient. Let's touch on amoeba now. Remember, an amoeba is an example of a protoctist. This is a single-celled organism which can use diffusion in order to obtain all the nutrients it needs. So, oxygen diffuses from outside that amoeba into the amoeba, from an area of high concentration surrounding the amoeba to a low concentration inside the amoeba. The reason why diffusion is appropriate is because it occurs very quickly because the amoeba is single-celled, which means it has a large surface area to volume ratio, and therefore the speed of diffusion is fast enough to allow oxygen in as and when it is required.

Larger organisms which are multicellular have a much smaller surface area to volume ratio. Diffusion is not suitable. It is too slow, which is why they develop the need for a circulatory system. Right. We're going to be talking about all things to do with plants, starting with that key plant process, photosynthesis. Remember, photosynthesis is carried out in the chloroplasts of plant cells. They contain chlorophyll, which absorbs that sunlight and actually carries out the process of photosynthesis. And this is the method by which green plants make their own food.

So let's start by looking at the word equation for photosynthesis. Remember that it's: carbon dioxide plus water forms glucose and oxygen. This is why it's such a great process – because of the huge volume of oxygen photosynthesis releases. In terms of the balanced symbol equation, try and learn this off by heart. And remember that six is very important. So you've got 6CO2 + 6H2O forms C6H12O6 + 602. Now, unfortunately, there are lots of different factors which can act to reduce or limit the rate of photosynthesis, and we call these limiting factors.

You need to know the definition of a limiting factor. It's a factor which in a reaction is in the shortest supply, and a lack of this factor is the reason why rate of reaction no longer increases. Now, in terms of photosynthesis, there are three limiting factors you need to know about. These are carbon dioxide, light intensity, and temperature. So, any one of these may act to limit how much photosynthesis can take place. And we're going to talk about each of these in turn in a different situation. So let's think about early morning. We have a green plant, and it's trying to photosynthesize in early morning. But what could be limiting how much photosynthesis takes place? Well, it's early morning, so it's quite cold, so obviously temperature is going to be a limiting factor here. Another limiting factor will most likely be the light intensity because it's not as light early morning as it is at midday.

So despite carbon dioxide levels increasing, you will find that low light levels, low temperatures limit the rate of photosynthesis, the reason being is that in the morning, low temperatures obviously mean low kinetic energy, so all those enzymes involved in those chemical reactions involved in photosynthesis can't actually come together. They don't collide as frequently with their substrate molecules, and that's all because of little kinetic energy. If we take midday now, we know that temperature will have increased. We know that the light levels will increase. So neither of these things will be limiting factors. The most likely limiting factor at midday will be carbon dioxide levels. And be sure that you can look at graphs on all of these factors and make sure you understand what is going on. Let's quickly look at the role of diffusion in gas exchange in plants. So, remember that carbon dioxide is needed by plants in photosynthesis. So, it will diffuse into the leaf through the stomata. Oxygen is released by photosynthesis. So that will diffuse out of the leaf, also by diffusion.

However, don't forget that plants are living organisms, so that they respire – which is a bit confusing – which means they need some oxygen, obviously, in order to carry out respiration. So if you look at it over the course of the day, you will find that during the night – because the plant can't photosynthesize because obviously there's no light – it will just be respiring, which means that more oxygen will enter the leaf than will exit the leaf. During the day, photosynthesis tends to occur at a faster rate than respiration, so in this case, more oxygen will leave the leaf compared with diffusing into the leaf. (no audio) Carrying on with the plant theme, we're now going to look at the structure of a leaf and how it is adapted for photosynthesis. Let's make some generic comments at the beginning by stating that a leaf has a large surface area, which is obvious, so that it can absorb more light. It's thin, so the gases don't have to diffuse too far. Looking very closely now at the structure of the leaf, you need to be to be able to label all those layers and discuss what each of them do, And you need to be very specific here.

So, we're going to start with the waxy cuticle. The waxy cuticle is there to prevent transpiration. And remember that transpiration is the loss of water from the leaf. So a nice, thick waxy cuticle prevents excess water loss. Next up, we have the upper epidermis. Remember that this is transparent, and it allows the light to enter the leaf. You don't need to say anything more than that. The layer beneath this is the palisade mesophyll. Mesophyll is just a fancy way of saying tissue. The palisade mesophyll is what your generic plant cell looks like. So it's your rectangle block. It's crammed full of chloroplasts, so it contains lots of chloroplasts for photosynthesis. And this is where photosynthesis takes place. Under this, you have the spongy mesophyll. The important thing to note here is the presence of plenty of air spaces, which allow gases such as carbon dioxide and oxygen to diffuse. You'll also find the vein here, and the vein contains the xylem and the phloem. The xylem brings water into the leaf; the phloem transports sugar away from the leaf. Then we have the lower epidermis.

Not a lot to say here. The next layer is the most important layer, and that contains the guard cells and the stomata. Now, the guard cells control whether the stomata are open or closed. And the stomata allow carbon dioxide into the leaf and oxygen and water to leave the leaf. So, we know photosynthesis is the method by which plants make food, and they make that in the form of glucose. So what do they do with that glucose? Well, remember that glucose contains carbon, hydrogen, oxygen, so obviously, it contains the components that can be used to make other biological molecules, and this includes fats and proteins.

So that glucose is used to make fats and proteins. It's used to make storage compounds, such as starch, which the plant can call upon in lean times. And it's also used to make the sugar cellulose, which is an important component of plant cell walls because it gives it its structural integrity. Relating to the plant, mineral ions – luckily you don't need to know too much. Just learn the role of nitrates and magnesium. So the mineral ions are present in the soil around the roots of the plant, and the plant obviously needs them to be healthy, so it absorbs both the magnesium and nitrates by active transport – so against the concentration gradient – and it uses the nitrates to build proteins, and it uses the magnesium to manufacture the chlorophyll found in chloroplasts. You need to know some deficiency signs.

So with magnesium, clearly, you won't be able to manufacture chlorophyll anymore, so you see yellow leaves. And if you've got a lack of nitrates, you will see a stunted, poorly grown plant, so it's very short. Now we need to look at digestion. Let's first of all discuss the definition of digestion, which is that it's the breakdown of large insoluble molecules into small soluble ones, the reason being that we need to take our food into our mouths, and we need to break it down into teeny, tiny pieces, change its structure so that it can be absorbed through the walls of the small intestine. So that's what we're on about when we're talking about digestion. Now, I've already told you about chemical digestion, which relates to enzymes entirely because that's totally altering the structure of the food molecule. We need to also look at mechanical digestion, which is a far more physical process that involves just chopping that food up into smaller pieces, but it doesn't alter the structure of that food. So if we think about where mechanical digestion takes place, it will be in the mouth.

Your teeth chew. It'll also be in your stomach, where your muscular walls churn that food up and break it up into smaller chunks. So we'll start at the mouth. So I've already said we've got physical digestion from the teeth. Chemical digestion comes from amylase, which digests starch into glucose. The food then passes down the food pipe, which we're going to have to call the esophagus. And peristalsis is a process whereby the muscular contractions of the esophageal wall force that bolus – that ball of food – down into your stomach.

Here, muscular contractions of the stomach lining help churn the food. You've got the secretion of hydrochloric acid. This has two jobs: it breaks down the food, and it also destroys pathogens. And that's the reason why you don't get sick all the time if you eat some slightly dodgy food. Sometimes the food you've eaten is so dodgy that you still get sick, but for the most part, you'll find your stomach acid will destroy those pathogens. Remember, a pathogen is a microorganism which causes disease. Protease is secreted. That breaks down proteins into amino acids. At this point, the stomach empties, and the food flows into the small intestine, where there'll be further peristaltic contractions – I don't know if that's the exact version of the word peristalsis, but it's similar enough – which force the food along. We've got more enzymes being added here: lipase, protease, and amylase.

And note that they'll have a different optimum pH from the protease in the stomach. They'll have a much higher optimum pH. We need to mention bile now. Now, bile is an interesting substance. I think it's green. It's made in the liver. You must remember that. It's stored in the gallbladder. And it's released into the small intestine. And it has two main jobs.

First of all, bile is an emulsifier, and what that means is that it breaks up large fat droplets into smaller fat droplets. Why? Because it creates a much larger surface area because small droplets have a much larger surface area than large droplets, and that means that lipase can work more easily on the lipid molecules, on the fat molecules. But if you can't be bothered to write all that, just say that it emulsifies. Its second role is to neutralise stomach acid. It brings up the pH to an alkaline pH, approximately seven or eight. And what that does is it means that those enzymes that have been released into the small intestine don't get denatured by all the acidic food coming along.

So that's bile's two main jobs: to emulsify and to neutralise. So at this point, we've done lots of digesting. Our food molecules are very small. They're very soluble, which means they can now pass through the walls of the small intestine into the bloodstream. They often ask you what the adaptations are of the small intestine lining. So you're going to talk about villi, primarily. So remember, villi are these structures shaped like this, and they provide a very large surface area for absorption.

And that's also maximized by the presence of microvilli. This video is detailed it's making me lose my voice. So yeah, microvilli help increase the surface area further. You've got a short diffusion distance. You've got a plentiful supply of blood capillaries. And you've also got the presence of lacteals, which are for fat absorption. So the small intestine is extremely adaptive for its role. (no audio) So once all that food has been absorbed and you've just got the leftover, undigestible food, it passes into the large intestine, and here, water is reabsorbed into the blood. Lastly, the faeces – because we're now at the faeces stage. That's the fancy word for poo. The faeces are stored in the rectum before they pass out of your body via the anus. We call this egestion, the removal of faeces from the anus, not to be confused with excretion.

You do not excrete faeces; you egest them. Excretion has a different definition. It is the removal of waste products of metabolism. And just to touch on a couple more definitions: ingestion is the taking in of food into the body, and metabolism is the rate at which chemical reactions take place in the body, and lastly, assimilation is building up large molecules from small molecules. Right. That was a really detailed video. I hope you found it helpful. Moving on to humans now, we're looking at balanced diets, so we need to look at the various nutrients that you need for a balanced diet, their roles within the body, the foods that they're found in, and any deficiency diseases. So let's have a look and start by listing these nutrients.

So we're looking at carbohydrates, fats, proteins, minerals, vitamins, water, and fibre. I hope I've listed them all. So starting with carbohydrates: foods which contain lots of carbohydrates include things like bread and rice and pasta. Carbohydrates are an important source of energy. Proteins now. Now, you find lots of protein in meat, such as chicken and beef. Protein is important for the growth and repair of muscles. Now, remember, lots of people take protein shakes to the gym. Why? Because they're trying to build their muscles at the same time, so they like to take a source of protein. So try and remember it from that point of view. If you have a lack of protein, you get a really horrible disease called kwashiorkor. People are seen with very distinctive ball-like stomachs, where your stomach comes out super far.

And that's a symptom of kwashiorkor. Fats: foods which contain a lot of fat include dairy foods, such as butter and cream. I'm just giving you a few examples; it's not exhaustive. Now, fats are a very concentrated source of energy, and they provide insulation, i.e. they help to keep you warm. Moving on to some specific examples of vitamins: so vitamin C, you find this in citrus fruits, such as oranges and lemons. Vitamin C is important for the repair of tissue, so it helps to stick together the cells in the lining of your mouth. And if you don't have enough vitamin C, you get a disease called scurvy, which was infamous in the 1500s. When sailors used to go out to sea, they never got enough oranges and lemons, and then you'd see very characteristic mouth bleeds. So scurvy is the deficiency disease, and the way to get rid of that is by eating lots of oranges and lemons. Vitamin D now. Now, vitamin D is important for strong bones. You find it in fish liver oils, which is gross. I hate cod liver oil. But it's also manufactured by the action of the Sun on the skin, so you can get a lot of vitamin D sunbathing.

But obviously not too much. Don't get burnt; that's not a good idea either. And lack of vitamin D leads to rickets in children. Then we need to talk about vitamin A. Vitamin A is important for good vision in dim light. Lack of vitamin A leads to night blindness. You find it in fish oils, again, liver – so lots of pleasant things to be eating – and also in margarine. Now we're going to move on to minerals, such as iron. Iron is found in red meats and spinach. It's important for healthy blood. It's a really important component of haemoglobin, which is found in red blood cells. Lack of iron leads to anemia, which is when you feel really tired and exhausted all the time. And lastly, calcium: calcium is a mineral which is important for strong teeth and bones. It's found in dairy products, such as milk. Lack of calcium will also lead to rickets.

Last two things. Fibre: so fibre is essential to help food move through your digestive system. Without fibre, you're liable to get constipation. Vegetables and fruit contain a lot of fibre. And water: water we know we need to survive; without it, we would die very quickly, and that's because it supports all the chemical reactions that take place in our bodies. Now, an important side note is to notice that we need a balanced diet full of all these nutrients in order to stay healthy. But obviously our requirements will vary depending on our age. So older people will need less food, less of each of these nutrients compared with teenagers, for example. Pregnant women, they'll need more because they're supporting the growing fetus. Teenage girls will need more iron because they've started their periods. People who exercise will need more protein to help them with the growth and repair of their muscles.

So it's a bit of a common-sense part of the specification, but just be aware that there are differing requirements. The next topic we need tackle is respiration: both aerobic and anaerobic respiration. Remember, it's the process carried out in mitochondria which releases energy. New for your specification is the fact that ATP is produced. And all ATP is is a posh way of describing the energy store that is created as a result of respiration. So, what is this ATP used for? Well, it's used in cell division. It's used to build up large molecules from small molecules. It's used in active transport. And it's used to contract our muscles. Let's look at the equation for aerobic respiration now. So you need to take oxygen into the body.

You need to add that to glucose. An arrow, and then what's produced is carbon dioxide and water, and energy is released. Again, you need the balanced symbol equation here, and it's again all the 6s. I hope you've noticed that photosynthesis and respiration are the same equation, just reversed. So in terms of the balanced symbol equation, you're looking at: (reading visual aid) Plus energy in a square bracket, if you so wish. Now, every aerobic respiration involves the use of oxygen, and that's the type of respiration that we carry out ordinarily. Now, sometimes, we have to carry out anaerobic respiration, which tends to be when we've exercised and when we can't take enough oxygen into our bodies, and this involves the incomplete breakdown of glucose, and we find that lactate is produced instead.

Now, lactate is pretty poisonous. It's what gives our muscles cramp. And we need to remove that lactate, so we have to take in more oxygen, which we call the oxygen debt, in order to break down that lactate to ensure that glucose is completely broken down. Now, there are two places where anaerobic respiration takes place that you need to know about. So, I've already mentioned one, which is in your muscles. So, I've given you all that information about the oxygen debt. It's when you can't take enough oxygen into your body, and that's because you've been doing something pretty strenuous, like sprinting.

You also need to know about anaerobic respiration in yeast. Remember that yeast is a type of fungus. And yeast anaerobically respires, and it breaks down glucose into ethanol plus carbon dioxide. And these are very useful industrial processes because remember, the carbon dioxide is used to help bread rise and that the ethanol is used in beer making. Moving on to the breathing system, so we are looking at our lungs and all the vessels relating to that. So we're going to start with our mouth. It leads down to our windpipe, which we're going to have to call the trachea.

The trachea branches to form two bronchi. Further branching occurs, which is the bronchioles, and that ends in lots of air sacs, which we call alveoli, and they're surrounded by a network of blood capillaries. Now, if you actually think about it, it is very much like a tree because the trachea is represented by the trunk, the big branches are the bronchi, smaller branches are the bronchioles, the alveoli are similar to the leaves. So do bear that in mind. Now, we need to talk about how those bronchi and bronchioles are kept clean. That's due to the presence of two types of cell: the goblet cell and the ciliated cell. The goblet cell, first of all, secretes mucus. That mucus is good because it traps bacteria – pathogens – and it stops them from entering your lungs. This is when the cilia come in. Because remember they have hair-like projections which waft, and they waft that mucus which is covered in bacteria up to your mouth, where it can be swallowed and destroyed by stomach acid.

Now, what is ventilation? Well, that's simply taking air into and out of your lungs. Let's look in great depth now at an inhalation, so a breath in. So, first of all, the external intercostal muscles contract. The ribs move up and out. The diaphragm contracts, and what this means is that it flattens. And together, all these processes increase the volume within your thorax. Because there's an increased volume, clearly the pressure will decrease because the same amount of air is now present in a larger volume, and this means that air will be sucked into your lungs, causing your lungs to inflate. (no audio) Now we want to take an exhalation. We want to breathe out, so the opposite takes place. This time, the internal intercostal muscles contract. The ribs move down and in. The diaphragm relaxes. This has the effect of reducing the volume inside the thorax.

The pressure increases relative to the pressure outside the body, and therefore, air is sucked out of your lungs and your lungs deflate. (no audio) A small thing on looking at the composition of gases in your lungs – clearly, air that you inhale will contain more oxygen than the air you exhale. Why? Because the whole point of breathing in is to get oxygen into your lungs so it can diffuse into your blood and then be taken around the body for respiration. Respiration obviously produces carbon dioxide, which is why when you breathe out, the air contains more carbon dioxide than the air you breathed in.

Looking more closely at the alveoli – because this is where gas exchange takes place, where oxygen moves into the blood, carbon dioxide leaves. We need to look at the adaptations of alveoli for gas exchange. So clearly, they have a very large surface area. They are thin, which gives you a short diffusion distance for that oxygen and carbon dioxide to move across. They are moist, which helps the gases to dissolve. And I can't think of anything else. So that is the adaptations of alveoli done. (no audio) Smoking, which is one of my favorite topics – I think this is really interesting. I'm going to give you a very detailed answer as to how smoking affects the body. I'm going to take the various components in turn and talk about the effect that that has on your body. So let's start by looking at why people smoke. It's because it contains the drug nicotine. Nicotine is addictive.

It's why people really struggle to give up. It also has adverse effects on the body by making your blood more viscous – that means it's stickier, it's thicker, and is more liable to have high blood pressure and cause blood clots. Looking at carbon monoxide now. You must have heard of carbon monoxide poisoning on TV adverts. It's a very dangerous gas. It's toxic, and that's because it combines irreversibly with the haemoglobin in your red blood cells. It forms carboxyhaemoglobin, and that prevents oxygen transport and effectively suffocates you. We also have tar inside cigarette smoke. Now, that acts as a carcinogen, which means it's a cancer-causing agent, so it contributes to lung cancer. It also coats your cilia, and by coating your cilia, it prevents them from wafting. It paralyzes them and prevents that mucus which is ladened with bacteria from leaving your lungs, leaving your airways, and because of that you're more susceptible to infections such as bronchitis, and that's where people get a really characteristic throaty cough because the removal of this mucus isn't happening passively anymore; they're actually actively having to cough it out, which is why they get that pretty unpleasant cough.

Emphysema is a pretty nasty disease relating to smoking. What happens here is the alveolar walls are damaged. They break down. This decreases the surface area available for gas exchange and effectively means that people can't get enough oxygen into their lungs and they get a very short of breath. (no audio) Let's look at transport in plants. So, we're going to start by looking at two vessels, two tissues you need to know lots about. That is the xylem and the phloem. Make sure you can label these both inside the root, at cross section, and inside the stem. Note that in the root the x matches with the xylem, which is why the x in the middle of the root is xylem. That's a nice way of remembering. Whereas the circles around that x are the phloem.

Inside the stem, slightly different. The outer layer of tissue is the phloem. The inner layer is the xylem. Try and remember this because the phloem transports sugar. And aphids are little insects which bite into that stem in order to steal some food, so they're biting in so that they can reach that phloem and take the food. So the phloem is on the outside of the stem. So, looking more closely at the roles, as I've already told you, phloem transports glucose, and it transports it from the leaves, where it's made in photosynthesis, to other parts of the plant – so to growing regions, such as flowers and the tops of stems, and to storage regions, such as the roots. And that's where it's stored as starch. Xylem transports water this time and mineral ions from the roots, where it's absorbed through the new hair cells, up the plant, to the leaves and various places. So this is an important thing for you to note. Xylem only transports water upwards; phloem transports both up and down the plant. In terms of the structures, because xylem is transporting water and mineral ions, it needs to be very strong.

And notice that it's made out of dead cells which are stacked on top of each other. So there's no organelles in there obstructing the flow of water. There's also lignin, which helps to reinforce those walls further. Phloem has a different structure. Just list some keywords here. Don't talk too much about it. It has sieve plates tubes, and it has companion cells, and those companion cells contain lots of mitochondria so that they can release energy so that sugar can be actively transported in and out of the phloem. Looking in slightly more detail – this is triple content. We're now going to look at how the mineral ions enter the plant. So I think I've already told you this, but they enter by active transport. They enter against the concentration gradient. The reason being is there's far fewer mineral ions in the soil compared with what's inside the plant.

The plant is greedy; it wants as many mineral ions as possible. So because it's against the concentration gradient – from a low concentration in the soil to the high concentration in the plant – it absorbs it using active transport into that root hair cell. Looking at how water is absorbed – again through the root hair cell – this is by osmosis. So it's from an area of high water potential in the soil to an area of low water potential inside that root hair cell.

Lastly, how does water move up through the plant? Well, just to give you a bit of chemistry background here, remember, the water leaves the leaf at the stomata by transpiration. So as the stomata open, the first droplet leaves. But because water molecules are all attracted to each other due to the presence of hydrogen bonding, this forms a continuous column within the xylem. So it's a huge chain of water molecules. So as one leaves the stomata, another one is drawn up from below. And we call this transpiration stream. (no audio) You need to know about how the rates of transpiration may be increased and decreased. So, transpiration, let's remind ourselves, is the loss of water vapour from the surface of the leaf, i.e. through the stomata. So, we're looking at how that water is leaving the leaf. So, how might we increase the rate at which that occurs? And I always say, think about washing/drying outside.

What sort of conditions would you want for your washing to make it dry nice and quickly? So first of all, we want it to be dry, and the same here. This increases transpiration rates when it is dry. Why is this? And that's because there are very few water molecules in the air surrounding the leaf. There's far more water inside the leaf; therefore, water will diffuse out through the stomata very quickly when it is dry. If we look at when it's humid – so when there's lots of water in there, it feels really yucky on your skin – there's a lot of water in the air surrounding the leaf, lots of water in the leaf.

There's not a lot of difference between the water molecules inside and outside of the leaf, so diffusion is going to occur very slowly when it is humid. Next up, what's going to make our washing dry nice and quickly? High temperatures. We know this. In hot countries, our beach towels dry nice and quickly. Same with transpiration. Increased temperature leads to increased transpiration. This is because the water molecules have more kinetic energy, and therefore, they can simply diffuse faster out of the leaf. Next up, we want wind for our washing. And again, windiness will increase transpiration rates, the reason being that the wind blows those water molecules away from the surface of the leaf, effectively increasing the concentration gradient, so diffusion will occur nice and quickly. And then the last point, which has nothing to do with washing/drying, is sun. So when it is sunnier, you will find that there is more transpiration, the reason being that because it's sunny, the plant wants to photosynthesize a lot, so it opens its stomata in order to let in that carbon dioxide, which is needed for photosynthesis.

And clearly, because the stomata is now open, water can automatically leave. (no audio) So what is the actual purpose of transpiration? Why is it a good thing? Because at the end of the day, we're just seeing water leaving the plant, which seems like a bit of a waste. Firstly, the water flowing in the xylem helps to support the plant; it keeps it upright. And this is why, when the plant doesn't have enough water, it starts to wilt. It just doesn't have enough water to make itself turgid. Secondly, it cools the plant. Thirdly, it provides a method for delivering mineral ions around the plant.

And number four, it supplies water for photosynthesis. (no audio) Let's now look at transport in animals. So primarily we're looking at the blood here. So we're going to start by looking at the components of blood. That will be red blood cells, white blood cells, plasma, and platelets. Now, remember, plasma is that liquid which actually acts as a suspension; it carries these various cells around the body. Platelets, these are small fragments of cell; they clot the blood at the site of a wound, so they're very important in forming scabs. The way in which they do this is they convert soluble fibrinogen into insoluble fibre, and that forms the meshwork, which seals the wound. What sorts of things are transported in plasma? These are going to be our products from digestion, so things like glucose, amino acids, so soluble products. They'll also be hormones being transported. They'll be urea from the liver, which needs to be taken to the kidneys so it can be excreted. And carbon dioxide. Looking more closely at red blood cells, we need to look at the structure of red blood cells so we can see how they're so well-adapted for their function.

They have a biconcave disc shape. This means they're doughnut-shaped, and what this does is, it maximizes the surface area to volume ratio, ensuring that they can transport as much oxygen as possible. The absence of a nucleus also means that there's more room for oxygen. And do mention that they contain a pigment called haemoglobin. Remember, iron is needed for haemoglobin production. I mentioned that earlier in balanced diets. And that haemoglobin binds to oxygen, forming oxyhaemoglobin. Next topic – white blood cells and the immune system. So let's first of all discuss how we prevent pathogen entry in the first place. So remember, our skin acts as a barrier. Our hydrochloric acid in our stomach helps to destroy pathogens. Our tears prevent pathogens entering our eyes – and also your eyelashes. But what happens once those pathogens actually enter our body, enter our bloodstream? Clearly, we can't stay ill forever and ever and ever. So there are mechanisms in place which actually act to remove those pathogens. The two mechanisms you need to know about are white blood cells, and they are the phagocytes and the lymphocytes. So starting with the phagocytes, remember that they engulf or ingest pathogens by enclosing them inside a vacuole, and then digestive enzymes are secreted, which destroy the pathogen.

The second type of white blood cell is the lymphocyte. The lymphocyte is far more complicated, and it works by recognizing the antigen on the pathogen. It secretes lots of antibodies which destroy that specific pathogen, and in this way the pathogen is destroyed. Now, it has various modes of action, which helps to increase the pathogen's destruction. First of all, it labels the pathogen, making it easier for the phagocyte to recognize it and therefore engulf it. It neutralises any toxins produced by the pathogen. And it also causes the bacterial cell to burst open on occasion. Lastly, it makes the pathogens stick together. With that answer relating to the lymphocytes, notice that I use lots of keywords: antigens, antibodies, for example. Try and include as many keywords as possible. Just shove them in your answer because if you look at the mark schemes, they'll be underlined as being worth a mark each, so it's worth writing them in anyway. Don't just keep repeating yourself. You need to insert lots and lots of keywords here. Lastly, vaccinations, vaccines. So obviously, when you're going on holiday somewhere tropical, you might need to go to the doctor to get some vaccines.

Now, what these are is they're injections containing either a dead, weakened, or attenuated form of the pathogen. That means it contains the pathogen's antigens. Now, when those antigens enter your body, clearly your lymphocytes are going to be set off, and they're going to produce antibodies, which actually respond to those pathogens, and some of those lymphocytes turn into what's called memory cells. Those memory cells remain in your body, and therefore, if you become infected at a later date with a much larger amount of that pathogen, there are memory cells already in place which can secrete antibodies very quickly – much faster, much sooner, and in a much larger quantity.

And what that means is that pathogen can be destroyed before it can take a hold of your body. So the whole point of vaccination is to inject a harmless version of that pathogen so that if you accidentally become infected at a later date, it can be destroyed before it can take hold. (no audio) So, let's look more closely at some vaccinations which are made. So, dead pathogens are used to immunise against whooping cough. We used a weakened form of the pathogen to treat measles and also tuberculosis And then lastly, we just inject the antigens themselves when we're treating influenza. So we've talked all about blood.

Now we're going to talk about circulatory system, so we need to look at the heart. I've already talked a little bit about multicellular versus unicellular organisms. The reason why organisms such as ourselves – humans – require a circulatory system is because our surface area to volume ratio is too small and diffusion is too slow, so we need a circulatory system which actually acts to transport oxygen around our bodies. So that's the reason why we have a circulatory system. The heart forms the epicenter of our circulatory system. It is the pump which delivers oxygen around our bodies. And you've got to know detailed information about how this actually happens. So we're going to divide the heart into four.

Do you remember that we switch over the left and right sides when we're looking at a diagram? Why? Because we're picking up the heart and pushing it into our bodies, so it is the opposite way around, if you actually think about it. So, the four chambers are: the left atrium, right atrium – they form the top two chambers – the left ventricle and right ventricle form the bottom two chambers. Now, do remember that pulmonary means relating to the lungs, so if you have used the word pulmonary, it means that blood must be flowing either to or from the lungs.

So we're going to start by picking up oxygen in the lungs. It's going to be delivered to the heart to the left atrium via the pulmonary vein. Remember that veins bring blood to the heart; arteries take it away. So the pulmonary vein delivers oxygenated blood to the left atrium. The left atrium contracts, forcing blood into the left ventricle. Do remember that the valves open here, and they are the bicuspid valves. They open to allow blood flow from the atria to the ventricles. The left ventricle contracts, forcing blood into an artery. This is the aorta.

It is the main artery, delivering oxygenated blood around the body. So, that blood goes and delivers oxygen. The oxygen is removed and used by respiring cells. Clearly, the blood will now be deoxygenated, and it needs to return to the heart so it can be pumped on further to the lungs. So it's going to return to the heart via a vein. The vein is the vena cava, and it's going to take blood into the right atrium. The right atrium contracts, forcing blood through the tricuspid valves, into the right ventricles.

The right ventricle contracts, forcing blood into an artery. This artery is clearly going to be flowing to the lungs so the blood can be oxygenated, and this is why it's the pulmonary artery. (no audio) So that's our overview of blood flow around the body. Let's notice a couple of things about the heart – firstly that the walls of the ventricles are thicker than the walls of the atria. Why? Because they need to pump at a much higher pressure to deliver the blood much farther. After all, they're delivering blood to both the lungs and the rest of the body. All the atria are doing is pumping to deliver the blood slightly lower – a couple of centimetres lower, into the ventricles.

Why is the wall of the left ventricle thicker than that of the right? Again, it's the distance thing. Blood from the left ventricle is going all around the body. Blood from the right ventricle is simply returning to the lungs. Your heart is here. Your lungs are here. It's not too far. Why do we call this system a double circulatory system? Well, that's because the blood passes twice into the heart for every once it travels around the body. Simple organisms such as fish have a single loop, so the blood just keeps passing from the gills to the heart, around the body – round and round and round. They're not as efficient at oxygenating their bodies as we are. You've got to know quite a lot about the various vessels that travel around the body.

Just remember that arteries carry blood away from the heart. And do remember that pulmonary means relating to the lungs and that hepatic means relating to the liver; renal means relating to the kidneys – so renal failure, kidney failure; hepatitis, disease of the liver. So these words, if you do know what they mean, it really helps because then when we're looking at, what's the name of the vessel supplying the liver? Well, it's hepatic.

It's coming from the heart, which means it's an artery, so it's the hepatic artery. What is the name of the blood vessel entering the kidneys? Well, it's coming from the heart, so it's an artery; it's going to the kidneys, so it's renal. So it's the renal artery. The vessels leaving the organs, well, these have obviously got to return to the heart to become oxygenated, so they're going to be veins.

So the vessel leaving the liver will be the hepatic vein. Just note: the hepatic portal vein, that's just the name of the vessel which shunts blood from the digestive system to the liver. And that's the only weird one you need to know about. Looking at coronary heart disease now, coronary arteries: coronary means relating to the heart. So the heart has its own special network of vessels which supply the heart with its own supply of oxygen. It can't actually obtain its oxygen needs from the blood flowing through it; it has to have its own special set of vessels.

We call these the coronary arteries. And they're famous because this is how people get heart attacks. They get blocked; they get obstructed. And it does mean that oxygen can't reach the heart muscle, so part of it dies, which is what a heart attack is. So first of all, what factors increase your chance of getting coronary heart disease? So that could be a sedentary lifestyle, so lack of exercise. Could be your diet, eating diets high in fat and sugar. It could be inheritance, so genes. Some people are just more susceptible than others because of genes that they've received from their parents. It could be diabetes. Diabetes and coronary heart disease are very closely linked. Stress, as well. People shouldn't get too stressed because that can put a strain on their heart too.

In terms of what happens in coronary heart disease – how a heart attack might occur, what you find is that fatty deposits get offloaded in the walls of the coronary arteries. This obstructs the blood, meaning that less oxygen can reach those respiring cells. Because they're not receiving enough oxygen, they have to anaerobically respire. Remember, that produces lactate, which slowly poisons the muscle cells, and then eventually, there isn't enough oxygen, so the heart dies and those cells die. (no audio) Let's look at how a heart rate increase is brought about during exercise, for example. So clearly, when you exercise, you're going to produce more carbon dioxide. Why? Because your muscles are respiring more. So that carbon dioxide flows in the blood, and it is detected by both the aorta and the carotid artery. This sends impulses, or messages, to the brain, particularly the medulla part of the brain, and specifically the accelerator nerve. Now that accelerator nerve causes an increase in heart rate so that more oxygen can be delivered to your muscles and so that more carbon dioxide can be removed.

(no audio) Looking now at the structure of arteries, veins, and capillaries – so let's start with the arteries. They have a narrow lumen. Remember, the lumen is the hole in the artery. It's like the hole in a straw. So it's narrow. This clearly means that blood is going to be forced through at a high pressure. Because it's at a high pressure, it means that the walls of the arteries need to be very thick to withstand this pressure. Be nice and detailed here and state that they have thick muscle and elastic fibre walls. Looking at veins now – in veins, blood travels at a much lower pressure. This is because veins have a much wider lumen. The walls, therefore, need to be much thinner. So they have thin muscle and elastic fibre walls. They contain valves, and these valves prevent the backflow of blood because that blood sometimes travels so slowly, it's liable to start backing up.

We don't want that to happen, so the valves force it to move on in the right direction. Capillaries now – capillaries are the tiny vessels that supply all our cells with oxygen. They are one cell thick, and this enables a very short diffusion distance. And they have an extremely narrow lumen. Moving on to excretion – so, both in plants and humans. Obviously, when we're talking about humans, we'll be really looking at the kidney. Let's start by looking at the definition of excretion. It's the removal of waste products of metabolism from the body. And if we look at the removal of waste products in both photosynthesis and respiration in plants – photosynthesis, obviously, produces oxygen, so that will be one of the waste gases which is lost through the stomata of the plant leaf. And in respiration, obviously, carbon dioxide is produced, so that will be, again, released out of the stomata. (no audio) Looking at excretion in humans – what sort of substances are excreted? First of all, sweat from the skin, urea from the kidneys, carbon dioxide from the lungs.

Do note that feces are not excreted; they are egested. And I already touched on this in the nutrition part of this video. Now we're looking at triple content, so, really, stuff related to the kidney. So, remember, with the kidney, we have the nephron. This is when you take the kidney and you zoom in and you see that there's a Bowman's capsule, followed by the proximal convoluted tubule; the loop of Henle; the distal convoluted tubule; and then lastly, the collecting duct. So let's have a look, first of all, at what happens at the Bowman's capsule. That is a process of ultrafiltration. Ultrafiltration is when small molecules are filtered out of the blood into the Bowman's capsule. Such molecules include urea, sodium and chloride ions, glucose, and water. And remember that these are forced out under pressure. So how does the structure of the Bowman's capsule and the blood vessel flowing into it – the glomerulus – actually aid this ultrafiltration? The reason is because the blood vessel entering the glomerulus is wider than the one leaving.

This creates a huge amount of pressure, which actually forces those small molecules out of the blood, into the Bowman's capsule. Which molecules don't enter the Bowman's capsule? Well, that's proteins. Why? Because they're too large to pass through the basement membrane. Now we've entered the proximal convoluted tubule, we need to know what happens there. The process of selective reabsorption occurs here. Now, this is when all glucose, some ions, and none of the urea is removed from the proximal convoluted tubule back into the blood. This is occurring against the concentration gradient, so it requires energy. Now we're moving on to the collecting duct because we now need to look at water reabsorption, and this is all to do with osmoregulation. Now, osmoregulation is the control of water content in the blood. And obviously, sometimes we drink lots, so we need to wee a lot; sometimes we drink very little, so our wee will be very, very concentrated and small in volume.

And now we're going to look at the mechanisms involved in controlling the amount of urine we produce. So let's start when we've not had very much to drink, so overnight, for example, or a hot day. Clearly, our bloodstream will contain little water. Now, the hypothalamus is a part of your brain that detects the water content of the body. What happens is it sends a signal to the pituitary gland, also found in the brain, and that secretes a large amount of a hormone called ADH, antidiuretic hormone.

That passes in the blood to your kidneys, specifically the collecting duct, and it basically tells the collecting duct to be more permeable to water. So it adds lots of channels into the collecting duct, which enables lots of water to leave the collecting duct and be reabsorbed back into the blood. This means that there's far less water left over to create urine. So in this situation, urine is very yellow; it's concentrated and low in volume. Let's look at the opposite effect, so when we have had lots and lots to drink. This time, the hypothalamus will detect high water levels in the blood. It will send a signal to the pituitary gland saying that there is lots of water in the blood. So therefore, little ADH will be released. ADH travels in the blood to the collecting duct of the kidney. It makes the walls of the collecting duct less permeable to water, meaning that more water stays inside the collecting duct and that less is available to be rebsorbed. This means that there is more water flowing from the collecting duct to the bladder, and therefore, resulting urine is high in volume, not very concentrated, and very pale in colour.

Just to touch on the name of some key vessels – the vessel linking the collecting duct to the bladder is known as the ureter. The vessel taking urine out of the body – so from the bladder – is known as urethra. And if they ask you, in men, what two substances does the urethra transport out of the body, that would be both sperm and urine. Definitely not water. Some people in the past have written water. That is really silly; that is not the answer. Looking at the general structure of a kidney, you can see the outermost layer is known as the cortex; the middle layer is known as the medulla; and then the ureter passes out; and then we've obviously got the renal artery and the renal vein supplying and removing blood.

Moving on to the coordination and response topic, again starting with plants, let's first of all discuss the meaning of the word homeostasis. That means the maintaining of a steady internal environment. So, we're going to take plants first of all, and we're going to look at tropisms. First of all, what is a stimulus? And that is change in the environment. So, what sort of stimuli do plants need to respond to? Clearly, they need to respond to the amount of light.

And they actually also respond to gravity. So we call a plant's response to light a phototropism. A plant's response to gravity is known as geotropism. So, how do organs of a plant react to various tropisms? So let's take phototropism, first of all. Clearly, a stem shows positive phototropism. Why? Because it grows towards the light. Roots show negative phototropism because they grow away from the light. Let's look at gravity now.

Roots obviously show a positive geotrophic response because they grow down, in the direction of gravity, whereas stems show a negative geotropism because they grow away from gravity. Looking more closely now at how these changes are brought about to the stem and the roots, we need to understand the role of auxins. So what are auxins? Well, they are plant hormones. Let's now explain how a plant stem may bend towards the light, towards the Sun – because you do see, particularly sunflowers, bend towards the Sun.

That's because the auxins concentrate on the side furthest from the light source. This causes cell elongation on that side. So the cells get longer, and therefore the plant stem bends towards the light. They do like to show you various experiments. And an important part of this will be the coleoptile, which are these really boring little seedlings. Now, note that they are cereal seedlings. And they are simple plants used to investigate tropisms. So do be aware of how they respond if you chop their tops off, if you put a mica sheet in between the top of the stem and the bottom of the stem, if you use agar jelly, you do need to know different responses. Lastly, the role of a clinostat – this is another boring piece of equipment.

So a clinostat is a device used to remove any stimulus. So, for example, it negates the effect of gravity or it will negate the effect of sunlight. Now, we've touched on plants; now we need to look at humans, so we'll talk about the nervous system here. So, let's start by looking at, again, what is a stimulus? It is a change in the environment. Now, that change in environment is detected by various sense organs, and you need to be aware of their names and what they're actually receiving information about. Starting with the eye, that receives the light energy. The ear receives sound energy and kinetic energy. The muscles in your skin receive kinetic energy. Your tongue receives chemical energy. So your nose, it also receives chemical energy. Why? Because that's chemicals in food, and that's what you smell.

And then lastly, your skin receives kinetic energy and heat energy. So, there are two types of communication you need to be aware of: the nervous communication and hormonal communication. So, hormones are chemicals which travel in your blood. They send signals and messages to various parts of your body, whereas the nervous system involves the use of electrical impulses. So let's have a quick comparison between the two. First of all, the nervous system is much faster than the hormonal system. Clearly, electrical impulses will get to their location much faster than hormones travelling in the blood. The nervous system involves very localised responses. So the electrical impulse will be locating a very specific effector muscle, whereas the hormonal system has far wider spread effects. The nervous system responses are short-lived, whereas the hormonal system involves much longer lived responses.

And the last thing which I've already mentioned, but just to really point it out to you – the nervous system involves electrical impulses, whereas the hormonal system involves the use of chemical messengers. (no audio) So, we're going to take the nervous system in greater detail now. So let's, first of all, look at what a stimulus is. So that's the change in the environment, and obviously that causes the response the nervous system brings about. Do be aware of what the central nervous system is. That consists of both the brain and the spinal cord. Let's go through all the steps involved in a regular nervous response, one which does not involve a reflex action. So I'm going to use picking up a book as an example. So first of all, we need to list the stimulus, which is seeing or viewing the book. This is picked up by receptors, and these receptors will be in your eyes – your photoreceptors on your retina. They'll send electrical impulses along your sensory neuron, to your central nervous system.

Electrical impulses then pass along your motor neuron, to your effector, and this will be muscles or glands, and in this particular case, it will be muscles, which will contract to pick up your book. Remember, an effector is either a muscle or gland. So, a muscle responds by contracting; a gland responds by secreting hormones. Don't forget the role of the synapse. So, the synapse is the gap between two neurons, and this is where a neurotransmitter is released. So the neurotransmitter diffuses across that synaptic gap, that synaptic cleft, and binds to the postsynaptic membrane. So, your electrical impulse is changed to a chemical or a neurotransmitter, and then changed back to an electrical impulse at a synapse. Looking now at reflex actions, remember these are faster and they are involuntary, so they do not involve a conscious part of your brain. And it tends to be in response to something painful. So, taking and putting your hand in the oven and accidentally touching one of the shelves which is hot, this would trigger a reflex action.

So, the stimulus this time would be the high temperature from the oven tray. Your receptors would be on your fingers, which would receive that information about it being too hot. Electrical impulses would then flow along your sensory neuron, to your relay neuron this time. So we're not involving the conscious part of our brains. The electrical impulse passes along the motor neuron, to your effector, which would be a muscle in your finger, which contracts to remove your hand or your finger away from that heat source. (no audio) Right. We're going to look, now, at the eye. You need to know lots about this. It's a very important sense organ. So I'm going to start by running through its various structures and their roles. So, first of all, light enters the eye, and it hits the cornea. So the cornea's role is to refract the light, to bend that light as it enters the eye. The light then has to pass through the pupil. So, the pupil's role is simply to allow light into the eye. The size of the pupil, however, is controlled by the iris, and that is the colourful part of your eye.

Mine's brown; other people's are green or blue. And the iris has circular and radial muscles which will actually help it control the size of the pupil. We'll go into that in slightly more detail further on. So now the light has entered the eye, it now hits the lens. The lens' role is to refract that light further in order to focus it on the retina. The retina contains photoreceptors. These are cells which are sensitive to light, and they are called rods and cones. Rods are sensitive in very dim light, and cones are sensitive to colour. Then we have the optic nerve, and that converts those light signals into electrical impulses which can be carried to the brain, where they can be computed. The white bit of your eye is the sclera, and that just forms the tough outer casing. If you've ever dissected an eye, you'll notice it's full of some black, inky stuff. That is the choroid layer, and that's role is to prevent the light bouncing around within your eye. Then we have a blind spot. The blind spot is simply the place on the back of the eye where the optic nerve leaves the eye – because clearly, there can't be any photoreceptors where you have the optic nerve.

The fovea is a place that is very concentrated with cones, so it's very good for seeing colour. (no audio) We need to look a bit more, now, at the lens. So, I've already told you that the lens focuses light onto the retina. It does this by a process of accommodation. So, accommodation is all about focussing light from different distances away so that an image can be formed on the retina. So, let's look at what happens when we're looking at an object far away.

So when it's far away, the light is going to be coming into your eyes fairly parallel, which means it doesn't need refracting very much. Therefore, the lens needs to be thin. In order to do this, the ciliary muscles relax, and the suspensory ligaments are taught. And this means you have a very nice, thin lens, so it doesn't refract the light too much. Looking at an object really close to us now, the light rays are going to be coming in at a far greater angle; therefore, the lens needs to do far more work in order to focus light onto the retina. In order to do this, the lens needs to be fat.

So in this case, our ciliary muscles contract and our suspensory ligaments slacken off, meaning that our lens is nice and fat. So that's the way in which we focus on light at different distances away. We now need to look at the pupil reflex. So the pupil changes its diameter dependent on light levels. So if there's lots of light, the pupil needs to be narrower, and that's to prevent damage to the retina because too much light entering the eye will damage the retina.

If we're in a dim or dark room, the pupil obviously needs to be nice and wide in order to allow as much light into the eye as possible. We can use this response as an example of a reflex action. So, previously, I told you all the steps involved in a reflex action. And we're just going to use those steps, but we're going to apply them specifically to this example. So I'm going to use an example which is that we have walked into a very bright room. So, the stimulus will be lots of light. The receptor will be the rods and cones on your retina.

Then the sensory neuron will pass along the optic nerve, to the brain, and there will be a relay neuron. Then we have a motor neuron, which is also passing along the optic nerve, and it will end in an effector. And in this situation, the effector is the muscles which you find in your iris, so your circular and radial muscles. And you'll find that your circular muscles will contract and your radial muscles will relax, and that will act to narrow or constrict the pupil. Let's look at the role of the skin now. So, first of all, the skin acts as a barrier, and that actually helps prevent pathogen entry because we often have viruses and bacteria landing on us, but because the skin is such an amazing intact organ, it does actually prevent those microorganisms entering our body.

It's also waterproof, which is why you don't swell up when you go swimming. It forms a very tough outer layer just generally to prevent you from physical harm if you knock into something. It's a sense organ for pressure, touch, and pain. And lastly, it controls our heat loss. How? By either sweating or the hairs stand up in order to trap some insulating air close to our bodies. (no audio) Let's now look at what happens when were too hot and too cold. So, we're looking further at the homeostasis topic, which, remember, is all about controlling a steady internal environment within our bodies So, let's start by looking at when we are too hot. Well, first of all, an uncomfortable thing happens, which is that we sweat, the reason being that when sweat evaporates, it acts to cool the body.

Vasodilation occurs, and that's the arterioles in our face; they dilate, they become wider. What that means is that the blood flows closer to our skin, and therefore, heat can be radiated more easily. And also, when we're too hot, the hairs lie down on our body on our arms, and that's just so that less insulating air is trapped close to our skin because remember, air is a good insulator; it keeps you warm. So by making our hairs lower, it means we're kept less warm.

(no audio) So, if we're too cold, clearly our hairs will stand up on end so that more insulating air is trapped close to our skin, and that actually acts to keep us warm. We shiver, the reason being that that muscle contraction releases heat energy. And lastly, the opposite of vasodilation occurs: vasoconstriction. The arterioles in our face constrict – they narrow, bringing blood away from the surface of our skin, so less heat is radiated. (no audio) Now looking at a separate hormone, which is adrenaline, remember that is the fight or flight hormone.

It's released in bucketloads when we're under conditions of stress. And it's good to imagine a really angry cat when you're thinking about the effect that adrenaline has on your body because if you imagine the cat, then it will actually help you remember every single thing that happens to you when you have adrenaline coursing through your body. So, this cat's hairs have stood on end, and that's because it wants to appear scarier to its opponent. Its pupils have dilated, and that's to allow more light into its eye so it can see more clearly. Its heart rate – not that you can see this – but adrenaline has caused its heart rate and breathing rate to increase. Why? The heart rate increases to deliver blood faster around your body. Your breathing rate increases to allow more oxygen into your body for respiration.

Blood is diverted from your digestive system to your active muscles in your arms and your legs, and that's so you can either fight or run away if needs be. So, yeah, try and remember the cat, at least for the hair standing up on end and the pupils dilating. (no audio) Another thing to do with homeostasis – we're looking at blood sugar.

So how do we decrease our blood sugar levels after we've eaten? Well, we release a hormone called insulin. Now, insulin is secreted from the pancreas, and it causes glucose to be converted from its soluble form into glycogen, which, remember, is insoluble, and that can be stored in the liver. We are now moving on to reproduction. So, we'll start by looking at the difference between sexual and asexual reproduction. Hopefully, sexual reproduction is nice and straightforward. Clearly, you need two parents. And it involves gametes. Remember, the gametes are eggs and sperm because these are sex cells. So, the sperm and egg meet at fertilization. The first cell formed is known as the zygote. And then it divides by mitosis to form your multicellular organism. And clearly, due to the production of gametes, these offspring will be genetically varied, which is a really important thing to note with sexual reproduction is that it produces genetically varied offspring.

So if the environment changes, it means that some individuals will be better adapted whilst others will be less well adapted. And that's where natural selection comes in, but we'll come across that later on in this video. With asexual reproduction, you only need one parent. It's a much quicker process. You end up with genetically identical offspring. And this is why it's good when conditions are unchanging. It's a great way of quickly producing large numbers of identical offspring, which we call clones. There is no gametes, no fertilization, and no zygote is formed.

Some key definitions now – so fertilization, that's obviously the joining of an egg and a sperm. A zygote is formed, and remember, it undergoes mitotic cell division, so lots of mitosis place to form two cells, four cells, eight cells, sixteen cells. So if they ask you how a 32-cell embryo is formed – sperm and egg, gametes – make sure you include lots of keywords – join at fertilization. The zygote is formed. It divides by mitosis to form 2 cells, 4 cells, 8 cells, 16 cells, 32 cells. And that's how you get lots and lots of marks by not writing very much – by including keywords such as gametes, mitosis, et cetera.

Now we're looking at asexual reproduction in plants. The examples you need to know about is a strawberry runner and potato tubers. So these will produce mini plants. You might have seen them on spider plants as well. They'll produce mini plants which are clones of the original plant, and you can snip them off and transplant them somewhere else. Looking at sexual reproduction now – so obviously we need to look at the structure of a flower, how sexual reproduction takes place. So, in order to look at reproduction in plants, we obviously need to look at the structure of a flower. You need to know the names of the different parts of the flower and know that the flower is separated into both male and female parts.

So, the male part consists of the anther and the filament. So, the filament is actually the thing that supports the anther, whereas the anther contains the pollen grains. And they're essential because they're the male gamete in a plant. The stigma and the style and the ovary are the female parts of the plant. The collective noun for them is the carpel. And everything else is just extra detail that will actually help attract an insect or will help with the wind pollination aspect of sexual reproduction in plants.

So now we know about the structure of the flower, that will help us understand what pollination is, fertilization, et cetera, et cetera. So, we're now going to look at seed formation. It's important that you know every step involved in this because you could be asked a five-marker on this. So, first of all, pollen from a male anther will land on the female stigma. A pollen tube will grow down the style. And digestive enzymes will help break down the wall of the ovary. At this point, the pollen will meet the ovule, which is the female gamete, and fertilization takes place. The ovule then goes to form the seed.

The ovule wall forms the seed coat. And lastly, the ovary wall will form the fruit. So make sure you list those last steps involved. (no audio) First of all, let's understand what we mean by the term pollination. You've got to be super specific here. So, remember that the male gamete in a plant is pollen; the female gamete is the egg. But all pollination is is transferring that pollen to a separate plant. So your perfect definition here is that pollen produced on the anther, which is a male part of the plant, is transferred to the stigma, which is a female part of the plant, on a second plant. And the perfect definition will pop up now so you can see exactly what you need to write.

Self-pollination, obviously, as the name suggests, that's when the plant does it itself. So that's where pollen from a single plant will land on the stigma of the same plant. Fertilization now – so remember, fertilization in humans is all to do with sperms and eggs fusing. This time, as we're talking about plants, it's about the pollen fusing with the egg. Looking now at the difference between insect- and wind-pollinated plants – so let's, when we're talking about the insect-pollinated one, think about all the ways in which these flowers, these plants make themselves appealing to insects. So, first of all, they have bright, large, colourful petals.

These are flag-like structures, and they literally draw the insects' attention. So they're like, "Right. Come and pollinate me." Secondly, they have a nectary. The nectary is essential because that's actually why the insect is visiting the plant in the first place. It's not doing the plant a favour; it doesn't give a damn about the plant. It's simply trying to obtain the sugar from that nectary, which is why insect-pollinated plants have a nectary. They have enclosed stigma and anther, and what that means is that the insect is forced to rub against to the pollen or the stigma when they enter the plant to find the nectary, so they're more likely to pick up the pollen. They have a strong scent so that the insect can smell them. And that's everything I can think of.

Looking now at a wind-pollinated plant, it's going to be very different from an insect-pollinated plant largely because it's the wind blowing that will actually blow that pollen away to a separate plant, which is why it makes sense, therefore, that the anther are very exposed, so that when the wind blows, the pollen will literally blow off the plant and be carried by the wind elsewhere. They tend to be dull-coloured, have small petals, no scent for the same reason: because the wind can't see these things because it's wind, so there's no need to have flag-like petals.

There'll be an absence of a nectary, again because the wind doesn't need sugar. They have small petals, small pollen grains, feathery stigma. Just list these all out and you'll be fine. Next up, we need to look at germination. So first of all, what is germination? This is, obviously, what happens when you plant a seed, and it effectively pops out and starts growing, but you need a detailed answer. So, first of all, the seed coat bursts.

The radical is the name for the small root that appears, and that starts to grow downwards. A small shoot will appear, and obviously that starts to grow upwards. And the seed's food store is used up because the plant can't photosynthesize until it grows its first leaves. In terms of the conditions needed for photosynthesis, remember the mnemonic "wow," standing for warmth, oxygen, and water. So all these things are needed in order to enable a seed to germinate. Moving on to the male and female reproductive system in humans, now, and there's a new emphasis for you guys on the various roles of different components of the reproductive system. So let's start with the female reproductive system. So, starting from the beginning, you've got the vagina. The entrance to the uterus is the cervix. And then branching off the uterus, you've got the fallopian tubes, or the oviduct … lastly ending in the ovaries. So, what is the role of the ovaries? Well, it's to manufacture eggs. The role of the fallopian tube is to deliver eggs to its entrance.

So that's where fertilization takes place. The uterus is obviously where the zygote embeds. It develops into an embryo, into a growing fetus, and it supports the embryo through the placenta. The cervix is simply the entrance to the uterus. And the vagina is a passage leading out of the woman, and it's where the penis inserts to deposit semen during sexual intercourse. And lastly, the urethra – it's not really to do with the female reproductive system, but do remember, it's a separate tube, and it transports urine out of the body, so it links to the bladder. Please don't think that a woman urinates out of her vagina. Lots of people seem to be very confused on this. No. There are two separate passageways. Moving on to the male reproductive system now, so we're going to start by looking at the testes, or testicles. These manufacture sperm and also the hormone testosterone. They link to the urethra via the sperm duct. The sperm duct is simply a tube which transports semen from the testes to the urethra. The urethra is a tube which links the sperm duct to the outside of the body, and in men, it transports both semen and also urine.

And then, lastly, there are some glands, such as seminal vesicles – some of you may not need to know this – the prostate gland, and these just contribute fluid to the semen so that it's not just made up of sperm. Lastly, the penis – it passes urine out of the body and deposits semen inside a woman's vagina. Now, let's be sensible about the topic and no being silly. Now we're describing the passage of sperm in the female. So, clearly, I just said the sperm is deposited in the vagina. It swims through the cervix, into the uterus, and lastly, it swims all the way to the entrance of the oviduct, or the fallopian tube, where fertilization actually takes place. So the sperm has a very long way to swim. The role of the placenta – remember, this is a huge organ which actually supports the growing fetus. So it provides the fetus with oxygen; digested nutrients, such as glucose and amino acids, to help it to grow; and it also removes waste products, such as urea. Later on in pregnancy, it takes over the role of producing the hormone progesterone.

And progesterone is a good place to link because we now need to look at the various hormones involved in the female reproductive system. So, we're going to start with FSH. Now, FSH is produced in the pituitary gland, and it stands for follicle stimulating hormone, and that's a really good way of actually helping you know what it does. So, its role is to stimulate the follicle.

The follicle is the egg. So its role is to mature the egg in the ovary. LH is also made in the pituitary gland. It is lutenising hormone, and its role is to cause ovulation. Now, ovulation is the release of an egg from an ovary. Next up, oestrogen. Oestrogen is produced by the ovaries. It's responsible for secondary sexual characteristics in females, so those are all the changes that occur during puberty, such as breast development, hips widening, pubic hair, armpit hair, all those sorts of things – sexual drive develops. Its other role is to inhibit FSH, so it actually slows the production of FSH, which makes sense, really, because if you've already had an egg released, you don't want to be maturing any more eggs because presumably you're going to be fertilizing that first egg. And its last and very important role is to repair the uterus lining, so it causes it to thicken in preparation for a fertilized egg. Progesterone, now – Progesterone is produced by, firstly, the corpus luteum. The corpus luteum is just the leftover husk, effectively – when the egg is ovulated, it's the leftover structure.

That produces progesterone. Later on in pregnancy, the placenta, as I've already told you, takes over the role, and its role is to maintain the thickness of that uterus lining. Without a thick lining, a woman will miscarry because it really needs to be thick in order to support the growing fetus. And then, because we need to mention men, testosterone – its role is to support the development of secondary sexual characteristics, so again the puberty changes. This includes pubic hair, armpit hair, widening shoulders, bigger muscles, voice breaking or deepening, sexual drive develops, sperm production occurs.

Moving on to the protein synthesis part of the specification – we need to start by looking at some key definitions, such as genome, and that is the entire DNA belonging to an organism. So, we're focussing on the nucleus of a cell. Remember that the role of the nucleus is to control the activities of the cell, and it does this because it contains lots of genetic material. So, within the nucleus, we know that there are chromosomes. There are 46 chromosomes, which is a diploid number because remember, they're arranged as 23 pairs. Remember, half the number of chromosomes is known as a haploid number.

However, I digress; I don't really want to talk about that now. So, the chromosomes are made up of DNA. You need to know the definition of a gene. A gene is a section of DNA which codes for a particular protein. Looking more closely at DNA now – this is triple content. So, we need to, first of all, know what the structure of DNA is. Remember, it looks like a ladder; we call this a double helix, which winds itself up. It's made up of a sugar-phosphate backbone, and remember that the sugar here is deoxyribose. Why? Because DNA stands for Deoxyribose Nucleic Acid, so the sugar here is deoxyribose. There's a phosphate and sugar backbone, and then linking the two backbones, the rungs of the ladder, are bases, and remember the names of these bases – adenine, thymine, guanine, and cytosine. Looking more in depth at the various names involved – nucleotide, a nucleotide is simply just three units made up of a deoxyribose sugar, a phosphate, and lastly a base. So it could be either one of those bases I've just mentioned, say adenine, thymine, cytosine, or guanine. (Martin) Sorry, is that how you pronounce it, guanine (ɡuː-ə-niːn)? How do you say it? (Martin) I've always heard it guanine (ˈɡwɑːniːn).

I got taught guanine (ɡuː-ə-niːn).
(Martin) Really? But it sounds weird now you're saying it. Martin's telling me that I'm pronouncing guanine (ɡuː-ə-niːn) wrong. My biology teacher said guanine (ɡuː-ə-niːn), and don't know how to say it any other way. Apparently, he says guanine (ˈɡwɑːniːn). Hmm So, you're like, "What an idiot." So, we need to now look at complementary base pairing. That's when the various bases pair up in a very specific way. Try and remember the shape of the letters match. So guanine (ɡuː-ə-niːn) or guanine (ˈɡwɑːniːn) always pairs with cytosine because they're both curly in terms of the G and the C.

Adenine and thymine always pair up, and that's because they're the straight letters. So use that to help you remember. Looking at the difference between RNA and DNA, the main difference here is the sugar involved. So RNA has ribose sugar as opposed to DNA's deoxyribose. And lastly, there's a change in base. There is no thymine in RNA. You've got to learn that this time, it's uracil. So adenine pairs with uracil in RNA, and cytosine and guanine still pair up. RNA is single-stranded compared with DNA's double strand, so we don't see a ladder with RNA; we just see one side of the ladder, effectively. Let's move on to looking at what a codon and an anticodon is. So a codon is just a fancy way of describing three bases which are found on the mRNA molecule, and they correspond to a single amino acid. An anticodon is the three complementary bases, which you find on tRNA.

And they pair up with the codon. So although I've touched on mRNA, tRNA, codons, and anticodons, we need to actually understand why we're even mentioning these words. Hopefully, you'll know this from your school classes. After all, this is a revision video. But now I'm going to go into great detail about protein synthesis. So remember, protein synthesis is all about making new proteins. It's about sorting out the arrangement, the sequence of amino acids so that it can actually produce a particular protein. And we're going to talk about how those amino acids line themselves up in the correct order, aka protein synthesis. So there are two main stages: transcription and translation.

And the first stage is transcription. So we need to go into that now. Bye, cat. So, with transcription, first of all, the DNA needs to unwind to expose a single strand. So we're going to expose those bases on the DNA. There are bases floating around inside the nucleus – mRNA bases – and they come along and they pair up with those exposed DNA bases. (no audio) And that will start forming the mRNA molecule. And it's a single strand, noticing that uracil has replaced thymine. So we call this complementary base pairing. And a codon is simply three of those mRNA bases in a line. So we have created our mRNA – our messenger RNA because it carries the message from the DNA and now needs to leave the nucleus, so it leaves via a nuclear pore and attaches itself to a ribosome, which we find within the cytoplasm, hence the summary definition of ribosome's role is that it carries out protein synthesis. So the mRNA attaches to a ribosome, and this is where translation begins. So there are tRNA molecules within the cytoplasm, and they have complementary anticodons, so they'll have the three bases which complementary pair up with the mRNA exposed bases.

And on the other end of that tRNA molecule will be an amino acid. And effectively, the tRNA brings that amino acid to the mRNA. The codon will match up with the anticodon, and we have our first amino acid in place. Then the next three bases on the mRNA will be read, and a different tRNA molecule will bring, probably, a different amino acid along because if there are different bases, it will correspond to a different amino acid. So the second amino acid has been brought, and they attach to each other using a peptide bond. So that is the start of our protein chain, and it keeps going and keeps going, and eventually a stop codon will be reached, and that's just three specific bases which correspond to no amino acid whatsoever, and that will signal the end of that growing protein chain.

Back to double award, so back to everyone now. We just need to know the definition of a mutation. That's a random change in the DNA of an organism. So, effectively, those bases, we know that they occur in a certain order within DNA. A mutation will cause a change to those bases. So first of all, it could just be a straight substitution. It could be a deletion, so a whole base could just be taken out.

And you find, with a deletion is that, effectively, all those bases move along one place, so you'll end up with totally different amino acids being produced. It's quite likely that you'll end up with huge disruption in the protein made. There might be an inversion, which is when the two sides, the two bases, in the DNA swap over. There could be a duplication, when a base is simply copied, so there could be two adenines in a row, for example. And again, that will most likely cause a change in the amino acid which is brought along to the ribosome. (no audio) We're going to talk about genetics now. So we need to know the definitions of lots of very key, important terms, and then I'm going to show you how to do Punnett squares and pedigree analysis. So let's start by looking at what a gene is. A gene is a section of DNA which codes for a particular protein. Now, there are different genes which control different traits – so, for example, eye colour. Now, different forms of the same gene we call alleles.

So, you must learn that definition. So, if we take eye colour for example, different alleles for eye colour could be blue or brown. You must now know the meaning of the word genotype. The genotype is the alleles an organism has. So, for example, when we're talking about blue eyes, it's two small b's (bb); if we're talking about brown eyes, it could be big B, little b (Bb). So when you're asked for genotype, you must provide letters. The phenotype is different. This is the physical appearance of a particular trait. So if you're asked for the phenotype of this eye colour, the answer here is blue.

So the genotype would be little b, little b (bb). The phenotype would be blue eyes. So be very aware of that distinction. Next up, we need to know the meanings of homozygous and heterozygous. Homo- means same, so it means having two of the same alleles. Whether that's two big B's (BB) or two little b's (bb), it doesn't matter, as long as they have the same case. So they both need to be upper or they both need to be lowercase. And that is the meaning of the word homozygous. Heterozygous means different, so that means containing different alleles. So in the case of eye colour, that would be a big B and a little b (Bb). Now, a dominant trait requires simply the presence of one allele for it to exhibit itself in an individual. So, brown is an example of a dominant trait because you can have two big B's or a big B and a little b (Bb), and the trait will still appear. Recessive requires the absence of the dominant allele. So a recessive trait could be blue eyes because you need two little b's, for example.

(reading visual aid) So, let's work out what we have here first of all. Blue-eyed – remember, blue is a recessive trait, which means that her genotype must be small b small b (bb). We've been told that the father is brown-eyed, which means he could be big B, small b (Bb), or big B, big B (BB). But the fact that he is heterozygous tells us that he must be big B, small b (Bb) and not big B, big B (BB). So I'll show you how to lay out your answer. This is the method you should always use. So start by writing mother and father at the top. And we know how much I love tables. So you're going to write in your table phenotype, genotype …

And lastly gametes. Remember, these are sperm and eggs. So, the phenotype, this is the physical appearance. We can see from the description that the mother has blue eyes … and the father has brown eyes. The genotype – so these are the alleles that each parent has. We've already written these out. So it's small b, small b (bb), big B, little b (Bb).

The gametes – just split these up because this is saying what the eggs and sperm will be, so just write out what you wrote on the genotype layer. But put circles around them to show that they're gametes because here are the sperm, and the mother's are eggs. And now we need to do the Punnett square. So mother … Father. I don't know why my iPad sometimes undoes what I've done already. So she's … small b, small b (bb). He's big B, little b (Bb). Let's cross them. So big B, small b (Bb), big B, small b (Bb), small b, small b (bb). And therefore, both of these will have blue eyes. Both of these will have brown eyes. So as a percentage likelihood, it's 50% will have blue eyes … and 50% will have brown eyes. (reading visual aid) So it's recessive, which means to have the disease you need this genotype – small c small c (cc). It doesn't matter what letter you use to assign, by the way, but I'm using C here because of cystic fibrosis.

"A carrier mother and a carrier father" – that automatically tells me that this is their genotype, and you must learn that they're effectively heterozygous if they are carriers. So we've worked out their genotypes, so we're ready to do the genetic cross by writing mother and father at the top again … phenotype … genotype … and gametes in the table. The phenotype is that they are both carriers. Their genotype, we know, is heterozygous, so it's big C, small c (Cc). This means that half of her eggs will be big C, half of them will be small c.

And the same with the sperm. Make sure you really show a difference in the size of your letters here. And now it's time to do the Punnett square. So, this child is big C, big C (CC), so they'll be healthy. This child is big C, small c (Cc), so they'll be carriers, but they'll still be healthy. Same for this one.

And lastly, this child here will have cystic fibrosis. So they have a 25% chance of having a child with CF. Codominance is when both alleles are expressed in an individual. The example here we use is red snapdragons. (no audio) So now we need to look at pedigree diagrams, and the best way to do this is by showing you an example. Always use this approach. And do notice, these are supposed to be really difficult, so don't worry too much if you're pining it too much. Question 3. "Familial Hypercholesterolemia (FH) is an inherited condition caused by a dominant allele." That is key. "People with the condition have high levels of cholesterol in their blood "[increasing] the risk of dying from blocked arteries.

"The diagram shows the pattern of inheritance in several generations of a family with familial hypercholesterolemia." So, do notice with a pedigree diagram that the squares are always the men in the family. You'll know this from the key. The circles always represent females. And in this case, from the key, we can see that the grey-shaded boxes are sufferers of FH whilst the white boxes or circles are non-sufferers. So, "Person A is heterozygous for FH. Use this information to complete the table." So, let's start by labelling the genotype of person A, and we are going to use the letter D.

I mean, it doesn't matter what letter you use, but I'm going to use the letter D because you can easily see the difference between a capital D and a small d. So, labelling their genotype … this is what they look like. So, labelling their genotype, this is what they look like. So big D, small d (Dd). So what is the question actually asking? How many people have the genotype which is homozygous recessive? So, homozygous recessive – homozygous meaning the same case, recessive meaning lowercase, which is why we're looking for small d, small d (dd) here. Homozygous dominant – homozygous meaning the same, dominant meaning that they're both capital. So that's what we're looking for. Now we're going to work out what the pedigree diagram tells us. First of all, I'm going to look at all the people without FH, so all the people that are either white circles or white squares. Because they don't have the disease, I know, therefore, that they are small d, small d (dd), so I can just label all of their genotypes straightaway.

And now we need to count them to work out the number of people with the genotype homozygous recessive. And once I've done that, I can see that it is 11. Now we're getting slightly more difficult by looking for the big D, big D (DD), so homozygous dominant. So we need to infer things from the pedigree analysis. First of all, look at woman C. So she got her genotype from parents A and B. Now, B is homozygous recessive, which means they must have passed on a small d. She has the disease, which means she must have a big D, so this is her genotype. E has the same issue, but they're a man. So they're going to be big D, small d (Dd). Person G inherited a small, lowercase allele from D. They have the disease, which is why they're capitalized. And the same goes for person J. And then looking at N, O, P, well, they inherited a small d from their mother. They have the disease, which is why they have a big D. So we're actually looking for people with big D, big D (DD).

The answer here is zero. "Person G and […] H have three children [all of whom] have FH. What is the probability of G and H having three children who all have FH?" This is a crazy amount of work for one mark because the only way I can see of doing this is to draw a Punnett square. So, we're going to do a Punnett square for G and H using my layout I already described. So we're looking at the phenotype, genotype, and gametes. From the key, we can see that person G has FH. H is, therefore, healthy from the key. We've already labelled their genotypes, so we can just copy that directly across and then separate these out to see the gametes.

Now just simply do a cross. And it's these two here that will have FH. Now, what is 50% as a probability? Well, it's 0.5. The question asks the probability of all three children having FH. So remember, when we're talking about probability, we have to multiply together our probabilities. Pop that into your calculator, and you'll get a value which is 0.125. Let's look at mitosis and meiosis now. So, remember, they're both types of cell division, but they're used for making very different things. So, meiosis is used to make gametes. So that means it's used to make sperms and egg. Mitosis is a completely different type of cell division. You need to learn that it's used in cloning, asexual reproduction, and the growth and repair of cells. So, for example, if you damage yourself, you cut yourself, it will be mitotic cell division which replaces those cells.

If you're carrying out asexual reproduction – so something like a strawberry runner producing baby strawberry plants – that will involve mitosis, the reason being is that it creates genetically identical offspring. I don't think you really need to know this, but some teachers like to just chat slightly about the different stages involved in mitosis and meiosis. I'm really only going to give you their names. And the first stage is prophase; the second stage is metaphase, then anaphase, and finally telophase. But this is a revision video, and I'm not willing to talk about it anymore at this point because I don't think it's necessary for lots of you. So, let's look at the differences between mitosis and meiosis. I always do this as a table because it allows me to make a direct comparison. So, look at the number of cell divisions, first of all. That will be one cell division in mitosis, two cell divisions in meiosis. The number of daughter cells now – so that's the number of cells produced once this cell division has taken place – in mitosis, you're looking at two daughter cells; in meiosis, you're looking at four daughter cells.

I've already touched on the sorts of cells that are produced, but just to recap – mitosis produces genetically identical daughter cells; meiosis produces genetically varying daughter cells, which makes sense. If we're using mitosis in cloning, that's genetically identical; if we're using meiosis in making gametes, it makes sense that we want our sperms and egg to all be different to each other. And do notice that the gametes will contain a haploid number of chromosomes, whereas the daughter cells produced by mitosis will contain a diploid number.

(no audio) Don't forget that haploid means containing one set of chromosomes. So in humans, that is 23. Diploid means containing two sets of chromosomes. In humans, that is 46. Looking at species now, so what is the definition of a species? Well, it's individuals which can reproduce to produce fertile offspring. And that's key. It's all very well having a horse and a donkey and they mate then they produce an offspring, which we call a mule, but the mule is sterile; it cannot reproduce. So that's the crucial thing about members of the same species: they can reproduce together to produce fertile offspring. Lyra! Hey! (cat meowing) Would you come in? (cat meowing) Oh. You're all soggy. Why isn't it focussing? You're so soggy. Is it wet out there? Super soggy. So how is variation within a species brought about, because we know the human race isn't full of billions of people that all look the same? That is brought about by a combination of things: first of all, genetics, and secondly environmental factors.

So two identical twins, regardless of the fact they have the same genes, if you move them to opposite parts of the world, it's very likely they'll have different heights, different masses, slightly different skin colour, and that's due to the environment they experience. That could be lots of sun; one of them could eat more; one of them could do less exercise. Are you all right? Are you too muddy? You're purring, aren't you? This makes a change. What is a mutation? It's a rare, random change to the genetic material of an organism.

So, a mutation can be brought about by a number of things: things like ionizing radiation, exposure to UV light, x-ray exposure, and various mutagens, which are just chemicals which cause mutations, and you find those in things like cigarette smoke. Now, the crucial thing with mutations is what they do is they alter the DNA of an organism. We've already looked at protein synthesis. So if you think about it, if you alter the DNA of an organism, what that will do is it potentially alters the sequence in which amino acids are assembled, and therefore, it can alter the end product, so the end protein which is produced.

Proteins are responsible for phenotypes, so our physical appearance. So mutation can therefore cause an alteration in our phenotypes. Now, not all mutations cause this alteration because sometimes a mutation occurs where the DNA – although it is changed, it doesn't actually alter the order in which the amino acids are assembled, so you end up with the same protein here. Very short topic now on evolution and natural selection. So first of all, your definition for evolution, it states that many organisms which are alive today and many more which are now extinct first evolved from very simple life forms that first evolved over 3.2 billion years ago. So that's basically saying that evolution states that we all evolved from small life forms like bacteria, which became multicellular, which became more and more complicated: they became reptiles, they became birds, and then they became mammals, and then we came about.

So that's really what evolution is stating. Natural selection links very nicely with this. Remember, this is Charles Darwin's theory. Now, he stated – and I do just want you to learn this as a five-mark answer off by heart. He stated that there is variation within a species due to mutation, which is what I've just discussed. So within a species, there is variety. This means that some individuals within the species are more likely to survive because they are better adapted. Because they're surviving, they're likely to reproduce, so produce offspring, and those offspring will inherit those favorable genes. So before you know it, you have many generations that go past, and they've all inherited this favorable gene, making them more likely to survive.

And I'm now going to bring up that perfect answer for you. (no audio) Natural selection can be seen pretty much everywhere on Earth, including bacteria. So we're just going to describe how bacteria may become antibiotic-resistant, and it does link to natural selection. So what happens is you have a colony of bacteria. You give them an antibiotic, and due to mutations, some of those bacteria are stronger. They are resistant. That means they are not killed by the antibiotic. So what happens is all the other bacteria are killed, leaving behind these very strong antibiotic-resistant bacteria. They soon replicate, and before you know it, you've got a colony of bacteria which is no longer treatable using antibiotics. And that's why everyone's so scared about antibiotic resistance and what it means for our future medicine.

(no audio) Ecology, now. Not my favorite topic mainly because it's full of discussing definitions which all seem to be very similar and all sound the same. So let's start by looking at the definition of environment. That is the total non-biological components of an ecosystem. So we're looking at the soil and the water, for example. The habitat is the place where a specific organism lives. Now, population – be very specific with your keywords here – this is all the organisms belonging to a particular species which you find within an ecosystem. What is the community? This is the population of all species found within a particular ecosystem. Now, what is a producer? Because remember, producers start all food chains and food webs. This is just a plant which photosynthesizes to produce its own food. A consumer is an animal which eats other animals or plants. What is a decomposer? It's an organism which decays dead material and helps to recycle nutrients.

Define a parasite. This is an organism which lives within another organism, causing harm to that organism and feeding off of them. What is a predator? It's an animal which kills and eats another animal. Gosh, these definitions do keep coming. What is biodiversity? That's the variety of plants and animals found within an ecosystem. What are biotic factors? These are living factors, so these are living factors which affect organisms' lives, so it could be other animals competing for food, competing for nesting sites, bringing disease and pathogens to other organisms. Abiotic factors are nonliving factors which affect organisms, such as soil pH, temperature, water, carbon dioxide availability, number of daylight hours, et cetera. So now we've looked at all the definitions, we now need to look at how we're going to sample an ecosystem. So say we've got a big field and we want to know about the variety of species living there, how are we going to do that? And we are going to use a quadrat.

So, remember that is a big metal frame which you're going to place randomly on the field, and you're going to take a sample of the organisms you can find within it. Make sure you can draw a quadrat. It's not a complicated diagram; just draw a nice metal grid. And be prepared to state how you would use it. So, first of all, you need to place it randomly using a random number generator because what you do, remember, is you get a field, you effectively mark it out with imaginary squares, and then you use your random number generator to work out where to place that quadrat. Don't say throw it because that's really biased. It means that it will ping off to the left or the right.

So don't say throw the quadrat. You're going to write down all the species you find within the quadrat, and then you're simply going to repeat and place it in many other places around the field so you've got a really good feel of the place. Looking more closely now at pyramids of numbers and pyramids of biomass, so remember, a pyramid of numbers simply shows you the number of each organism at each trophic level. So, a trophic level is just the stage in a food chain. So, for example, a pyramid of numbers could start with grass. The grass could be eaten by a rabbit, so that will be the next tier, and lastly, you'll have foxes.

But we don't like using pyramids of numbers because often they end up looking a really strange shape and not being pyramidal at all, and that can be due to the producer only being one organism, such as a tree; that will be very small in comparison to the number of sparrows living on it, which is why you end up with very funny-shaped pyramids. So we use pyramids of biomass because that actually shows the mass of living material available, and therefore, the oak tree, for example, will appear much larger, and therefore, the pyramid will be the correct shape. I'm just going to talk you through a couple of food web questions just so you know how to answer those.

Why is so little energy passed from one organism to another, so, for example, from the producer to the primary consumer? So, let's take grass, the producer, and a rabbit as an example. Remember, the rabbit is the primary consumer. The issue here is that only part of the grass is digestible. Much of it passes out of the rabbit as faeces. Some of the grass isn't eaten. So the rabbit won't even eat the root. So, I don't know if that's true, but it can be a reason.

The rabbit moves. It keeps itself warm. It respires. It egests; I've already mentioned this; it poos. So these are all ways in which energy is lost within a food chain. In fact, 90% is lost at each stage of the food chain. (no audio) If they ask you where all that energy originates from, remember, that is the Sun. (reading visual aid) This has been drawn quite horribly, so rather than looking at where the layers are, you're going to need to count the arrows and make sure you remember things about what a producer is and what a primary consumer is. So starting with the number of organisms, which they've done for us, which is annoying because that's the easy question, just count how many different organisms there are. There are eight, so we do agree with them. "[N]umber of different types of plant". You're looking for the producers here So what here is a plant? Well, you can see from the picture, even if you don't know what a cattail is, that it's a cattail and marsh grass.

So the answer here is two. The "number of animals". So these are the animals which form anything from the primary, secondary, tertiary consumer level. So the animals, therefore, are grasshopper, cricket, shrew, frog, snake, and hawk. And if you count those up, you'll get six. "[N]umber of primary consumers". So remember, they appear straight after the producers. so we've got an arrow leading to the cricket and the grasshopper, which is why the answer here is two. "[N]umber of food chains". Well, this is more difficult. So food chain one will be marsh grass, grasshopper, shrew, hawk. Food chain two is the same but includes the snake before the hawk, so that will be the second one. Food chain three is the cattail, cricket … shrew, hawk. Food chain four will be the same but include the snake. And then food chain five will include cattail, cricket, frog, snake, and hawk.

So that was pretty tricky, but the answer here is five. (reading visual aid 1.b.i) I've already mentioned that a lot; that is a producer. (reading visual aid 1.b.ii) So, I told you earlier in my definitions, that something that catches and eats something is a predator. (reading visual aid 1.c.i) So, we need to have a look. Oh, right. So the grasshoppers are eaten by the shrews, so clearly if we reduce the number of grasshoppers, the number of shrews will reduce because there is less food for them. (reading visual aid 1.c.ii) So if we have fewer grasshoppers, we can see that they won't be eating the marsh grasses much, which means that the marsh grass population will increase. Let's do the carbon cycle now. I do like this topic. If you just learn it as a list of steps, it's far easier than learning it as the whole cycle unless you're a very artistic, kind of, pictorial person because I really struggle in that way, but I find learning this list of steps works well every time.

So, we're looking at how carbon is cycled in our atmosphere and within living organisms. So, the place I like to always start is carbon dioxide in the air. So, what happens to that carbon dioxide in the air? Well, it gets absorbed by green plants in photosynthesis, and it is used to make glucose. Those green plants then respire because they're living organisms, and that releases carbon dioxide back into the atmosphere. So, here's the first step, of CO2 moving in, CO2 moving out. The plants are eaten by animals, and so that carbon that was part of the plants becomes part of the animal body. And then the animal respires again, releasing carbon dioxide into the atmosphere. Lastly, plants and animals inevitably die, and then this is where decomposers come in. They break down that dead material, and they respire, again releasing carbon dioxide. So, we can see carbon dioxide went in in the first place with green plants photosynthesizing, and then it left via respiration by plants, animals, and microorganisms.

Do notice that combustion, which is burning of fuels and things, also releases carbon dioxide into the air. (no audio) Now the nitrogen cycle. This is for triple people only. Again, start in one place and allow your answer to feed in from there. So, I like to start with nitrates in the soil. So, remember, those nitrates get absorbed by active transport into the root hair cells of the plants. Nitrates are important in the plant's manufacture of proteins. So the nitrogen in the air becomes locked up in proteins within the plant. The plant is eaten by animals, and therefore, the nitrogen moves from the plant into the animal.

And then both the plants and animals die, and microorganisms become important again. So the ones you need to know about are nitrifying bacteria. And they convert the ammonium within the dead matter from nitrites to nitrates. So, nitrates have been returned to the soil; nitrogen has been returned to the soil. Do notice there are some really annoying bacteria called denitrifying bacteria. And what they do is they take nitrates in the soil and convert it back to nitrogen in the air, so they're not very popular with farmers because, after all, the farmer wants lots of nitrates in the soil; they don't want nitrogen gas being released into the air because there's plenty of nitrogen in the air. So that's why denitrifying bacteria are particularly frustrating. The last thing to notice is nitrogen-fixing bacteria, which we find on root nodules – so small bumps on the roots of leguminous plants. A legume is just a plant such as a bean or pea or … Legumes are a type of plant. Legumes are a family of plants which include peas, beans, and clover. So they're super good to plant because they provide a lot of nutrients to the soil, the reason being that the root nodules containing nitrogen-fixing bacteria, they convert nitrogen gas in the air to nitrates, so effectively, they make the plant its own supply of fertilizers.

So that's really great because, by planting clover or peas or beans, you're basically fertilizing the soil. So say the farmer regularly plants wheat as their cash crop. They might choose to crop rotate and plant clover every four years just to add a nice input of nitrates to the soil. (no audio) Let's now look at human impact on the environment. And we're going to start with eutrophication, which is an effect brought about when farmers use too much fertilizer on their land and when sewage – this is disgusting – washes into rivers and streams and lakes. So, do remember, fertilizers and sewage contain a lot of nitrates. So what happens is the plants use those nitrates to build proteins, and they grow extremely quickly. Because they've grown so quickly, they end up dying. The reason for this is due to lack of light. Basically, they block all the light for each other, and they die because they can't photosynthesize. (crash) My notes have gone.

The death of the plants obviously provides lots of food for decomposers and microorganisms, so they grow hugely in number because they're feeding on this dead material. Because they're aerobic respirers, they use up all the oxygen in the watercourses, and this means there's no oxygen available for aquatic animals, and they die. And that's how leaching of nutrients, of fertilizers and sewage can end up with death of all aquatic animals within watercourses. The greenhouse effect now.

The greenhouse effect is very famous. We're always talking about it because of environmental change. Do remember, this is due to human activity, or we think it's due to human activity – so burning of fossil fuels, which releases carbon dioxide. Other greenhouse gases include methane. Do remember sources of methane. So, some of that comes from the digestion of bovine animals, such as cows. Effectively, when they fart and burp – that's disgusting – they release lots of methane. Rice paddy fields – the microorganisms that are found in rice paddy fields contribute an awful lot of methane to the atmosphere. And remember, the other two greenhouse gases you need to know about are water vapour and nitrous oxide. So there are four greenhouse gases: carbon dioxide, methane, nitrous oxide, and water vapour. So, what effect does this increase in greenhouse gas have on the environment? What you find is that the whole of the Earth's atmosphere heats up, and this leads to widespread melting of the polar ice caps. This means much more water is added to our seas and oceans, and consequently, you get a rise in sea level.

This floods low-lying land – so towns and cities close to the coast. And it will automatically lead to loss of biodiversity because animals have less habitat, less places to live in, and you can see extinction of some species. We also are seeing a knock-on effect with extreme weather – huge storms, typhoons, et cetera. This is all a result of enhanced greenhouse effect and global warming.

There's a change in bird migration patterns because it's getting warmer throughout the year, so they're confused as to what month it is because obviously, they don't know the month; they just base their migration on the temperature. So we see a real change in bird migration patterns. (no audio) We're going to touch now on CFCs. Remember, we find these in aerosols and fridges, although much less now they're supposed to be banned – the reason being is because they damage the ozone layer. And the ozone layer is important because it protects us from the Sun's UV rays. Acid rain you need to know about in GCSE chemistry. Remember, it originates because of sulfur impurities in petrol, which, when burnt, release sulfur dioxide into the air. The high temperatures found in car engines cause the nitrogen and oxygen in the air to react, again forming nitrogen oxides.

So nitric acid and sulfuric acid is produced, which are components of acid rain. So what effect does acid rain have on the environment? Well, first of all, it damages trees, and it can literally dissolve away their leaves. It damages limestone buildings. And lastly, it gets into lakes and rivers and makes them too acidic. It's quite a bitty topic, as we're again going to move on, and now we're looking at carbon monoxide. Just remember that carbon monoxide is again released by car engines, and it combines irreversibly with the haemoglobin found in red blood cells, meaning that they can no longer transport oxygen, and that's why carbon monoxide is such a toxic, poisonous gas.

Deforestation – so, geographers, you ought to know a lot about this. Remember, this is the cutting down of trees. Why do we deforest? Well, it provides more land for animal rearing – so, farms. It provides more land for crops to be grown. It provides building materials used in houses. And that's everything I'm going to say because I can't think of anything else. And then, what effect does deforestation have on the environment? Well, its got some horrendous effects, and you can talk quite in-depth about these effects. Firstly, by cutting down trees, what you're doing is you're releasing an awful lot of carbon dioxide into the air, which is obviously going to enhance the greenhouse effect and contribute to global warming. Cutting down trees means that animals' habitats are destroyed. There's nowhere for them to nest anymore, if we're talking about birds, or for animals to hide in the undergrowth if we clear the land. You get leaching of nutrients because remember, those tree roots hold those nutrients, those nitrates in place. Without the trees, you end up with the rain falling on the ground, washing away the nutrients into streams and rivers, meaning that the whole land remaining is barren and infertile.

And tied to this is soil erosion, where because of the rain falling, you get widespread flooding and landslides. And really, thinking about it, the whole of the water cycle is disturbed because there's less transpiration occurring, which means because less transpiration is occurring due to lack of leaves – meaning that less clouds form, less rain – and you get a whole disruption to the weather found over forested areas. We need to look at the use of biological resources – so, this is food production – starting with a look at greenhouses and how they increase crop yield. Remember, crop yield is just about how many plants or vegetables or flowers that farm is able to grow. So we use greenhouses, glasshouses, polytunnels, et cetera, all so that we can increase the amount of product or plant that we can grow.

So, why does using a greenhouse increase the amount of plants we can grow? Because first of all, you can artificially control the temperature. So in the cold winter months, you can add heaters, and they heat up the atmosphere, which increases the rate of photosynthesis. More photosynthesis means more crops. Don't forget, as well, that the glass actually traps some of the heat energy from the Sun, so you end up with an enhanced greenhouse effect actually going on inside the greenhouse, which again increases the temperature. You can control the carbon dioxide levels … water levels, et cetera.

So you can make sure that none of these is a limiting factor. And the water levels you can do by increasing the humidity inside the greenhouse. By increasing the humidity, what you're actually doing is reducing the rates of transpiration, which I've already touched on earlier in this video. And lastly, light – light is a limiting factor of photosynthesis, especially in the winter when there's more dark hours.

You can literally add electric lighting so the plants carry on photosynthesizing throughout the day and also the night. So we're looking for maximum growth here. (no audio) What effect does increasing the carbon dioxide and temperature have on crop yield? Well, this is obvious: increasing both of these will increase the crop yield. Why? Because increasing temperature means that the enzymes involved in photosynthesis have more kinetic energy, so they catalyse reactions faster. Obviously, don't forget that if the temperature gets too hot, they will be denatured. So there's always a sweet spot – an optimum temperature which must be used. And then, the carbon dioxide, of course – that is a reactant involved in photosynthesis. So if we increase the levels of CO2, then we increase the rate of photosynthesis.

This is a bit of a repeat of what I was just talking about, but it is a separate specification point, which is why I'm addressing it here. (no audio) So, how does using fertilizers increase crop yield? Well, the addition of fertilizers to the soil replaces leached or lost nitrates and mineral ions from the soil, because remember, fertilizers are very rich in nitrogen – nitrates – and those nitrates are used by plants to build proteins.

What is a pesticide? Remember, it is a chemical which kills pests, so anything which feeds on plants will be counted as a pest. Killing pests obviously reduces the damage to the crop, and it also helps to increase crop yield. Looking at how we control pests further, remember, we can add chemicals – so pesticides – or we can use biological control, which is about using other animals which kill and eat the pests. And we need to look at their various advantages and disadvantages. So let's, first of all, look at the advantages of using pesticides. Now, these are easy to use, so they're easy to apply; they're effective, which means they do a pretty good job of killing the pests; and they're readily available. Issues, though – there's lots of issues with using pesticides. Firstly, that they can be very expensive. They're persistent, which means it takes a while for them to decompose, so once you apply them to your soil, you've got to be aware they may hang around for many, many years.

And the problem here is that they can often kill animals which aren't even pests, which is really, really bad because these are innocent animals getting killed by the pesticides because the pesticide does not discriminate correctly. So what happens here is it kills other animals. Some of these animals get eaten by large animals – so we're talking about food chains here – and this is called bioaccumulation, where the pesticides become stored in these animals. And then, as this pesticide works itself up the food chain, we call this biomagnification. And the famous case study of this is DDT, which was used to eradicate malaria and typhoid in the Second World War, and there are still areas of the world where DDT is killing huge amounts of flora and fauna – and that just means animals and plants. Another disadvantage is that you have to keep reapplying this pesticide. (no audio) Looking at biological control now, so let's name a few examples. So, like I said before, this is using animals to kill pests.

The most famous one probably is using ladybirds because ladybirds are predators to aphids. So they come along and munch on the aphids, which would otherwise be destroying things like cabbages. So what are the advantages of using ladybirds – using biological control? Well, they tend to be quite specific and kill the pest that you're after. Secondly, they're self-sustaining; they tend to reproduce, which is great because you don't have to keep reapplying ladybirds; they tend to grow into new populations which will continue to eat the aphids. And clearly, they'll be non-toxic, especially when compared with things like DDT. However, there are disadvantages. They have been known to not just eat the pests that you're after; they can go around eating other things. So that can be both an advantage and a disadvantage. They never fully eradicate the pest, so there will still be some aphids which survive the purge because the ladybirds don't go eat all of them.

When you add extra animals to an environment, to an ecosystem, they can have undesired effects. You don't really know what they're going to do. They can have major effects on food chains; they really disrupt them. So you do have to be careful before you decide to apply these animals to your ecosystem. And lastly, compared with using pesticides, it's pretty damn slow waiting for ladybirds to go and eat all the aphids, whereas with pesticides, you tend to find everything gets wiped out immediately. (no audio) Right. Looking at microorganisms involved in food production, so primarily yeast.

So how is yeast used in bread making? Don't forget that yeast is a fungus. When it's forced to respire anaerobically, it breaks down glucose – or it respires glucose – into carbon dioxide and ethanol. That carbon dioxide is very important in bread making. It creates bubbles, which actually helps that bread dough to rise. Looking at the role of yeast in beer making – so the anaerobic respiration of yeast. Remember that produces ethanol; I've just told you that. An ethanol is an alcohol, so that's where the alcohol found in beer originates. I think there's been a change in emphasis with the new specification. So with yoghurt making – I'm not going to talk too much about it- just be aware that the bacterium lactobacillus bulgaricus is used.

It carries on anaerobic respiration, breaking down lactose, which is the sugar found in yoghurt, into lactic acid, which gives yoghurt that very distinctive flavor. And that can be done inside a fermenter. Moving on to fermenters, so what is a fermenter? Well, it's a vessel which contains microorganisms which are involved in fermentation reactions. So, let's describe the structure of a fermenter and how it is optimized to ensure as much of the product is produced as possible.

So, first of all, we control the temperature, and that's through a cooling jacket, because microorganisms when they respire produce a lot of heat, and if too much heat is produced, then it can denature the enzymes and actually kill themselves. So the cooling jacket has cold water flowing around it, which helps to remove excess heat from the fermenter. Coupled with this, you have temperature and pH monitors because obviously you need to determine if the temperature is too high, you need to determine if the pH is too alkaline or acidic.

So it's important that we keep watch on the fermenter. We also have stirring paddles to mix up the contents, and that ensures that the nutrients and heat are evenly distributed. You often have an air inlet – not always, but sometimes – and that's to allow oxygen into the fermenter for any microorganisms which respire aerobically. And lastly, nutrient supply because obviously, the microorganisms need something to respire. (no audio) So, what is fish farming? This is when you artificially keep fish in cages and nets. So, Scottish lochs, for example, have a lot of fish farms. And what you can do here is you separate the large fish from the small fish to keep fish of equal sizes together.

You carefully monitor their diets, so you make sure they're fed protein – very protein-rich diet – and they're fed little and often, the reason being you don't want loads of food dumped into the water because that will increase the microorganism growth, which will cause disease amongst the fish. You keep an eye on diseased fish. So if you notice any that are dead or diseased, you remove them to stop them infecting the rest of the fish population, and you also give the fish antibiotics to keep them healthy. No one thinks fish farming is a really good idea, by the way. This is just me telling you what fish farming involves. The water quality is constantly monitored because the fish are so close together, so it needs to be kept clean, needs to be kept oxygenated. Waste products, such as faeces, are removed with nets or filtered out. And lastly, in terms of the sorts of fish which are fish farmed, they do a lot of selective breeding here, so they'll be selectively breeding fish with the larger masses, that have more flesh because obviously these will sell for higher money.

And don't forget that these fish farms often have nets over the huge areas where the fish are kept, and that's to stop birds predating, so it's to stop birds coming along, poking their beaks and stealing a fish and flying off with them. (no audio) So, what is the advantages and disadvantages of fish farming? And we often use fish farming for commercial fish varieties, such as cod, haddock, tuna, for example, where there's a huge demand for them by humans to consume them.

So the advantages are firstly, that you can control their diet, so you can give them those high-protein diets. You can control their oxygen levels. You can remove diseased fish. You can give them antibiotics to keep them healthy. You can prevent pests from killing and catching them. And selective breeding programs can increase the quality of the fish. Disadvantages are increased risk of disease due to how close together the fish are, the fact that antibiotics might enter the food chain if they get consumed by other animals, any pesticides used are likely to be toxic to other animals, and that's where you might see eutrophication occurring.

Now selective breeding – so, remember, this is when humans use animals or plants with desired characteristics, they force them to breed, and then they repeat this process over many generations, so before you know it, you have animals with desired characteristics. So if we're looking at animals, let's, for example, look at the dairy industry. So dairy cows – clearly a good animal here will produce a lot of milk. So humans – make sure you point out that it's humans – they select a dairy cow that produces a high yield of milk. They mate her with a bull. It's quite hard to determine the bull because obviously they don't produce milk. But they'll mate her with a bull, and then her calves are likely to produce more milk – the female calves – because of their high-yield mother.

Then you take those calves, and you keep repeating the process until you have lots of calves and lots of cows that produce lots of milk. And you can do the same with plants. So you can selectively breed plants to be a particular colour. So you pick flowers that are a particular colour. You force them to cross-pollinate. And then before, you know, you've got a load of plants with your desired characteristic, such as petal colour. (No audio) Genetic engineering now. This is quite a complicated topic. It is chock-a-block full of key scientific words, but if you learn them off by heart, you should be fine. So remember, we genetically engineer things like insulin. So, insulin is a hormone produced by our pancreas, and it's responsible for lowering our blood sugar levels after we've eaten. And for type 1 diabetics, they find that they don't produce insulin, so they really struggle to maintain their blood sugar levels – which is where genetic engineering comes in.

Because in the olden days, they used to obtain pig insulin – so they used to chop into pigs, remove the insulin, and not only do you have major ethical issues with this, but obviously the insulin wasn't particularly fit for purpose because it came from pigs. So it was important that we found a way of producing insulin from humans, and so that's where genetic engineering came in. And when we're talking about genetic engineering, we're talking about using bacterial cells because bacterial cells contains small rings of genetic information called plasmids, which we can manipulate so that you can insert the insulin gene and force the bacteria to produce lots of insulin.

So let's go into great detail how that is done. So we obtain the bacterial cell, and we cut open the plasmid using a restriction enzyme which acts as a pair of biological scissors. Then we use a restriction enzyme to cut the insulin gene away from the rest of the cell, and we insert that insulin gene into that bacterial plasmid using a ligase enzyme. And we stick it together, and that's why we say it has sticky ends. Once we've done that, we're ready to put the bacterial cell into a fermenter, and it's been done many, many times.

And then I've already mentioned the fermenter, and you need to provide it with the optimum conditions, so the right temperature, the right pH, the optimum amounts of oxygen and nutrients, et cetera, and before you know it, your bacteria has made huge amounts of insulin. Don't forget a few keywords here concerning genetic engineering. Once that plasmid has a different gene inserted into it, we call it a recombinant plasmid, which means it's been recombined – so it's been changed. And don't forget also that the bacterial cell – the plasmid – is acting as a vector, which means it transports biological material from one place to another. So I've already given the bacterial plasmid as an example. And we met a different example earlier on in this video when I mentioned malaria.

The mosquito is the vector when we're talking about malarial infection because it carries the plasmodium – remember, that's the protoctist that causes malaria – from one organism to another. Be prepared to give those lists of steps for any named human protein. (no audio) We can also genetically modify plants so that they can have desired characteristics. This could include being frost-resistant, so that stops them dying when frost hits in winter. It could be to extend their shelf life to stop them going off so that they have a longer shelf life and are more fit for human consumption after many days. You might actually want to make plants resistant to weed killers. This sounds really strange, because why would you want to make a plant resistant to weed killer? But think about it. A farmer applies the weed killer. By the way, the best name for weed killer is herbicide: -cide meaning kill, herb- meaning to do with plants. So the farmer applies the herbicide. He or she wants to kill the weed.

However, some of it will inevitably fall on the plant that they're trying to grow, and obviously you don't want that to happen because it will actually kill the plant you're trying to grow. So if you can make that crop plant resistant to herbicide, then that's great because you applied that herbicide or weed killer; it kills the weeds; and your actual plant that you're after stays alive and continues to grow. You can also genetically modify plants so that they can actually have health benefits. One of the most famous examples of this is golden rice.

So, when poorer countries grow lots of rice – unfortunately, rice doesn't have a huge amount of nutritional value, so what you can do is genetically modify it so that it contains vitamin A. And therefore, when people eat that rice, they get a huge amount of vitamin A in their diet, and that stops them getting night blindness. This is something you should remember from the balanced diet topic of the specification. So golden rice is an excellent example where genetic modification has been used really well. And then the most strange example of genetic modification is when we talk about tobacco plants, and these have been modified so they actually produce hepatitis antigens, therefore have a potential vaccine against hepatitis. So this is, like, crazy science, but just remember that tobacco plants may be modified to produce hepatitis vaccines.

So, we've already touched on this. We've given lots of examples of genetic modification in plants. Just to reiterate the advantages and to throw in a few more, you can have increased salt tolerance. (no audio) Let's now define a transgenic organism, and that's where genes have been transferred from one species to another, such as in goats – they've had a gene from spiders inserted into them so that when they produce milk, they actually produce spider webs. This is crazy, but it's actually true. Because spider web is so strong, even when compared with steel: if you made a spider web as large as a steel frame, you would find the spider web was much stronger. So that's why scientists are interested in working out artificial ways of manufacturing spider web. I digress.

Don't worry about this too much. The point here to notice is that a transgenic organism is one that has had a gene inserted into it from another organism. Oh, final stretch, cloning. So remember, a clone is a genetically identical organism. One of the most famous clones produced was Dolly the sheep. She was genetically identical to her mother. So, how was she cloned? Well, this was by a process called adult cell cloning. And let's talk in great detail now about how this occurs. So, first of all – let's use Dolly as our example – you obtain a body cell from her mother.

So that could be a skin cell. And it just needs to have a diploid number of chromosomes, i.e. a full complement. Then you get an egg cell from a sheep. It doesn't matter what sheep, but you need an egg cell; it needs to be the same species. And you remove its genetic material, so you now have an empty egg cell. And then you have the body cell from Dolly's mother. So you insert the nucleus from the body cell into that now empty egg cell.

And we use an electric shock in order to do this. Do notice the word enucleate, and that means just removing the nucleus from a cell. So now we've inserted the body cell nucleus into the empty egg cell, we now have a complete genetic match for the original mother. And we need to insert that egg into the uterus of another sheep, and we call that sheep a surrogate. So that egg is implanted into the sheep's uterus, and it divides by mitosis, and before you know it, the sheep gives birth. She is not related to her lamb, but that lamb is a genetic copy – so a clone of Dolly's mother. (no audio) Looking at cloning in plants, there's two methods you need to know about: first of all, taking cuttings. This is a really basic way of cloning a plant. It's when you use a pair of scissors, you snip off a side shoot from a plant, you dip it in rooting powder, you plant it in soil, and it starts to develop roots, and then it grows into a new plant which is genetically identical to the original plant.

A more complicated, more scientific way of cloning is called micropropagation, otherwise known as tissue culture. So you need to learn a list of steps which will enable you to write a nice, long answer as to how this is carried out. So first of all, small parts of plants are obtained, and we call these explants. And we pop these into agar jelly, which contains nutrients and hormones, and before you know it, the small explants start to develop roots, and then we transfer these into soil.

It's crucial that we have sterile conditions to prevent the growth of microorganisms. I've already said we provide them with hormones, such as auxins. And we need to control the temperature, pH, and water levels to maximize the growth. When we look at the advantages of micropropagation versus taking cuttings, first of all, the clear advantage is that you can make large numbers of plants very quickly because a cutting will only produce one plant, whereas micropropagation can produce thousands from a single plant. It can occur at any time of the year, so we're not restricted by the growing season. And that's everything I can think of to do with micropropagation. But just say that large numbers can be produced very quickly at any time of the year. This is a really strange point: How can cloned transgenic animals be used to make human proteins? Really, really, really strange point, but I just need to say it for my own mental health to make sure I've completed the specification because I am a perfectionist.

So, using cloned transgenic animals means that you can create large numbers of genetically identical – so cloned – animals. Because they are transgenic, they have that gene from another species. They can, therefore, be used to produce huge amounts of protein. (no audio) And my final point is ridiculous. It is about examples of using transgenic animals to create proteins. This is ridiculous. Just know that trypsin is a protein which damages the human liver and lungs. Now, in order to prevent trypsin from causing all this damage, we produce a second protein called AAT, which destroys the trypsin. However, some people don't produce enough AAT, so they're liable to have their lungs and livers damaged by trypsin. And as an answer to this, we can genetically modify sheep – so we can make them transgenic – so that when they produce milk, that milk actually contains AAT, and we can harvest that, and then give it to people who don't produce enough AAT by themselves.

This is such an annoying, strange point because the rest of the specification, I think, is very well planned. This is bizarre. (no audio) Right. I hope you found my video helpful, guys. These are so difficult to make, but I know you guys really like to just sit and watch the whole thing in one. So, please give me a like. Like this video if you found it helpful. It is a good incentive for me to continue my work. And don't forget to sub. I'll be back soon with another video. Bye. (music).

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