Hello everyone! Welcome and welcome back on Out of a Puddle. Today, I'd like to talk about the fundamental unit of life, the cell. The cell was discovered by Robert Hooke in 1665.
In his manuscript 'Micrographia', Hooke reported the microscopic observation of a thin section of cork, which he noticed was made of multiple, tiny pores closely resembling hive cell chambers and actually, what the Italian term "cellula" means is exactly "small chamber". What Hooke observed at the time, and that he called "cells", were not the same cells we are referring to today, instead, they were empty spaces between cell walls of a plant tissue, therefore paradoxically by using the word "cell" he didn't mean to indicate anything alive.
Antoni van Leeuwenhoek soon after observed
single-celled organisms for the first time which nowadays we would call "protists", but instead he called them
"animalcules", by using a microscope he invented, composed of a single spherical lens
capable of up to 270x of magnification a huge step forward compared to the previous
magnification threshold of only 50x The Cell Theory was finally postulated by
Schwann and Schleiden during the 1800s when the main three points of the theory were established:
1. All living organisms are made up of at least one cell; 2. The cell is the most fundamental unit of life;
3. All cells originate from other pre-existing cells through cell division. The modern interpretation of the theory adds further points, for this reason I recommend you to look into the links I left in the description But then what is a cell essentially?
Cells are often represented as bags full of water and enzymes, especially when talking about bacterial cells.
Although it is the fundamental unit of life, the cell is still an extremely interesting and complex system in its own right, that being because of a 4 billion year evolutionary history, which led to a stratification of functions and adaptations that produced an inextricable network of complex interactions. All this inside each single one of the existing cells. One of the cell fundamental components is
the plasma membrane, mainly composed by a double layer of phospholipids delimiting the internal environment, or cytoplasm, and separating it from the external environment; but the cell membrane is not an impenetrable barrier, nor can anything get through: indeed, it is characterized by a "selective permeability" due to the occurrence of membrane proteins, which mediate the movement of ions and molecules across the membrane, so that chemical potential differentials are maintained between the two faces of the membrane They are differences in concentration that, essentially, make a cell "alive" and not an ordinary fatty vesicle; indeed, the cell maintains itself in a state called "steady-state", which reminds a little of running on a tapis-roulant: you need to keep running to stay in place, however, since running consumes energy you also need to get the energy to keep running, in order to avoid the abyss of thermal equilibrium 🙂 Hence, a very important task, provided to a large extent by the plasma membrane Another fundamental element is the genome, which includes the entire information content of the cell and it is made of DNA in all cellular organisms,
a strong evidence to a single common origin The DNA is a polymer, which is a molecule made of a sequence of repeated fundamental units forming long code strings The code is interpreted only after the DNA is
transcribed into messenger RNA (mRNA), a copy of the DNA string that can be read by an extremely sophisticated
molecular machinery called ribosome localized in the cytoplasm.
At the ribosome,
the message contained in the mRNA is translated into aminoacidic chains, which form the
proteins, among which there are structural proteins, membrane proteins, DNA-binding proteins that affect gene expression, or enzymes, which catalyze all of the chemical reactions making up the cell metabolism The arrangement of the genome within a cell represents one of the main differences between the two types of cells: those which are definitely prevalent as well as the first ones to appear on Earth, the prokaryotic cells – including Bacteria and Archaea – and the eukaryotic cells, appeared only 1.5 billion years later, however with a more complex organization. The genome in the prokaryotes is typically found free in the cytoplasm, and it had a super-coiled circular shape. Let's consider to have a rubber band: you firmly grab one extremity and you coil the other, until you obtain something closely resembling the topology of a bacterial genome.
The DNA is transcribed into RNA, which is also translated into proteins at the same time. In the eukaryotic cell you have a completely different situation: indeed, there are at least two types of genomes: the nuclear genome, the mitochondrial genome, and – in plant cells – also the plastidial genome. The mitochondial and the plastidial genomes are
circular, just like those from bacteria, one of the main pieces of evidence supporting the prokaryotic and symbiotic origin of these organelles Instead, the nuclear genome is something completely different: first of all, the DNA is not circular, but linear; moreover, each segment is complexed to proteins functioning both as scaffold as well as for the regulation of gene expression
– called histones.
The DNA winds around these proteins forming spool-looking structures called nucleosomes. This ball of string made of proteins and DNA is called chromatin, and each
segment of chromatin is a chromosome. The nuclear genome is called like that because – big surprise – it is found within the nucleus, which is delimited by a double… double porous membrane, dividing the nuclear compartment from the cytoplasmic compartment, therefore separating the processes of transcription and translation as well. Indeed, a feature that makes the eukaryotic cells unique compared to the prokaryotic cells is a complex compartimentalization Indeed, the eukaryotic cells are typically full of organelles, membrane-bound structures specialized in exerting certain functions, for example mitochondria and the Golgi apparatus; this specialization allowed the eukaryotes to reach a high level of efficiency with no rivals in the living world. However, even the prokaryotes can produce compartimentalization, to a lesser extent: indeed, the cell membrane can fold, providing areas specialized to exert certain functions, like – for example – energy production. But one of the most studied examples of compartimentalization in prokaryotes are the carboxysomes:
they are structures occurring in some cyanobacteria and other autotrophic bacteria, made of proteinaceous shells shaped like d20s, which contain enzymes for carbon organication.
Essentially, they are compartments specialized for the production of organic compounds using CO2. The Archaea represent another exception: a group of prokaryotes, different from the common bacteria, from which the eukaryotes likely originated. These are now represented by the so-called "extremophiles", organisms which live in environmental conditions we consider extreme, such as high temperature, high pressure, high salinity, or high acidity, and for this reason they are particularly difficult to study. The Archaea, just like the eukaryotes, can show genomes complexed to histonic proteins as well, but unlike the eukaryotes, they vary a lot in the kind of histonic proteins, in the combinations of these proteins, and in the number of times the DNA winds around them.
Pluricellular organisms are characterized by a further level of specialization, according to which different cell types differentiate throughout the development of a single organism, thus producing different tissues, making up different organs, and finally apparatuses with integrated functions. It is worth noticing that, more often than not, cells in some tissues play their role only once they are dead: in general, we refer with "programmed cell death" for all of the occurrences in which the death of the cell is expected for in the natural development of an organism. A striking example is the one provided by vascular plants: indeed, the tissue specialized for the transport of water and minerals from the roots up to the leaves is composed by bundles of tubes, formed by the walls of plant cells which are dead once they reached maturation.
There's still an infinite amount of things to say about the cell, about the differences between the eukaryotic and the prokaryotic cell, but I hope I was able to give you a starting point from which you can better interpret the world, and possibly get more into it. We could also possibly face some in-depth analyses in the next videos, in case you're interested please let me know. As for the rest, if you enjoyed the video as usual click 'like', please leave a comment for any question, subscribe and… see you next time!.