The Endoplasmic Reticulum Bio 3

the endoplasmic reticulum according to authors Diane Schwarz and Michael blower the endoplasmic reticulum is the largest organelle in the cell and is a major site of protein synthesis and transport protein folding lipid and steroid synthesis in calcium storage these functions can occur because of the two types of endoplasmic reticulum or er that exist the rough and smooth ER the rough ER functions specifically in protein synthesis whereas the smooth ER functions in lipid synthesis detoxification and calcium storage the image below gives you an idea as to what each structure looks like the rough ER contains ribosomes which are cell structures that help make protein and their location indicates what kind of protein will be made smooth ER doesn't contain the structure for its job is not to make proteins ribosomes are very important structure and a fun fact is that they are found in all organisms the image presented in the slide demonstrates the role of the ribosome in protein synthesis which we'll look at more closely in the next couple slides protein synthesis begins at transcription where RNA polymerase opens up the double helix of DNA and adds new nucleotides in the form of RNA transcription occurs in the nucleus but as it ends the mRNA leaves through the nuclear pore to connect with our RNA or ribosome in the cytoplasm here the cytoplasm is where translation occurs translation translates codons into their amino acid sequence with that being said codons are three consecutive nitrogenous bases on the mRNA if we look at the example I drew we can examine this process in more depth the first image demonstrates transcription where the one strand of DNA molecule has nitrogenous bases a c g g a a t a a the single strand demonstrates that RNA polymerase has already split the double helix furthermore the same image also shows the complimentary mRNA in which the RNA polymerase added on the nitrogenous bases for the mRNA our ug c c uu a you you notice how DNA has thymine but mrna uses u or so next to go onto translation which is shown in the second image it begins as the our RNA or BRABUS own starts to read the mRNA next the tRNAs join where they produce anticodons based on the codons on the mRNA in our example the first tRNA produces anticodon a cg which is associated with amino acid ser 9 i want to point out that typically the first one would produce Aug which is the start codon but for the sake of saving room on the image I'm just going to mention it more verbally furthermore the second tRNA has the anticodon e excuse me GAA which codes for the amino acid gluten acid we can determine which amino acid will be forming with the help of the amino acid sequence chart as the tRNA does this the amino acids from each one will join the polypeptide chain and hold together through peptide bonds the RNA will then move down the mRNA and kick off the previous tRNA until it reaches the stop codon in our case the stop codon is UAA here our chain breaks away and translation is complete although we completed both translation and transcription protein synthesis is not quite done yet we talked about the important role of ribosomes in protein synthesis but we still need to talk about the rough ER itself there's a site called the translocator or translocon on the rough ER where ribosomes finished the production of proteins and decide whether that protein will be a part of the plasma membrane or will be stored in their soluble form in the ER not only talks about protein synthesis we can talk about the mistakes or mutations that can occur there are three types of mutations they substitution which replaces one nitrogenous base with another nucleotide deletion which removes a nitrogenous base and nucleotide insertion which adds a nitrogenous base take note that in nucleotide deletion and insertion every amino acid after that base will be affected and that could possibly change the location of our stop codon there are times however when we have a silent mutation which is a change in the mRNA but is unaffected because it produces the same amino acid this can be and this can be seen in the following example codon CCU produces the amino acid proline but if space substitutions were to occur and change the codon to CCC the amino acid will still remain proline in the image I've drawn you can see base substitution that changes the amino acid more closely you can see that substituting the a to a C in the second codon will change the amino acid for that codon instead of having the normal amino acid of glutamine it will now have proline moving forward we have the smooth ER in its function of lipid synthesis this process synthesizes cholesterol and phospholipids which are very important because their components that make up the fluid mosaic model remember that the fluid mosaic model which is the cell membrane is made up of phospholipids transport proteins polysaccharides and cholesterol in addition lipid synthesis can also produce steroid hormones therefore an abundance of smooth ER can be found in steroid producing cells such as those in the testes and ovaries when discussing the responsibilities of the smooth ER lipid metabolism is also something to consider as a reminder metabolism is the ability to use energy so an exam all of this in lipid synthesis would be in the liver where it allowed glycogen to be broken down into glucose in addition there's a large quantity of smooth ER in the liver due to its detoxifying ability the smooth ER is capable of converting organic chemicals such as alcohol into water-soluble products that are much safer through detoxification finally the last function of our smooth ER that I'll be discussing is its ability to store calcium the ER contains several calcium channels that are responsible for releasing calcium 2 plus from the ER into the cytosol when intracellular levels are low this is vital for cells in the nervous and muscular system that use calcium mediated signaling for excitation and contraction

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