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Hi friends, hope you are fine. Which is the largest organelle inside the cell
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We often think of nucleus in animal cell and vacuole in plant cell. As per the latest studies
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the entire endoplasmic reticulum enclosed by a continuous membrane is the largest organelle of
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most eukaryotic cells. Its membrane may account for about half of all cell membranes and the space
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enclosed by the ER lumen may represent about 10% of the total cell volume. In the last video of the
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in-depth series, we discussed about animal cell structure. This video will give you an in-depth
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understanding of ER structure and function within five minutes. First of all, beginning with what is
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endoplasmic reticulum. Endoplasmic reticulum is an interconnected network of flattened sacs and
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tubules that is flattened sacs in the case of RER as you see here and tubules in the case of SER
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Forming a continuous sheet enclosing a single internal space called ER lumen or ER cisternal
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space. It consists of sheets, tubules and nuclear envelop as the outer nuclear membrane is in
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continuous connection with the rough endoplasmic reticulum as you see here and shares a common
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lumen with RER. In short endoplasmic reticulum is a network of tubes that produce, modifies and
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transport proteins, lipids and other molecules throughout the cell. There are two types of
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endoplasmic reticulum. The two distinct types of ER that performs different functions within the
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cell are the rough ER or rough endoplasmic reticulum which is covered by ribosomes on its
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outer surface, therefore called as RUF, functions in the protein synthesis and processing. The second
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one is the smooth ER as you see here. It is not associated with ribosomes and is involved in lipid
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metabolism that is why called as smooth Now let us move into the detail of functions of RUF endoplasmic reticulum One of the major functions of RAR is to serve as a site for protein synthesis for secretory and integral membrane proteins as well as a
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subpopulation of cytosolic proteins as per the recent studies. Let's move into the detail of
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each of this function. Function number one is protein synthesis. Protein synthesis requires
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localization of ribosomes to the cytosolic phase of endoplasmic reticulum. Translation of secretory
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or integral membrane proteins initiates in the cytosol even though the ribosomes are seen as
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attached to the RAR. Actually, translocation of secretory or integral membrane proteins
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initiates in the cytosol then ribosomes containing these mRNAs are recruited to
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the ER membrane and fuses with signal recognition proteins and are synthesized
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and released into the rough endoplasmic reticulum. You can refer our video on
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secretory and membrane protein synthesis in ER for more. Following protein synthesis and translocation into the ER lumen, a protein destined for secretion must undergo
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protein folding and modifications with the aid of chaperones and folding enzymes
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So the second function of ER, RER is protein folding, protein folding using chaperones and
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other folding enzymes. Binding immunoglobulin protein BIP is a is an HSB70 chaperone that is located in the lumen of ER. BIP assists in the folding
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of newly synthesized polypeptides as you see here by binding to the exposed
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hydrophobic side chains and subsequently coordinating the formation of their correct tertiary and quaternary structure. BAP's association with nascent
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polypeptides is stabilized by high concentration of calcium ions in the ER lumen and likely involves calcium binding Third function is protein processing that involves disulfide bond formation
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initial glycosylations like N-linked glycosylation. Let's move to disulfide bond formation. Disulfide bond formation in protein occurs extensively between 16 side chains via the
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oxidation of thiol group that is the SH group that is required for protein
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tertiary and cotary structure as you see this is the SH group and when two
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cysteine residues come together a bond is formed and is the disulfide bond. The
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cytosol is an unfavorable environment for disulfide formation because it contains NADPH dependent reducing pathways to maintain proteins in a
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reduced form. But ER or RER contains distinct enzymatic pathways involving the protein like
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disulfide isomerase protein PDI or protein disulfide isomerase family of proteins that
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helps in disulfide bond formation and also ER lumen is more oxidizing than the cytosol
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favoring disulfide bond formation. The next function is glycosylation. Glycosylation or addition of sugar to proteins is an important modification to eukaryotic proteins
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because the added sugar residues are often used as molecular flags or recognition signals to other
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cells that come in contact with them. N-linked glycosylation actually begins in the endoplasmic
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reticulum but O-linked glycosylation does not occur until the polypeptide has been transported
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into the Golgi apparatus and the final function of RER is quality control that means misfolded
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proteins are degraded in ER called as ER associated degradation. Now let us move into the functions of
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SCR or smooth endoplasmic reticulum. SCR is involved in lipid metabolism and is in continuous connection with RER
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Smooth endoplasmic reticulum synthesizes membrane lipids including phospholipids and cholesterol The second function is synthesis of steroid hormones such as cortisol estrogen and testosterone
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ACR is abundant in cortisol producing cells of adrenal gland, leading cells of testes that
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produce the hormone testosterone and follicular cells of the ovary that produces estrogen
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In all these cells, SCR is abundant. The third function of SCR is detoxification reaction
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SCR is abundant in liver where it detoxifies drugs, often involves enzyme-catalyzed hydroxylation or
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addition of OH group to hydrophobic drugs making them more soluble and easier to remove or excrete
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from the body. And the fourth function is calcium metabolism, especially sarcoplasmic reticulum
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that is found in muscle cells. That is SCR that is present in muscle cells called a sarcoplasmic
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reticulum. It's an SCR that is involved in calcium storage, calcium ion storage. It's a major store
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for of intracellular calcium ions, a major signaling molecule with diverse functions
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The SCR contains several calcium channels, rhinodin receptors, IP3 receptors and all such things
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that are responsible for releasing calcium ions from the ER into the cytosol
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when the intracellular calcium levels are low. And finally, the final function of SCR
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SCR of liver cells is also involved in enzymatic breakdown of stored glycogen
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using glucosic phosphatase. It's a membrane bound enzyme that is unique to SER or endoplasmic reticulum
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Often used as an ER marker enzyme for ER localization using fluorescent
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antibodies. So these are the functions of RER and SER. Hope you are benefited from
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this video. Take care. Stay blessed. Thank you so much. You are with biology
