Endoplasmic reticulum and cell membrane relationship questions

Plasma Membrane and the Endoplasmic Reticulum

endoplasmic reticulum and cell membrane relationship questions

interpretation. Rather, CPL should interpret “consisting of” as the correct relationship, region containing Y. Similarly for "A plasma membrane consisting of lipids and proteins" Smooth endoplasmic reticulum, mitochondria, and ribosomes. An organelle in the cell is called endoplasmic reticulum (ER) found in the eukaryotic cells. Especially in eukaryotic cells, the cytoplasmic matrix itself is pervaded by membrane-bordered channels that comprise the endoplasmic reticulum or ER.

The RER is generally a series of connected flattened sacs.

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It plays a central role in the synthesis and export of proteins and glycoproteins and is best studied in secretory cells specializing in these functions.

The many secretory cells in the human body include liver cells secreting serum proteins e. What is the function of the endoplasmic reticulum? The endoplasmic reticulum ER serves important functions particularly in the synthesis, folding, modification, and transport of proteins.

Ribosomes on RER, which give RER its rough appearance, specialize in the synthesis of proteins that possess a signal sequence that directs them specifically to the ER for processing. Proteins synthesized by the RER have specific final destinations, such as the cell membrane, cell exterior, or the ER itself. SER is involved in the synthesis of lipids, including cholesterol and phospholipidswhich are used in the production of new cellular membrane.

endoplasmic reticulum and cell membrane relationship questions

In cells of the liver, SER contributes to the detoxification of drugs and harmful chemicals. The sarcoplasmic reticulum is a specialized type of SER that regulates calcium ion concentration in the cytoplasm of striated muscle cells.

How does the endoplasmic reticulum work with the cell membrane?

Learn more about ribosomes. When was the endoplasmic reticulum discovered? The ER was first noted in the late 19th century, when studies of stained cells indicated the presence of some type of extensive cytoplasmic structure, then known as the gastroplasm. The electron microscope made possible the study of the morphology of the ER in the s, when it was given its present name.

Learn more about the electron microscope. Rough ER is named for its rough appearance, which is due to the ribosomes attached to its outer cytoplasmic surface. Rough ER lies immediately adjacent to the cell nucleusand its membrane is continuous with the outer membrane of the nuclear envelope.

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The ribosomes on rough ER specialize in the synthesis of proteins that possess a signal sequence that directs them specifically to the ER for processing. Protein synthesis and folding One of the major functions of the ER is to serve as a site for protein synthesis for secreted and integral membrane proteins [ 8 ], as well as a subpopulation of cytosolic proteins [ 1 ].

Protein synthesis requires localization of ribosomes to the cytosolic face of the ER, and the canonical pathway that regulates protein synthesis involves co-translational docking of the mRNA: Translation of secretory or integral membrane proteins initiates in the cytosol, then ribosomes containing these mRNAs are recruited to the ER membrane via a signal sequence within the amino terminus of the nascent polypeptide that is recognized and bound by the signal recognition particle SRP [ 910 ].

endoplasmic reticulum and cell membrane relationship questions

The complex of mRNA: Translation continues on the ER and the emerging polypeptide can co-translationally enter the ER through the translocon [ 2 ], which is a channel that contains several Sec proteins and spans the lipid bilayer [ 13 ].

Also during this time, or in some cases once translation is complete [ 3 ], a signal peptidase cleaves the short signal peptide allowing the free protein to enter the ER lumen [ 14 ]. If the protein is destined to be an integral membrane protein, determined by the presence of a stretch of hydrophobic residues or stop-transfer membrane anchor sequence, translocation will pause [ 15 ]. At this point the protein will be shifted laterally and become anchored within the phospholipid bilayer where it remains [ 15 ].

Transmembrane proteins can either contain one hydrophobic stretch of amino acids, and are classified as single pass transmembrane proteins, or contain multiple regions that cross the membrane and are classified as multi-pass transmembrane proteins [ 3 ]. If the protein is not destined to be integrated into the membrane, but instead enter the secretory pathway or the lumen of membrane-bound organelles, the protein begins the process of transport.

endoplasmic reticulum and cell membrane relationship questions

Once translation is complete and the signal peptide has been cleaved the ribosomes are released back into the cytosol [ 1617 ]. For cytosolic proteins translated on ER-bound ribosomes it is not clear how these mRNAs are recruited to the ER or what populations of ribosomes are utilized to initiate translation, although a recent study indicates that the ER-resident protein p may play a role in the translation-independent recruitment of mRNAs to the ER [ 20 ].

Following protein synthesis and translocation into the ER lumen, a protein destined for secretion must undergo proper folding and modifications, with the aid of chaperones and folding enzymes.

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These modifications include N-linked glycosylation, disulfide bond formation and oligomerization [ 3 ]. At this point the fate of the secretory proteins is determined. If the protein functions in the ER, for example as a chaperone, then proper folding will commence.

If the protein is destined for secretion, it will be released by the chaperones and packaged for travel through the Golgi on to a final destination such as the plasma membrane or secreted or move into peroxisomes [ 21 ].

Additionally, the cytosolic regions of the transmembrane protein may interact with cytosolic proteins or chaperones to properly fold these domains.

On the other hand, even with several proteins and complexes dedicated to folding proteins properly, a fraction of proteins do not achieve native and functional form and are either misfolded or aggregated [ 22 ]. These proteins can either remain in the ER or enter the ER-associated degradation ERAD pathway mediated by the proteasome, assuring that aberrant polypeptides do not inadvertently enter the secretory pathway [ 23 ].

Recognition of misfolded proteins, followed by clearing of these aggregates through the ERAD pathway, needs to be tightly controlled so as not to affect cellular function [ 23 ]. Interestingly, there are several connections to activation of ER stress response pathways and pathological human conditions.