Endoplasmic Reticulum Flashcards
What is the er
Network of membranes thats penetrate much of the cytoplasm
Envolved from imaginations in the membrane
Divided into two compartments: rer/ser
Rough us defined by the presence of ribosomes bound to its cystolic surface
Whereas sr lacks these associated ribosomes
SER
Is an irregular network of membrane-bounded channels that lack ribsomess on its surface= smooth
1- steroid hormone synthesis occurs in sr rich cells- leydig of the testies - testosterone
2- cells synthesising fatty acids/ phospholipids are rich in sr
3- drug detoxification- in hepatocytes - in response to drug pheno barbital- carried out by oxygen transferring enzymes p450 family- which are known for thier lack of substrate specificity- being able to oxidise thousands of different hydrophobic compounds and convert them into more hydrophilic/more readily excreted derivatives
4- muscle contraction/ elaxation involve the release and recapture of ca2 by ser in skeletal muscle cells= sarcoplasmic reticulum
RER
System of membrane bound scacs/ cavities
Outer surface studded with ribosomes = rough apprehended
Is where membrane packaged proteins are synthesised including- secretory/pm and lysosomal proteins
Rer also monitors retention- degration of certain protiens
Proteins to be retained in rer cisternae are marked with presence of small peptide- composed of specific sequence of 4 amino acids at thier c terminus
Synthesis of proteins on membrane bound ribosomes
Proteins synthesised on r attached to cystolic surface of rer include
Secreted p, integral membrane p, soluble p that reside within compartments of endomembrane system including er/Golgi - complex lysosomes, endosomes, vesicles, plant vacuoles
Free ribosomes
Not attached to rer]
Are released into cytosol includes:
P to be destined to remain in cytosol- enzymes of glycolysis and p of cytoskeleton
Perephiral proteins of cystolic surface of membranes- spectrins- ankyrins that are only weakly associated with plasma membranes systolic surface
Proteins that are transported to the nucleus
What determines if protein is synthesised for excretion?
Sequence of a IN THE N TERMINAL portion of the polypeptide, which is the first part to emerge from the ribosome during p synthesis
1- secretory p contain signal sequence at n terminus that directs emerging polypeptide and ribosome to er membrane
2- the polypeptide move into the cisternae of er through protein lined, aqueous channel in er membrane
What is the signal hypothysis
The sh proposes that proteins destined for secretion, which involves the movement of protein across a biological membrane , are originally manufactured with an initial sequence of aaa that may/ not present in the mature protein
Deleting a ss from an er protein converts itinto cystolic p- placing on at beginning of cystolic protein redirects p into er
Synthesis of secretory, lysosomal,plant vacuolar proteins on membrane bound ribsosmes
Synthesis of polypeptide begins after a messenger rna bind to free ribosome, that is not attached to a cytoplasmic membrane
1-synthesis begins on free ribsosome
2- as signal seq emerges from ribosome it binds to signal recognition particle which stops further translation until the srp- ribsosome nascent chain complex can make contact with the er membrane
3- the srp complex then collides and binds to an srp receptor situated within the er membrane
4- attachment of this complex to srp receptor is followed by the release of the srp and the association of the ribosome with a translocation of the er membrane
5- after nascent polypep passes into l;Ulmen of er, protein undergoes folding with the aid of er chaperones
Membrane biosynthesis in er
Most lipids are synthesised entirely in er
Major exceptions- sphingomyelin, glycolipids- begins in er finished in the Golgi complex
2- some of the unique lipids of mitochondrial and chloroplast membranes are synthesised by enzymes that reside in those membranes
Newly synthesised phospholipid are insertyedf into half of bilayer facing the cytosol
