what is the endoplasmic reticulum?
a continuous network of flattened sacs, tubules, and associated vesicles that stretches throughout the cytoplasm of the eukaryotic cell
what are the ER cisternae?
what is the ER lumen?
the membrane-bound sacs
the space enclosed by the ER cisternae
what are the functions of the ER?
enzymes associated with the ER are involved in synthesis of proteins for: incorporation into the PM, organelles in the endomembrane system, and export from the cell
what are the two types of ER?
what are their functions?
Smooth and Rough ER
Rough ER is studded with ribosomes which synthesize polypeptides that accumulate within the membrane or are transported across the lumen
Smooth ER has no role in protein synthesis, involeved in the syntheses of lipids and steroids, also inactivates and detoxifies potentially harmful substances
What forms the endomembrane system?
rough and smooth ER
why are there variations in the amounts of RER to SER?
both types of ER are present in most cells
cells involved in synthesis of secretory proteins have more RER
cells producing steroid hormones tend to have extensive SER
what are microsomes?
when tissue is homogenized for subcellular fractionation, the ER membranes often break into smaller fragmetnsthat spontaneously close to form sealed vesicles called microsomes
useful for studying both types of ER
DO NOT form naturally in cells
how is the RER involved in the biosynthesis and processing of proteins?
ribosomes on the cytosolic side fo the RER membrane synthesize both membrane-bound and soluble proteins for the endomembrane system
newly synthezied proteins are inserted into the endomembrane system through a porecomplex as they are synthesized
what is the RER a site for?
initial steps of addition of carbohydrates to glycoproteins
folding of polypeptides
recognition and removal of misfolded proteins
assembly of multimeric protein complexes
what is ER-associated degradation? (ERAD)
-proteins that are incorrectly folded, midified or assembled are exported for degradation
degradation occurs in cytosolic proteasomes (enzymes that break up bad proteins)
how is the smooth ER involved in drug detoification?
-what is it catalyzed by?
drug detoxification involves enzyme-catalyzed hydroxylation because the addition of hydroxyl groups to hydrophobic drugs makes them more soluble and easier to excrete from the body
catalyzed by monoxygenases (member of the protein family cytochrome P-450)
how is the SER involved in drug detoxification?
how does hydroxylation work?
e- from NADPH or NADH are transferred to a heme group in cytochrome P-450, later donated to O2
-increase in drugs/poison=increase in SER=greater detoxification
how is the SER involved in carbohydrate metabolism?
involved in the breakdown of stored glycogen
liver stores glucose as glycogen in granules associated with SER
glucose needed,glycogen is broken down by phosphorylysis
how is the SER involved in calcium storage?
the sarcoplasmic reticulum found in muscle cells is an example of SER that specializes in calcium storage
ER lumen contains high concentrations of calcium binding proteins
calcium ions pumped into the ER by ATP-dependent calcium ATPases; released when needed for muscle contraction
how is the SER involved in steroid biosynthesis?
SER is the site of cholesterol and steroid hormone synthesis
SER associated with plastids in some plants
cholesterol, cortisol and steroid hormones share a four-ring strucutre but differ in the number and arrangement of carbon side chins and hydroxyl groups
what are exceptions to other organelles receiveing lipids from the ER?
Mitochondrion synthesize phosphatidylethanolamine
Peroxisomes synthesize cholesterol
Chloroplasts contain enzymes for chloroplast-specific lipids
how does the ER synthesize the membrane?
fatty acids for membrane phospholipids are synthesized in the cytoplasm and incorporated intothe ER membrane on the cytosolic side
transferred to the lumenal side by flippases
type of phospholipid transferred across the membrane depends on the particular translocator present (membrane asymmetry)
movement of lipids from ER to mitochondria, cholorplasts or peroxisomes is problematic
phospholipid exchange proteins convey specific phospholipids to these organelles