Endomembrane system Flashcards
components of the endomembrane system (6)
RER
SER
Golgi
lysosomes
transport and secretory vesicles
nuclear envelope
How can ER fragments be isolated and obtained?
-centrifugation of sample at high speed
-collection of the tissue homogenate
-different homogenate samples are harvested at diff speeds and henc include different components of the cell
- MICROSOMAL FRACTION obtained: fragments (microvesicles) of the ER called microsomes
-staining and microscopy for visualisation (RER microsomes have dark ribosomes in the sample that are absent in SER)
!! usually SER microsomes have a lower density (and hence are found abive) RER microsomes
morphology of SER
-no ribosomes on membrane (hence no involvement in protein synthesis)
-tubular arrangement
-membrane bound
General functions of the SER (5)
- lipid/steroid synthesis and metabolism
- compound detoxification
- glycogen metabolism
- Ca2_ storage
- biogenesis of cell membranes
process by which SER detoxifies compounds
usually by the addition of OH groups to toxic compounds, favouring their elimination
!!! very frequent in liver cells
process of plasma membrane biogenesis occuring in SER
-lipids (that form part of phospholipids) are synthesised by enzymes in SER
-phospholipids inserted into the cytosolic leaflet of membrane bilayer
- composition of bilayer odified by 2 ENZYMES:
- scramblase: restores the same number of PLs in layers
- flipase/flopase: flips phospholipids to have specific PL composition on each layer as needed
!! both these enzymes are transmembrane
purpose of Ca2+ storage achieved by SER
-monitors the ratio of calcium accummulation and release from/into cytoplasm
!! VITAL IN SKELETAL MUSCLE CELLS:
-SER is called sarcoplasmic reticulum
-Ca2+ ions allow muscle contraction
-SR releases Ca2+ upon depolarisation to contract muscle
-SR uptakes Ca2+ when the signal is done to stop contractions
process by which SER synthesises steroids and lipids
!!occurs in gonads and adrenal gland
-cooperation with mitochondria
process by which SER metabolises glycogen
OCCURS IN LIVER CELLS:
-SER used in a step of the process which degrades glycogen into glucose
-Glycogen turns into glucose-6-phosphate which cannot be transported in the bloodstream due to its connection to a phosphate group
-SER membrane contains permeases (enzymes) that remove the Pi group
-hence the compound is changed into glucose and is able to be transported in the bloodstream
Morphology of RER
-cisternae (flattened tubes)
-connected with nuclear envelope
-ribosomes on surface
-membrane bound
general functions of RER (5)
-protein synthesis
-folding of proteins into secondary and tertiary structure
-Nglucosylation modifications
-holds integrity of cell due to being connected with nuclear components
-addition of OH groups (hydroxylation) to lysine residues of tropocollagen molecules
RER in neurones name
NISSL BODIES: clumps of RER and free ribosomes in neurons
-taken as markers for these cells
-not present in axon or dendrites (only on the body where protein synthesis occurs)
2 areas where proteins can be synthesised and the differences between the proteins made
- synthesis on cytosolic polysomes (activated aggregates of free ribosomes):
-nuclear proteins (TFs/histones)
-soluble proteins (final destination being the cytoplasm)
-mitochondrial proteins - synthesis on RER bound ribosomes:
-secreted proteins (antibodies)
-integral membrane proteins (ion channel subunits)
-ER, Golgi and lysosome lumen resident proteins
how are proteins imported into the RER
SRPS: signal recognising particles
-proteins incoming to RER contain signal sequence
-SRPs on membrane of RER recognises this signal sequence
-after recognition, protein moves through translocation channel and is inserted into RER lumen
