Membrane Synthesis & Asymmetry

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MEMBRANE COMPONENTS

Phospholipids: synthesized on the cytosolic face of the ER
Glycolipids
Membrane proteins

MEMBRANE SYNTHESIS 1. Phospholipids are synthesized on the cytosolic face of the ER and glycolipids on the lumenal face of the Golgi apparatus 2. Vesicles bud from organelles and transport them to cell membrane 3. Vesicles fuse with cell membrane the lipids they transport retain same orientation unequal distribution of molecules generates curvature 4. Flippases flip some phospholipids to the extracellular face of bilayer 5. Glycolipids remain on extracellular face (no flippase action)

TOPOLOGICALLY EQUIVALENT SPACES Endoplasmic reticulum Golgi Apparatus Vesicles Extracellular space

MEMBRANE LIPIDS DISTRIBUTION

Extracellular layer

Phosphatidylcholine: most common, structural
Sphingomyelin: less abundant, variable head groups
Glycolipids: carbohydrate attached to membrane lipid

Cytosolic layer

Phosphatidylethanolamine: small head group that generates curvature
Phosphatidylinositol: minor lipid, binds proteins (signal transduction)
Phosphatidylserine: binds proteins to membrane

Variable head groups & fatty acid tail length/saturation

CLINICAL CORRELATION

Apoptosis (programmed cell death)

Phosphatidylserine in extracellular leaflet of bilayer is signal for phagocytosis

Membrane Synthesis & Asymmetry

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