Distribution of Membrane Lipids

1. Neutral fats are largely absent from cell membrane. Neutral fats such as triglycerides are found mainly in the cytoplasm and represent the major form of storage lipids.

2. Glycerphospholipids are the principal constituents of bacterial membrane. Sphingolipids and sphingophospholipids comprise the polar lipids, while the principle non-polar membrane lipids are neutral sterols.

3. Use of chemical markers has shown that phospholipids are asymmetrically arranged in human erythrocyte membranes. The outer membrane leaflet contain the zwitterionic phospholipids shpinngonnmyelin and phosphatidyl choline, while the inner membrane leaflet contains phosphatidyl ethanolamine, phosphatidyl serine and, when present, phosphatidyl inositol. The head groups of phospholipids in the inner membrane leaflet are largely anionic.

4.  In rate liver membranes phospholipids comprise about 60% of the lipids in plasma membranes and Golgi membranes, about 80% in the rough endoplasmic and nuclear membranes, and about 90% in mitochondrial membranes.

5. Phosphatidyl choline (lecithin) and phosphhatidyl ethanolamine (cepalin) are the most abundant glycerophospholipids in higher plants and animals. Cephalin is the main phospholipid of bacteria: lecithin is rarely present.

6. Phosphhatidyl glycerol is present is small amounts in animal mitochondrial membranes. In plant membranes it constitutes 20-30% of phospholipids and in chloroplasts 40-60%. The amount of phosphatidyl glycerol varies in bacterial membranes, being as high as 70% of the membrane phospholipids in Gram positive bacteria. Phosphatidyl glycerol is a precursor of diphosphatidyl glycerol which is found I mitochondria, chloroplasts and bacteria.

7. Choline Plasmalogen is abundant in heart muscle. Ethanolamine plasmalogen is present in relatively high concentration in myelin and in lesser amount in hear muscle. In bacteria plasmalogens are confined to anaerobes.

8. The membrane of eukaryote cell organelles contains sterols in varying amounts. The plasma membrane is comparatively very much richer in cholesterol.

9. Cholesterol: Phospholipids ratio. In human erythrocyte membranes the outer membrane leaflet is richer in cholesterol than the inner leaflet. In myelin outer membrane leaflet the cholesterol: phospholipids ratio is equimolar, while in the inner leaflet the raio is 3:7.

10.  Liver plasma membranes have a low phosphatidyl choline (lecithin) content and high phosphatidyl serine, sphingomyelin and cholesterol content.

11. Cholesterol controls the fluidity of fatty acid chains of phospholipids. It enhances the stability of phospholipid bilayers and also reduces their permeability. 


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