1. Natural Fats are largely absent from cell membranes. 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 membranes. sphingolipids and cholesterol are absent. In animal cell membranes glycerophospholipids and sphingophospholipids comprise the polar lipids, while the principal 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 contains the zwitterionic phospolipids sphingomyelin 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 rat 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 mitochondiral membranes.
5. Phosphatidyl choline and phosphatidyl ethanolamine are the most abundant glycerophospholipids in higher plants and animals. Cephalin is the main phospholipid of bacteria. lecithin is rarely present.
6. Phosphaidyl glycerol is present in small amounts in animal mitochondrial membranes. In plant membranes it contitutes 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 bacterial.
7. choline plasmalogen is abundant in heart muscle. Ethanolamine plasmalogen is present in relatively high concentration in myelin and in lesser amount in heart muscle. In bacteria plasmalogens are confined to anaerobes.
8. The membranes of eukaryote cell organelles contain sterols in varying amounts. The plasma membrane is comparatively very much richer in cholesterol.
9. Cholesterol:phospholipid ratio. In human erythrocyte membranes the outer membrane leaflet is richer in cholesterol than the inner leaflet. In myelin outer membrane leaflet the cholesterol:phospholipid ratio is equimolar, while in the inner leaflet the ratio is 3:7.
10. Liver plasma membranes have a low phosphatidyl choline 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.