Question

Are the following lipid samples likely to correspond to the inner or outer leaflet of a eukaryotic plasma membrane? (a) 20% phosphatidylcholine, 15% phosphatidylserine, 65% other lipids. (b) 35% phosphatidylcholine, 15% gangliosides, 5% cholesterol, 45% other lipids.

Answer 1

The plasma membrane of eukaryotic cells are made up of both lipids and proteins. The fundamental block of the membrane is the phospholipid bilayer. The plasma membranes of eukaryotes contain 4 major phospholipids namely phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, and sphingomyelin which contribute to more than two halves of the lipid in the membrane. The outer leaflet consists mainly of phosphatidylcholine and sphingomyelin on the other hand  phosphatidylethanolamine, phosphatidylserine and phosphatidylinositol are the predominant phospholipids of the inner leaflet.

Together with these phospholipids, the plasma membrane contains glycolipids and cholesterol that are found in the outer leaflet.Cholesterol forms a major component and occurs in the same amount as the phospholipids. The cholesterol is inserted into the phospholipid bilayer.

Therefore (b) 35% phosphatidylcholine, 15% gangliosides, 5% cholesterol, 45% other lipids corresponds to the outer leaflet of the phospholipid bilayer due to the dominance of phosphatidylcholine which forms a major part of the outer leaflet as stated above.

Whereas (a) 20% phosphatidylcholine, 15% phosphatidylserine, 65% other lipids corresponds to the inner leaflet due to presence of phosphatidylserine.