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The types of lipids that are present in the phospholipid bilayer define many of the physical attributes of the membrane (e.g., how fluid it is at any temperature). The phospholipid bilayer also contains lipids that are involved in cell communication (signal transduction) as detailed in future chapters. The figure below (Figure 2.3) reveals that there is an uneven distribution of phospholipids in the cell membrane with certain types preferentially on one side and others on the opposite side. These “sides” are called leaflets: the outer leaflet, where glycolipids can be found, faces the environment while the inner leaflet, rich in phosphatidylserine, is oriented towards the cytoplasm.

Figure 2.3. Types of phospholipids and their localization within the cell membrane.

Some typical examples of phospholipid localization:

•Outer leaflet: more phosphatidylcholine and sphingomyelin

•Inner leaflet: more phosphatidylserine and phosphatidylethanolamine

•Glycolipids: only on outside face