Lipids are an essential component of all biological membranes. The average lipid content in mammalian membranes is 50%, though it can be as low as 20% in the inner mitochondrial membrane or as high as 80% in the myelin sheath present around the nerve cells.
Phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, and sphingomyelin are the most common phospholipids present in mammalian membranes. At physiological pH, phosphatidylserine is negatively charged, while the other three remain neutral. In membrane lipids, the numbers of carbon atoms present in the fatty acids and their degree of saturation may vary. The fatty acids can be fully saturated with no double bonds, monounsaturated with only one double bond, or polyunsaturated with multiple double bonds.
Glycolipids comprise a physiologically important class of membrane lipids that are always present on the outer or extracellular side of the membrane. In eukaryotes, they are found both in the plasma membrane and the intracellular membranes. Galactocerebroside, a simple glycolipid, exists abundantly in myelin sheaths along with sphingomyelin. Gangliosides, the most complex glycolipids, are negatively charged due to the presence of sialic acid and can constitute up to 10% of the total lipids in nerve cell membranes. Gangliosides are studied for their roles in viral and genetic diseases. The ganglioside GM1 acts as a cell surface receptor for Cholera toxin, while an excessive accumulation of GM2 gangliosides is observed in Tay-Sachs disease, a genetic disorder.
Another class of membrane lipids is the sterols. Plant-specific sterols constitute a significant component of the plant plasma membrane and can comprise 30-50% of total membrane lipids. In eukaryotic membranes, cholesterols are also present, constituting almost 50% of the total lipids. The structure of cholesterol in human cell membranes is slightly different from ergosterol, a common lipid constituent of the fungal membranes.
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