膜脂

Cell membranes contain phosphoglycerides, sphingolipids, and sterols. Phosphoglycerides have a glycerol backbone with hydrophilic phosphate groups and hydrophobic fatty acyl chains. Sphingolipids contain a sphingosine backbone and form ceramide when a fatty acid attaches to sphingosine. Sterols like cholesterol intercalate within phospholipid bilayers to regulate membrane fluidity.

Both are common phosphoglycerides in mammalian membranes, but they differ in charge. Phosphatidylcholine and phosphatidylethanolamine remain neutral at physiological pH, while phosphatidylserine is negatively charged. These charge differences affect membrane properties and protein interactions, making phosphatidylserine particularly important for membrane asymmetry regulating transporters.

Glycolipids are always positioned on the outer or extracellular side of membranes. Gangliosides, the most complex glycolipids, are negatively charged due to sialic acid and constitute up to 10% of nerve cell membrane lipids. GM1 gangliosides serve as cell surface receptors for Cholera toxin, while GM2 accumulation causes Tay-Sachs disease.

Sterols cannot independently form lipid bilayers but instead intercalate within phospholipid bilayers as fluidity buffers. Cholesterol comprises almost 50% of total lipids in eukaryotic membranes, while plant-specific sterols constitute 30-50% of plant membrane lipids. Ergosterol serves a similar role in fungal membranes.

Fatty acids in membrane lipids vary in saturation levels: fully saturated with no double bonds, monounsaturated with one double bond, or polyunsaturated with multiple double bonds. These variations influence membrane properties and fluidity. The carbon chain length typically ranges from 16 to 22 atoms in phosphoglycerides.

Ceramide is the simplest sphingolipid, formed when a fatty acid attaches to the amino group of sphingosine. The remaining hydroxyl group of ceramide can form esters with phosphocholine to create sphingomyelin or with carbohydrates to form glycolipids. Sphingomyelin is abundant in myelin sheaths alongside galactocerebroside.

Lipids comprise approximately 50% of average mammalian membrane content, though this varies significantly by membrane type. The inner mitochondrial membrane contains as little as 20% lipids, while myelin sheaths contain up to 80% lipids. This variation reflects different membrane functions and structural requirements.