| Structural and metabolic role of membranes
|
| A major role of membranes is to maintain the structural integrity and barrier function of cells and organelles. However, membranes are not rigid or impermeable: they are fluid, and their components move around, and they are subject to metabolic turnover. The turnover of membrane components is especially important for the cellular response to information from inside and outside the cell: recognition, transfer, amplification, and signal transduction processes all occur in or on the membranes. Both small and large molecules must pass through the membrane. With few exceptions, specific membrane proteins mediate these transport processes.
|
| Phospholipids not only provide a fluid environment, but also regulate the activities of membrane enzymes. Particular phospholipids are required for specific membrane structures, such as curved regions and junctions with adjacent membranes. The inside surface of the membrane is more suited to phosphatidylethanolamine and phosphatidylserine, in which the polar heads are small and the hydrocarbons are more spread out, because of their larger contents of polyunsaturated fatty acids. As a result of such differing requirements, phospholipids are distributed asymmetrically between outer and inner leaflets of membranes: phosphatidylcholine and sphingomyelin are more abundant in the outer leaflet, whereas phosphatidylethanolamine and phosphatidylserine are enriched in the inner leaflet. Such asymmetries are actively maintained by flippases, and cell damage often leads to loss of this membrane lipid asymmetry. Exposure of phosphatidylserine in the outer leaflet of the erythrocyte plasma membrane increases the cell's vascular adherence and is a signal for macrophage recognition and phagocytosis. Both of these processes probably contribute to the natural process of red cell turnover.
|
|
