• Membranes are complex structures composed of lipids, carbohydrates, and proteins.
• The basic structure of all membranes is the lipid bi-layer. This bilayer is formed by two sheets of phos-pholipids in which the hydrophilic polar head groups are directed away from each other and are exposed to the aqueous environment on the outer and inner surfaces of the membrane. The hydrophobic nonpolar tails of these molecules are oriented toward each other, in the direction of the center of the membrane.
• Membrane proteins are classified as integral if they are firmly embedded in the bilayer and as peripheral if they are loosely attached to the outer or inner surface.
• The 20 or so different membranes in a mammalian cell have intrinsic functions (eg, enzymatic activity), and they define compartments, or specialized environments, within the cell that have specific functions (eg, lysosomes).
• Certain molecules freely diffuse across membranes, but the movement of others is restricted because of size, charge, or solubility.
• Various passive and active mechanisms are employed to maintain gradients of such molecules across different membranes.
• Certain solutes, eg, glucose, enter cells by facilitated diffusion, along a downhill gradient from high to low concentration. Specific carrier molecules, or transporters, are involved in such processes.
• Ligand- or voltage-gated ion channels are often employed to move charged molecules (Na+, K+, Ca2+, etc) across membranes.
• Large molecules can enter or leave cells through mechanisms such as endocytosis or exocytosis. These processes often require binding of the molecule to a receptor, which affords specificity to the process.
• Receptors may be integral components of membranes (particularly the plasma membrane). The interaction of a ligand with its receptor may not involve the movement of either into the cell, but the interaction results in the generation of a signal that influences intracellular processes (transmembrane signaling).
• Mutations that affect the structure of membrane proteins (receptors, transporters, ion channels, enzymes, and structural proteins) may cause diseases; examples include cystic fibrosis and familial hypercholes-terolemia.
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