Physiology Topics
The basic framework of the plasma membrane is the lipid bilayer made of phospholipids, cholesterol and fatty acids. The bilayer arrangement occurs because the lipids are amphipatic. In phospholipids, the hydrophilic part is the phosphate-containing head, linked by a glycerol to two fatty acid groups which are hydrophobic. Glycolipids appear only in the membrane layer that faces the extracellular fluid. Cholesterol molecules are interspersed among the other lipids, their hydroxyl groups forming hydrogen bonds with polar heads of phospholipids and glycolipids. The stiff steroid rings of cholesterol are nonpolar.
The phospholipid's fatty acid chains affect membrane fluidity. Unsaturated chains increase fluidity. At normal body temperature, cholesterol decreases fluidity.
Membrane proteins may be integral (extend into or though the bilayer) or peripheral (associated with either surface of the bilayer). They may function as channels, transporters, receptors, enzymes, identity markers or linkers. Channels allow specific substances to move through a pore. Transporters move specific substances across the membrane by changing conformation. Receptors recognize specific ligands and alter cell's function in some way. Enzymes catalyze reactions inside or outside the cell. Cell identity markers enable cells to recognize each other. Linkers anchor filaments inside and outside the membrane providing stability and shape for the cell and may participate in movement of the cell or linking cells together.
Materials may cross the plasma membrane by passive transport or active transport. In passive transport, a substance moves down its concentration gradient, whereas in active transport energy (ATP) is used to move the substance against its concentration gradient. Passive transport may occur by diffusion, facilitated transport or osmosis. Small, relatively nonpolar molecules diffuse freely through the membrane. Diffusion of larger substances is facilitated by channels and transport proteins. Channels are usually open and selective to specific ions. Transport proteins change conformation to move a molecule across the membrane.
The difference in concentration of a chemical between one side of the plasma membrane and the other is called a concentration gradient. Oxygen, chloride and sodium are more concentrated in the extracellular fluid, while proteins, carbon dioxide and potassium are more concentrated in the cytosol.
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