embrane, which is important to its function.
             The fluid-mosaic model suggests that the plasma membrane is highly flexible and can change its shape and composition through time. The lipid bilayer functions as a liquid in which other molecules such as proteins "float". The fluid nature of the lipid bilayer is very important. It provides an important means of distributing molecules within the plasma membrane. In addition, slight damage to the membrane can be repaired because the phospholipids tend to reassemble around damaged sites and seal them closed. The fluid nature of the lipid bilayer enables membranes to fuse with one another.
             Although the basic structure of the plasma membrane is determined mainly by its lipids, the functions of the plasma membrane are determined mainly by its proteins. Integral, or intrinsic proteins, penetrate the lipid bilayer from one surface to the other. Peripheral, or extrinsic proteins, are attached to either the inner or outer surfaces of the lipid bilayer. Integral proteins consist of regions made up of amino acids with hydrophobic R groups and other regions of amino acids with hydrophilic R groups. The hydrophobic regions are located within the hydrophobic part of the membrane, and the hydrophilic regions are located at the inner or outer surface of the membrane or line channels through the membrane. Peripheral proteins are usually bound to integral proteins. Some membrane proteins form channels through the membrane or act as carrier molecules. Other membrane proteins are receptors, markers, enzymes, or structural supports in the membrane. The ability of membrane proteins to function depends on their three-dimensional shape.
             Channel proteins are one or more integral proteins arranged so that they form a tiny channel through the plasma membrane. The hydrophobic regions of the proteins face outward toward the hydrophobic part of the cell membrane, and the hydrophilic regions of the pr...