The amphiphilic nature of membrane lipids results in bilayer structures that tend to form spheres.
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CHAPTER 3 Cell Structure and Function 29
associate with polar lipid head groups. The three-dimensional structure of many membrane proteins is complex, with numerous twists and turns through the lipid bilayer (Fig. 3.8).
The type of membrane proteins in a particular cell depends on the cell’s primary functions. For example, a kidney tubule cell has a large proportion of transmembrane proteins, which are needed to perform the kidney’s function of electrolyte and nutrient reabsorption. In contrast, the human red blood cell (RBC) contains mainly peripheral proteins attached to the inner surface of the membrane. One of these proteins, spectrin, has a long, thin, flexible, rodlike shape that forms a supportive meshwork, or cytoskeleton, for the cell. It is this cytoskeleton that enables the RBC to withstand the membrane stress of being forced through small capillaries.
Although proteins and lipids are generally free to move within the plane of the cell membrane, many cells are able to confine certain proteins to specific areas. Using the example of the kidney tubule cell again, it is important for the cell to keep transport proteins on its luminal side to reabsorb filtered molecules (Fig. 3.9). This segregation of particular proteins is accomplished primarily by intercellular con- nections called tight junctions, which connect neighboring cells and function like a fence to confine proteins to an area of the membrane. Membrane proteins also can be immobilized by tethering them to cytoskeleton or extracellular matrix structures.
Membrane Proteins Approximately 50% of the mass of a typical cell membrane is composed of protein. The specific types of membrane proteins vary according to cell type and environmental conditions. Some membrane proteins, called transmembrane proteins, extend across the membrane bilayer and are in contact with both the extracellular and the intracellular fluids. Transmembrane proteins serve a variety of functions, including transport of charged and polar molecules into and out of cells and transduction of extracellular signals into intracellular messages. Other peripheral membrane proteins are less tightly anchored to the membrane. The common structural orientations of membrane proteins are shown in