Plasma Membrane: Structure and Function PDF

The outer cell membrane (Figure 2.2) is called the plasma membrane or plasmalemma (plaz′mah-lem′ah; lemma = sheath, husk). This thin, flexible layer defines the extent of the cell, thereby separating two of the body's major fluid compartments: the intracellular fluid within the cells and the extrace...

The outer cell membrane (Figure 2.2) is called the plasma membrane or plasmalemma (plaz′mah-lem′ah; lemma = sheath, husk). This thin, flexible layer defines the extent of the cell, thereby separating two of the body's major fluid compartments: the intracellular fluid within the cells and the extracellular fluid that lies outside and between cells. To return to our analogy, you can think of the plasma membrane as a security fence surrounding the manufacturing plant (cell). This boundary contains specific checkpoints (receptors) that influence cellular activity in various ways. Figure 2.2 The plasma membrane according to the fluid mosaic model. Figure 2.2 Full Alternative Text 2.2a Structure The fluid mosaic model of membrane structure depicts the plasma membrane as a double layer, or bilayer, of lipid molecules with protein molecules embedded within it (Figure 2.2). The most abundant lipids in the plasma membrane are phospholipids. Like a lollipop on two sticks, each phospholipid molecule has a polar "head" that is charged, and an uncharged, nonpolar "tail" made of two chains of fatty acids. The polar heads are attracted to water---the main constituent of both the cytoplasm and the fluid external to the cell---so they lie along the inner as well as the outer face of the membrane. The nonpolar tails avoid water and line up in the center of the membrane. The result is two parallel sheets of phospholipid molecules lying tail to tail, forming the membrane's basic bilayered structure The inner and outer layers of the membrane differ somewhat in the kinds of lipids they contain. Sugar groups are attached to about 10% of the outer lipid molecules, making them "sugar-fats," or glycolipids (gli\"ko-lip′ids). The plasma membrane also contains substantial amounts of cholesterol, another lipid. Cholesterol makes the membrane more rigid and increases its impermeability to water and water-soluble molecules. Proteins make up about half of the plasma membrane by weight. The membrane proteins are of two distinct types: integral and peripheral (Figure 2.2). Integral proteins are firmly embedded in or strongly attached to the lipid bilayer. Some integral proteins protrude from one side of the membrane only, but most are transmembrane proteins that span the whole width of the membrane and protrude from both sides trans = across. Peripheral proteins, by contrast, are not embedded in the lipid bilayer at all. Instead, they attach rather loosely to the membrane surface. The peripheral proteins include a network of filaments that helps support the membrane from its cytoplasmic side. Without this strong, supportive base, the plasma membrane would tear apart easily. Short chains of carbohydrate molecules attach to the integral proteins to form glycoproteins. These sugars project from the external cell surface, forming the glycocalyx (gli\"ko-kal′iks; "sugar covering"), or cell coat. Also contributing to the glycocalyx are the sugars of the membrane's glycolipids. You can You can therefore think of your cells as "sugar-coated." The glycocalyx is sticky and may help cells to bind when they come together. Because every cell type has a different pattern of sugars that make up its glycocalyx, the glycocalyx is also a distinctive biological marker by which approaching cells recognize each other. For example, a sperm recognizes the ovum (egg cell) by the distinctive composition of the ovum's glycocalyx. 2.2b Functions The functions of the plasma membrane relate to its location at the interface between the cell's exterior and interior: The plasma membrane provides a protective barrier against substances and forces outside the cell. Some of the membrane proteins act as receptors; that is, they have the ability to bind to specific molecules arriving from outside the cell. After binding to the receptor, the molecule can induce a change in the cellular activity. Membrane receptors act as part of the body's cellular communication system. The plasma membrane controls which substances can enter and leave the cell. The membrane is a selectively permeable barrier that allows some substances to pass between the intracellular and extracellular fluids while preventing others from doing so. The processes involved in moving substances across the plasma membrane are described next. Membrane Transport Small, uncharged molecules, such as oxygen, carbon dioxide, and fat-soluble molecules, can pass freely through the lipid bilayer of the plasma membrane through a process called simple diffusion. Diffusion is the tendency of molecules in a solution to move down their concentration gradient; that is, the molecules move from a region where they are more concentrated to a region where they are less concentrated (Figure 2.3a). Water, like other molecules, diffuses down its concentration gradient. The diffusion of water molecules across a membrane is called osmosis (oz-mo′sis, Figure 2.3b). Figure 2.3 Membrane transport mechanisms. Figure 2.3 Full Alternative Text Most water-soluble or charged molecules, such as glucose, amino acids, and ions, cannot pass through the lipid bilayer by simple diffusion. Such substances can cross the plasma membrane only by specific transport mechanisms that use integral proteins to carry or pump molecules across the membrane. Some of these molecules move down their concentration gradient, diffusing through the plasma membrane by moving through a specific integral protein. This transport process is called facilitated diffusion (Figure 2.3c). Other integral proteins move molecules across the plasma membrane against their concentration gradient, a process called active transport, which requires the use of energy