Since the plasma (cell) membrane consists primarily of a double layer of phospholipids, molecules that are nonpolar, and thus lipid-soluble, can easily pass from one side of the membrane to the other. The plasma membrane, in other words, does not present a barrier to the diffusion of nonpolar molecules such as oxygen gas (O2) or steroid hormones. Small molecules that have polar covalent bonds, but which are uncharged, such as CO2 (as well as ethanol and urea), are also able to penetrate the phospholipid bilayer. Net diffusion of these molecules can thus easily occur between the intracellular and extracellular compartments when concentration gradients exist.
The oxygen concentration is relatively high, for example, in the extracellular fluid because oxygen is carried from the lungs to the body tissues by the blood. Since oxygen is combined with hydrogen to form water in aerobic cell respiration, the oxygen concentration within the cells is lower than in the extracellular fluid. The concentration gradient for carbon dioxide is in the opposite direction because cells produce CO2. Gas exchange thus
Interactions Between Cells and the Extracellular Environment 129
■ Figure 6.3 Gas exchange occurs by diffusion. The colored dots, which represent oxygen and carbon dioxide molecules, indicate relative concentrations inside the cell and in the extracellular environment. Gas exchange between the intracellular and extracellular compartments thus occurs by diffusion.
osmosis is the simple diffusion of solvent instead of solute, a unique terminology (discussed shortly) is used to describe it.
Larger polar molecules, such as glucose, cannot pass through the double layer of phospholipid molecules and thus require special carrier proteins in the membrane for transport. The phospholipid portion of the membrane is similarly impermeable to charged inorganic ions, such as Na+ and K+. However, tiny ion channels through the membrane, which are too small to be seen even with an electron microscope, permit passage of these ions. The ion channels are provided by some of the proteins that span the thickness of the membrane (fig. 6.4).
Some ion channels are always open, so that diffusion of the ion through the plasma membrane is an ongoing process. Many ion channels, however, are gated—they have structures ("gates") that can open or close the channel (fig. 6.4). In this way, particular physiological stimuli (such as binding of the channel to a specific chemical regulator) can open an otherwise closed channel. In the production of nerve and muscle impulses, specific channels for Na+ and others for K+ open and close in response to membrane voltage (discussed in chapter 7).
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Cystic fibrosis occurs about once in every 2,500 births in the Caucasian population. As a result of a genetic defect abnormal NaCI and water movement occurs across wet epithelial membranes. Where such membranes line the pancreatic ductules and small respiratory airways, they produce a dense, viscous mucus that cannot be properly cleared, which may lead to pancreatic and pulmonary disorders. The genetic defect involves a particular glycoprotein that forms chloride (Cl-) channels in the apical membrane of the epithelial cells. This protein, known as CFTR (for cystic fibrosis transmembrane conductance regulator), is formed in the usual manner in the endoplasmic reticulum. It does not move into the Golgi complex for processing, however, and therefore, it doesn't get correctly processed and inserted into vesicles that would introduce it into the cell membrane (chapter 3). The gene for CFTR has been identified and cloned. More research is required, however, before gene therapy for cystic fibrosis becomes an effective therapy.
■ Figure 6.4 Ions pass through membrane channels. These channels are composed of integral proteins that span the thickness of the membrane. Although some channels are always open, many others have structures known as "gates" than can open or close the channel. This figure depicts a generalized ion channel; most, however, are relatively selective—they allow only particular ions to pass.
occurs by diffusion between the cells and their extracellular environments (fig. 6.3).
Although water is not lipid-soluble, water molecules can diffuse through the plasma membrane to a limited degree because of their small size and lack of net charge. In certain membranes, however, the passage of water is aided by specific channels that are inserted into the membrane in response to physiological regulation. The net diffusion of water molecules (the solvent) across the membrane is known as osmosis. Since
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