transport

transport In a physiological sense transport generally means the movement of substances across the membranes of cells. This is an important process as, without transport, products of digestion would be unable to move from the alimentary tract into the body. Clearly the bounding membranes of cells cannot be generally permeable to all bodily substances, otherwise important cellular components would be able to leak out. Some substances, such as weak acids or bases in their undissociated form, are soluble in lipids and will dissolve in the lipid bilayer of the cell membrane. Later the substances may dissociate from the membrane, but statistically more molecules will move by diffusion across the cell membrane from a high concentration to a lower one, than in the reverse direction. This process is known as non-ionic diffusion. However many important substances, such as sugars, amino acids, and ions, are completely insoluble in cell membranes, and cross them by specialized processes. Many cell membranes contain a number of specialized molecules which combine specifically with one of the substances to be transported. Such a molecule is called a carrier, and the complex resulting from the combination can cross the membrane and release the substrate. As with non-ionic diffusion, more carrier–substrate complexes cross the membrane in a direction such that the substrate moves from high to low concentration. This carrier-mediated transport process is known as facilitated diffusion.

Neither non-ionic diffusion nor carrier-mediated diffusion require the expenditure of energy, relying simply on the concentration gradients existing across the cell membranes. However, some transport processes require the ‘uphill’ movement of substances. An example here will be useful, by considering how the body maintains a constant internal environment. We take a small amount of salt (sodium chloride) in the diet to replace that lost in the urine, sweat, saliva, and other secretions. To move salt from a low concentration in the gut, into the blood where it is at high concentration, means that the movement is up a concentration gradient, and therefore cannot occur by diffusion. The body deals with this by using a two-stage process in which sodium ions are actively transported. The first stage is the movement of sodium ions from the gut cavity across the face of the cells lining the gut; since the concentration of sodium ions inside these cells, as in all cells, is low, movement is by diffusion using specific sodium ion channels. The second stage is the movement of the sodium ions from these lining cells, across the membrane on their opposite face, away from the gut, into the tissue fluid, where the sodium ion concentration is high. This is achieved using a molecular pump, called the sodium pump (otherwise known as sodium– potassium ATPase: a protein molecule that spans the cell membrane). The pump causes a net movement of sodium ions, along with the expenditure of energy, yielded by the hydrolysis of ATP. This transfer of sodium ions across the gut epithelium results in the transfer of positive charge to the outer side of the cells. Because the pump transfers electrical charge in this way, it is said to be electrogenic. The transfer of positive charges provides the driving force for the movement of the negatively-charged chloride ions across the gut lining; thus the transfer of salt is achieved.

Similar two-stage active transport processes are responsible for the absorption or secretion of other salts, as well as sodium chloride, across many epithelial membranes. They occur in glands (such as salivary glands, the pancreas, and sweat glands) in organs such as the kidneys and the liver, as well as in epithelial membranes over the cornea and covering the brain.

Transport processes are also involved in other homeostatic processes, such as the regulation of cellular pH. Here carrier-mediated processes are used which, for instance, exchange a sodium ion for a proton (hydrogen ion) or exchange a chloride anion for a bicarbonate anion. These carriers are said to facilitate exchange-diffusion. As well as the sodium pump described above there are other molecular pumps which consume energy (obtained by the hydrolysis of ATP); for example, the calcium pump maintains low levels of calcium ions inside cells, and the proton pump is involved in generating the hydrochloric acid secreted into the stomach.

Although we refer to ‘the sodium pump’ and others in the singular, a single cell may have for example, hundreds of thousands of sodium pumps, with the number varying to suit local conditions. The body's energy requirement for these active transport processes accounts for at least a fifth of the metabolic rate of the whole body at rest.

Thus carriers, exchangers, pumps, and ion channels are the molecular machines which drive the body's transport processes.

Alan W. Cuthbert

See also cell membranes; diffusion; ion channels.