ATP-dependent transport systems for organic anions

Biologcal membranes evolved because of the need to define "self" as opposed to "the outside." Living systems cannot, however, function in isolation from their surroundings, and massive amounts of material continually cross membranes. Much of this material is organic: nutrients are taken up by cells, waste products are excreted and metabolites enter and leave numerous cell organelles and intracellular compartments. Except for very hydrophobic compounds, transport is mediated by proteins. In some cases, facilitated (i.e., transportermediated) diffusion of the solute is adequate; for example, this is the case in the uptake of glucose by erythrocytes. The concentration of glucose in serum is high and relatively constant over time, and the modest requirements of the erythrocyte are covered predictably and comfortably by glucose flowing in down its concentration gradient. The situation is quite different in the case of some other tissues (e.g., the intestinal epithelium). These cells are exposed to varying but usually low glucose concentrations in the intestinal lumen, and their function is to take up the nutrient efficiently. In this example, as in many other cases, the uptake is active (i.e., it proceeds against the concentration [or, more generally, electrochemical1 gradient of the solute and requires the expenditure of free energy).