Gradient coupling in isolated intestinal cells.

Work with isolated intestinal cells summarized below shows that: 1) An answer to the "energetic adequacy" question for Na+ -dependent transport systems cannot be provided unless solute fluxes by all routes are taken into account. 2) The conventional view of transport efficiency is in fact a measure of membrane leakiness to solute rather than a measure of innate efficiency for the Na+-dependent carrier. 3) Uncontrolled sugar-induced changes in membrane potential introduce a systematic error in measurements of the Na+: sugar coupling stoichiometry. 4) With appropriate control of the membrane potential the measured coupling ratio is 2 Na+ per sugar molecule. 5) A 2:1 coupling ratio allows for theoretical maximum sugar gradients of several hundred fold via a mechanism sharply dependent on the membrane potential.