Figure 1.

Basic mechanisms of pump and channel function. (a) Alternating access model of pump function. A conformational change in the membrane-spanning parts of the protein (grey) results in substrate (red) being accessible to the intracellular side of the membrane in one conformation (inward-facing), and to the extracellular side of the membrane in the other conformation (outward-facing). (b) This same mechanism can be expressed in terms of outer and inner gates that are never open at the same time (outer gate closed in the inward-facing state, inner gate closed in the outward-facing state). (c) Ion channels can operate with a single gate, which is closed in the channel closed state and open in the channel open state, allowing a continuous aqueous pathway for ion electrodiffusion across the membrane.