Although diffusion through channels accounts for some of the transmembrane movement of ions, it does not account for all. Moreover, there are a number of other molecules, including amino acids and glucose, that are able to cross membranes yet are too polar to diffuse through the lipid bilayer and too large to diffuse through ion channels. The passage of these molecules and the nondiffusional movements of ions are mediated by integral membrane proteins known as transporters (or carriers). Movement of substances through a membrane by these mediated-transport systems depends on conformational changes in these transporters.
The transported solute must first bind to a specific site on a transporter (Figure 6-8), a site that is exposed to the solute on one surface of the membrane. A portion of the transporter then undergoes a change in shape, exposing this same binding site to the solution on the opposite side of the membrane. The dissociation of the substance from the transporter binding site completes the process of moving the material through the membrane. Using this mechanism, molecules can move in either direction, getting on the transporter on one side and off at the other.
The diagram of the transporter in Figure 6-8 is only a model, since we have little information concerning the specific conformational changes of any transport protein. It is assumed that the changes in the shape of transporters are analogous to those undergone by channel proteins that open and close. The oscillations in conformation are presumed to occur continuously whether or not solute is bound to the transport protein. When solute is bound, it is transferred across the membrane, but the binding of the solute is not necessary to trigger the conformational change.
Many of the characteristics of transporters and ion channels are similar. Both involve membrane proteins and show chemical specificity. They do, however, differ in the number of molecules (or ions) crossing the membrane by way of these membrane proteins in that ion channels typically move several thousand times more ions per unit time than do transporters. In part, this reflects the fact that for each molecule transported across the membrane, a transporter must change its shape, while an open ion channel can support a continuous flow of ions without a change in conformation.
Vander et al.: Human I I. Basic Cell Functions I 6. Movement of Molecules I I © The McGraw-Hill
Physiology: The Across Cell Membranes Companies, 2001
Mechanism of Body Function, Eighth Edition
Movement of Molecules Across Cell Membranes CHAPTER SIX
Intracellular fluid m
Transporter Q protein
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