Figure

Model of mediated transport. A change in the conformation of the transporter exposes the transporter binding site first to one surface of the membrane then to the other, thereby transferring the bound solute from one side of the membrane to the other. This model shows net mediated transport from the extracellular fluid to the inside of the cell. In many cases, the net transport is in the opposite direction.

There are many types of transporters in membranes, each type having binding sites that are specific for a particular substance or a specific class of related substances. For example, although both amino acids and sugars undergo mediated transport, a protein that transports amino acids does not transport sugars, and vice versa. Just as with ion channels, the plasma membranes of different cells contain different types and numbers of transporters and thus exhibit differences in the types of substances transported and their rates of transport.

Three factors determine the magnitude of the solute flux through a mediated-transport system: (1) the extent to which the transporter binding sites are saturated, which depends on both the solute concentration and the affinity of the transporters for the solute, (2) the number of transporters in the membrane—the greater the number of transporters, the greater the flux at any level of saturation, and (3) the rate at which the conformational change in the transport protein occurs. The flux through a mediatedtransport system can be altered by changing any of these three factors.

For any transported solute there is a finite number of specific transporters in a given membrane at any particular moment. As with any binding site, as the concentration of the ligand (the solute to be transported, in this case) is increased, the number of occupied binding sites increases until the transporters become saturated—that is, until all the binding sites become occupied. When the transporter binding sites are saturated, the maximal flux across the membrane has been reached, and no further increase in solute flux will occur with increases in solute concentration. Contrast the solute flux resulting from mediated transport with the flux produced by diffusion through the lipid portion of a membrane (Figure 6-9). The flux due to diffusion increases in direct proportion to the increase in extracellular concentration, and there is no limit

FIGURE 6-9

The flux of molecules diffusing into a cell across the lipid bilayer of a plasma membrane (blue line) increases continuously in proportion to the extracellular concentration, whereas the flux of molecules through a mediated-transport system (orange line) reaches a maximal value.

FIGURE 6-9

The flux of molecules diffusing into a cell across the lipid bilayer of a plasma membrane (blue line) increases continuously in proportion to the extracellular concentration, whereas the flux of molecules through a mediated-transport system (orange line) reaches a maximal value.

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART ONE Basic Cell Functions since diffusion does not involve binding to a fixed number of sites. (At very high ion concentrations, however, diffusion through ion channels may approach a limiting value because of the fixed number of channels available just as there is an upper limit to the rate at which a crowd of people can pass through a single open doorway.)

When the transporters are saturated, the maximal transport flux depends upon the rate at which the con-formational changes in the transporters can transfer their binding sites from one surface to the other. This rate is much slower than the rate of ion diffusion through ion channels.

Thus far we have described mediated transport as though all transporters had similar properties. In fact, two types of mediated transport can be distinguished—facilitated diffusion and active transport. Facilitated diffusion uses a transporter to move solute downhill from a higher to a lower concentration across a membrane (as in Figure 6-8), whereas active transport uses a transporter that is coupled to an energy source to move solute uphill across a membrane—that is, against its electrochemical gradient.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook


Responses

  • veli
    Why does an increased number of transporters increase the rate of diffusion?
    6 years ago

Post a comment