102
Chapter 4
Mediated-Transport Systems
Although diffusion through channels accounts for some of
the transmembrane movement of ions, it does not account for
all. Moreover, a number of other molecules, including amino
acids and glucose, 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 car-
riers). The movement of substances through a membrane by
these
mediated-transport
systems depends on conforma-
tional changes in these transporters.
The transported solute must fi rst bind to a specifi c site
on a transporter, a site exposed to the solute on one surface
of the membrane (
Figure 4–8
). A portion of the transporter
then undergoes a change in shape, exposing this same bind-
ing site to the solution on the opposite side of the membrane.
The dissociation of the substance from the transporter bind-
ing 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 4–8
is only a model, because the specifi c conformational changes
of any transport protein are still uncertain.
Many of the characteristics of transporters and ion chan-
nels are similar. Both involve membrane proteins and show
chemical specifi city. They do, however, differ in the number
of molecules or ions crossing the membrane by way of these
membrane proteins. Ion channels typically move several thou-
sand times more ions per unit time than do transporters. In
part, this is because a transporter must change its shape for
each molecule transported across the membrane, while an
open ion channel can support a continuous fl ow of ions with-
out a change in conformation.
There are many types of transporters in membranes,
each type having binding sites that are specifi c for a particular
substance or a specifi c class of related substances. For exam-
ple, although both amino acids and sugars undergo medi-
ated transport, a protein that transports amino acids does not
transport sugars, and vice versa. Just as with ion channels, the
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Extracellular fluid
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Plasma
membrane
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Lipid bilayer
Open ion channel
Closed ion channel
Intracellular fluid
Extracellular fluid
Channel proteins
Figure 4–6
The separation of electrical charge across a plasma membrane (the
membrane potential) provides the electrical force that drives positive
ions into a cell and negative ions out.
Figure 4–7
As a result of conformational changes in the proteins forming an ion channel, the channel may be open, allowing ions to diffuse across the
membrane, or may be closed. The conformational change is grossly exaggerated for illustrative purposes. The actual conformational change is
more likely to be just suffi cient to allow or prevent an ion to fi t through.
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