Movement of Molecules Across Cell Membranes
103
plasma membranes of different cells contain different types
and numbers of transporters, and thus they exhibit differences
in the types of substances transported and in their rates of
transport.
Three factors determine the magnitude of solute fl
ux
through a mediated-transport system. The fi rst of these is the
extent to which the transporter binding sites are saturated,
which depends on both the solute concentration and the
affi nity of the transporters for the solute. Second, the number
of transporters in the membrane determines the fl
ux at any
level of saturation. The third factor is the rate at which the
conformational change in the transport protein occurs. The
fl ux through a mediated-transport system can be altered by
changing any of these three factors.
For any transported solute there is a fi nite number of
specifi c transporters in a given membrane at any particular
moment. As with any binding site, as the concentration of
the solute to be transported is increased, the number of occu-
pied binding sites increases until the transporters become
saturated—that is, until all the binding sites are occupied.
When the transporter binding sites are saturated, the maxi-
mal fl ux across the membrane has been reached, and no fur-
ther increase in solute fl
ux will occur with increases in solute
concentration. Contrast the solute fl ux resulting from medi-
ated transport with the fl ux produced by diffusion through
the lipid portion of a membrane (
Figure 4–9
). The fl
ux due
to diffusion increases in direct proportion to the increase in
extracellular concentration, and there is no limit because dif-
fusion does not involve binding to a fi xed number of sites. (At
very high ion concentrations, however, diffusion through ion
channels may approach a limiting value because of the fi xed
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 transporters are saturated, however, the maximal
transport fl ux depends upon the rate at which the conforma-
tional 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
de
sc
r
ibed
med
ia
ted
tran
spo
r
t
as
though all transporters had similar properties. In fact, two
types of mediated transport exist—
facilitated diffusion
and
Figure 4–8
Model of mediated transport. A change in the conformation of the transporter exposes the transporter binding site fi rst 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 fl uid to the inside of the cell. In many cases, the net transport is in the opposite direction. The size of
the conformational change is exaggerated for illustrative purposes in this and subsequent fi
gures.
Intracellular fluid
Extracellular fluid
Binding site
Transporter
protein
Transported solute
Flux into cell
Extracellular solute concentration
Diffusion
Maximal flux
Mediated transport
Figure 4–9
The fl ux of molecules diffusing into a cell across the lipid bilayer of a
plasma membrane (green line) increases continuously in proportion
to the extracellular concentration, whereas the fl ux of molecules
through a mediated transport system (purple line) reaches a
maximal value.
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