104
Chapter 4
active transport.
Facilitated diffusion uses a transporter to
move solute “downhill” from a higher to a lower concentra-
tion across a membrane, as in Figure 4–8. Active transport
uses a transporter coupled to an energy source to move solute
“uphill” across a membrane—that is,
against
its electrochemi-
cal gradient.
Facilitated Diffusion
As in ordinary diffusion, in facilitated diffusion the net fl
ux of
a molecule across a membrane always proceeds from higher to
lower concentration and continues until the concentrations on
the two sides of the membrane become equal. At this point,
equal numbers of molecules are binding to the transporter at
the outer surface of the cell and moving into the cell as are
binding at the inner surface and moving out. Neither diffusion
nor facilitated diffusion is coupled to energy (ATP) derived
from metabolism. Thus, they are incapable of moving solute
from a lower to a higher concentration across a membrane.
Among the most important facilitated-diffusion sys-
tems in the body are those that move glucose across plasma
membranes. Without such glucose transporters, cells would
be virtually impermeable to glucose, a relatively large, polar
molecule. It might be expected that as a result of facilitated
diffusion the glucose concentration inside cells would become
equal to the extracellular concentration. This does not occur
in most ce
lls, however
, because glucose is metabolized to
glucose 6-phosphate almost as quickly as it enters. Thus, the
intracellular glucose concentration remains lower than the
extracellular concentration, and there is a continuous net fl
ux
of glucose into cells.
Several distinct transporters are known to mediate the
facilitated diffusion of glucose across cell membranes. Each
transporter is coded by a different gene, and these genes are
expressed in different types of cells. The transporters differ
in the affi nity of their binding sites for glucose, their maxi-
mal rates of transport when saturated, and the modulation of
their transport activity by various chemical signals, such as the
hormone insulin. As you will learn in Chapter 16, although
glucose enters all cells by means of glucose transporters,
insulin primarily affects only the type of glucose transporter
expressed in muscle and adipose tissue. Insulin increases the
number of these glucose transporters in the membrane and,
therefore, the rate of glucose movement into cells. When insu-
lin is not available, as in one type of the disease
diabetes mel-
litus,
muscle and adipose cells cannot effi ciently transport
glucose across their membranes. This contributes to the accu-
mulation of glucose in the extracellular fl uid that is a hallmark
of the disease.
Active Transport
Active transport differs from facilitated diffusion in that it uses
energy to move a substance
uphill
across a membrane—that is,
against the substance’s electrochemical gradient (
Figure 4–10
).
As with facilitated diffusion, active transport requires a sub-
stance to bind to the transporter in the membrane. Because
these transporters move the substance
uphill,
they are often
referred to as pumps. As with facilitated-diffusion trans-
porters, active-transport transporters exhibit specifi
city and
saturation—that is, the fl ux via the transporter is maximal
when all transporter binding sites are occupied.
The net movement from lower to higher concentra-
tion and the maintenance of a higher steady-state concentra-
tion on one side of a membrane can be achieved only by the
continuous input of energy into the active-transport process.
Therefore, active transport must be coupled to the simultane-
ous fl
ow of some energy source from a higher energy level to
a lower energy level. Two means of coupling an energy fl ow to
transporters are known: (1) the direct use of ATP in
primary
active transport,
and (2) the use of an electrochemical gra-
dient across a membrane to drive the process in
secondary
active transport.
Primary Active Transport
The hydrolysis of ATP by a transporter provides the energy
for primary active transport. The transporter is actually an
enzyme called an ATPase that catalyzes the breakdown of ATP
and, in the process, phosphorylates itself. Phosphorylation of
the transporter protein is a type of covalent modulation that
changes the conformation of the transporter and the affi nity
of the transporter’s solute binding site.
One of the best studied examples of primary active trans-
port is the movement of sodium and potassium ions across
Low concentration
High concentration
Diffusion
Membrane
Facilitated diffusion
Active transport
Figure 4–10
Direction of net solute fl ux crossing a membrane by: diffusion
(high to low concentration), facilitated diffusion (high to low
concentration), and active transport (low to high concentration).
The colored circles represent transporter molecules.
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