Human Physiology, 14th edition (2016)

Interactions Between Cells and the Extracellular Environment 143

saturation. The diffusion of glucose through a plasma mem-

brane must therefore be mediated by carrier proteins. In the

conceptual model shown in figure  6.16 , the carrier protein

has a site that can bind specifically to glucose, and such bind-

ing causes a conformational change in the carrier so that a

pathway is formed through the membrane. As a result, glu-

cose is allowed to diffuse down its concentration gradient

into the cell.

Like the isoenzymes described in chapter 4, carrier pro-

teins that do the same job may exist in various tissues in slightly

different forms. The transport carriers for the facilitative diffu-

sion of glucose are designated with the letters GLUT, followed

by a number for the isoform. For example, the GLUT3 isoform

is the major glucose transporter in neurons, but GLUT1 is also

present in the central nervous system and is increased under

certain conditions. The pancreatic beta cells, which secrete

insulin, and the hepatocytes of the liver produce GLUT2. The

GLUT2 transporters allow an exceptionally high rate of glu-

cose transport into these cells from the external environment.

GLUT4 is present in adipose tissue and skeletal muscles, and

the insertion of GLUT4 carriers into the plasma membrane of

adipocytes and skeletal muscle fibers is regulated by exercise

Facilitated Diffusion

The transport of glucose from the blood across plasma mem-
branes occurs by facilitated diffusion. Facilitated diffusion,
like simple diffusion, is powered by the thermal energy of the
diffusing molecules and involves net transport from the side of
higher to the side of lower concentration. ATP is not required
for either facilitated or simple diffusion.
Unlike simple diffusion of nonpolar molecules, water,
and inorganic ions through a membrane, the diffusion of glu-
cose through the plasma membrane displays the properties
of carrier-mediated transport: specificity, competition, and

Figure 6.15 Characteristics of carrier-mediated
transport. Carrier-mediated transport displays the
characteristics of saturation (illustrated by the transport
maximum) and competition. Since molecules X and Y compete
for the same carrier, the rate of transport of each is lower when
they are both present than when either is present
alone.

Transport maximum

(Tm) Saturation

Saturation

Concentration of X

Molecule

X

Molecules

X + Y

Rate of transport of

X

CLINICAL APPLICATION

In diabetes mellitus, caused by the inadequate secretion

and/or action of insulin, the person has fasting hypergly-

cemia (high plasma glucose). Normally the glucose that

gets filtered out of the plasma when the kidneys form urine

enters the renal tubules, but then nearly all of it is returned

to the blood—a process called reabsorption —by membrane

transport carrier proteins. As a result, there is normally little

or no glucose in the urine. In the hyperglycemia of diabe-

tes, however, the carriers become saturated; their transport

maximum is exceeded. When this occurs, the untransported

glucose continues through the renal tubules and appears in

the urine, a condition called glycosuria.

Figure 6.16 A model of the facilitated diffusion

of glucose. A carrier—with characteristics of specificity and

saturation—is required for this transport, which occurs from

the blood into cells such as muscle, liver, and fat cells. This is

passive transport because the net movement is to the region of

lower concentrations, and ATP is not required.

Outside of cell

Higher concentration

Inside of cell

Lower concentration

Glucose

Membrane

Carrier

protein