Human Physiology, 14th edition (2016)

Interactions Between Cells and the Extracellular Environment 147

each membrane and connect to the cytoskeleton of each cell;

and (3) desmosomes, where the plasma membranes of the two

cells are “buttoned together” by interactions between particu-

lar desmosomal proteins ( fig. 6.22 ).

Although the space between the plasma membranes of

two cells joined by tight junctions appears to be occluded,

physiological evidence suggests that the barrier can be

selectively permeable. Some tight junctions are indeed

tight, but some may be leaky, selectively allowing ions and

molecules to pass. There are interconnected protein strands

between the cells in the tight junctions, and the tight junc-

tions that are leakier appear to have fewer of those strands.

The paracellular movement through tight junctions varies in

different membranes, and their leakiness may be subject to

regulation.

happens if a cell is poisoned with cyanide, so that it cannot pro-
duce ATP. After the primary active transport Na^1 /K^1 (ATPase)
pumps stop working, the concentration gradient for Na^1 grad-
ually decreases. As this occurs, the transport of glucose from
the intestinal lumen into the epithelial cells, as well as other
examples of secondary active transport, likewise declines. This
differs from passive transport, such as the facilitated diffusion
of glucose from the blood into tissue cells, which does not
depend on ATP.

Transport Across Epithelial Membranes

Epithelial membranes cover all body surfaces and line
the cavities of all hollow organs (chapter 1, section 1.3).
Therefore, in order for a molecule or ion to move from
the external environment into the blood (and from there to
the body organs), it must first pass through an epithelial
membrane. The transport of digestion products (such as
glucose) across the intestinal epithelium into the blood is
called absorption. The transport of molecules out of the
urinary filtrate (originally derived from blood) back into
the blood is called reabsorption.
The cotransport of Na^1 and glucose described in the last
section can serve as an example. The cotransport carriers for
Na^1 and glucose are located in the apical (top) plasma mem-
brane of the epithelial cells, which faces the lumen of the intes-
tine or kidney tubule. The Na^1 /K^1 pumps, and the carriers for
the facilitated diffusion of glucose, are on the basal (bottom)
plasma membrane of the epithelial cell facing the location of
blood capillaries. As a result of these active and passive trans-
port processes, glucose is moved from the lumen, through the
cell, and then to the blood ( fig.  6.21 ). Amino acids are simi-
larly transported across the epithelial lining of the small intes-
tine and kidney tubules. Some amino acids are cotransported
by a carrier that uses the Na^1 electrochemical gradient, similar
to the cotransport of glucose; however, other amino acids are
transported by a carrier that uses a proton (H^1 ) electrochemi-
cal gradient. This H^1 gradient is created by a different carrier,
a Na^1 /H^1 pump, which uses the inward movement of Na^1 to
transport H^1 out of the cell.
The membrane transport mechanisms described in this
section move materials through the cytoplasm of the epithe-
lial cells, a process termed transcellular transport. However,
diffusion and osmosis may also occur to a limited extent in
the very tiny spaces between epithelial cells, a process termed
paracellular transport.
Paracellular transport between cells is limited by the
junctional complexes that connect adjacent epithelial cells.
Junctional complexes consist of three structures: (1) tight
junctions, where the space between the two adjoining plasma
membranes appears to be occluded and strands of proteins
penetrate the plasma membranes to bridge the cytoskeleton
actin fibers in each cell; (2) adherens junctions, where the
plasma membranes of the two cells come very close together
and are “glued” by interactions between proteins that span

Figure 6.21 Transport processes involved in the

epithelial absorption of glucose. When glucose is to be

absorbed across the epithelial membranes of the kidney tubules

or the small intestine, several processes are involved. ( 1 ) Primary

active transport (the Na^1 /K^1 pumps) in the basal membrane use

ATP to maintain a low intracellular concentration of Na^1.

( 2 ) Secondary active transport uses carriers in the apical

membrane to transport glucose up its concentration gradient,

using the energy from the “downhill” flow of Na^1 into the cell.

Finally, ( 3 ) facilitated diffusion of glucose using carriers in the

basal membrane allows the glucose to leave the cells and enter

the blood.

Basolateral

surface

Apical surface

Glucose

(lower

concentration)

Na+

(higher

concentration)

Lumen of kidney tubule

or small intestine

Epithelial

cells of kidney

tubule or small

intestine

Cotransport

Glucose

(higher)

Glucose

(lower)

Primary

active

transport

Blood

Facilitated

diffusion

AT P

ADP

Na+

(lower)

Na+

Na+

K+

K+

Junctional

complex

ATP used to move both

Na+ and K+ against their

concentration gradients

Transport of Na+ down

its concentration gradient

provides energy for

glucose to be moved

against its concentration

gradient

1

2

3