116
Chapter 4
changes in the water concentration on the two sides because
a change in solute concentration, as we have seen, produces a
change in water concentration. The water concentration differ-
ence will cause water to move by osmosis from the low-sodium
side to the high-sodium side of the epithelium (
Figure 4–24
).
Thus, net movement of solute across an epithelium is accom-
panied by a fl ow of water in the same direction. If the epithe-
lial cells are highly permeable to water, large net movements of
water can occur with very small differences in osmolarity. As
you will learn in Chapter 14, this is a major way in which epi-
thelial cells of the kidney reabsorb water from the urine back
into the blood.
SUMMARY
Diffusion
I. Diffusion is the movement of molecules from one location to
another by random thermal motion.
a. The net fl ux between two compartments always proceeds
from higher to lower concentration.
b. Diffusion equilibrium is reached when the concentrations
of the diffusing substance in the two compartments become
equal.
II. The magnitude of the net fl
ux
J
across a membrane is directly
proportional to the concentration difference across the
membrane
C
o
C
i
, the surface area of the membrane A, and
the membrane permeability coeffi
cient
P
.
III. Nonpolar molecules diffuse through the lipid portions of
membranes much more rapidly than do polar or ionized
molecules because nonpolar molecules can dissolve in the lipids
in the membrane.
IV. Ions diffuse across membranes by passing through ion
channels formed by integral membrane proteins.
a. The diffusion of ions across a membrane depends on both
the concentration gradient and the membrane potential.
b. The fl ux of ions across a membrane can be altered by
opening or closing ion channels.
Mediated-Transport Systems
I. The mediated transport of molecules or ions across a membrane
involves binding the transported solute to a transporter
protein in the membrane. Changes in the conformation of the
transporter move the binding site to the opposite side of the
membrane, where the solute dissociates from the protein.
a. The binding sites on transporters exhibit chemical
specifi city, affi nity, and saturation.
b. The magnitude of the fl ux through a mediated-transport
system depends on the degree of transporter saturation, the
number of transporters in the membrane, and the rate at
which the conformational change in the transporter occurs.
II. Facilitated diffusion is a mediated-transport process that
moves molecules from higher to lower concentration
across a membrane by means of a transporter until the two
concentrations become equal. Metabolic energy is not required
for this process.
III. Active transport is a mediated-transport process that moves
molecules against an electrochemical gradient across a
membrane by means of a transporter and an input of energy.
a. Primary active transport uses the phosphorylation of the
transporter by ATP to drive the transport process.
b. Secondary active transport uses the binding of ions (often
sodium) to the transporter to drive the secondary transport
process.
c. In secondary active transport, the downhill fl ow of an ion
is linked to the uphill movement of a second solute either
in the same direction as the ion (cotransport) or in the
opposite direction of the ion (countertransport).
Osmosis
I. Water crosses membranes by (1) diffusing through the lipid
bilayer, and (2) diffusing through protein channels in the
membrane.
II. Osmosis is the diffusion of water across a membrane from a
region of higher water concentration to a region of lower water
concentration. The osmolarity—total solute concentration in a
solution—determines the water concentration: The higher the
osmolarity of a solution, the lower the water concentration.
III. Osmosis across a membrane that is permeable to water but
impermeable to solute leads to an increase in the volume of
the compartment on the side that initially had the higher
osmolarity, and a decrease in the volume on the side that
initially had the lower osmolarity.
IV. Application of suffi cient pressure to a solution will prevent the
osmotic fl ow of water into the solution from a compartment of
pure water. This pressure is called the osmotic pressure. The
greater the osmolarity of a solution, the greater its osmotic
pressure. Net water movement occurs from a region of lower
osmotic pressure to one of higher osmotic pressure.
Figure 4–24
Net movements of water across an epithelium are dependent on
net solute movements. The active transport of sodium across the
cells, into the surrounding interstitial spaces, produces an elevated
osmolarity in this region and a decreased osmolarity in the lumen.
This leads to the osmotic fl ow of water across the epithelium in
the same direction as the net solute movement. The water diffuses
through water channels in the membrane and across the tight
junctions between the epithelial cells.
Na
+
H
2
O
H
2
O
H
2
O
H
2
O
H
2
O
H
2
O
Na
+
H
2
OH
2
O
Tight junction
Tight junction
Epithelial cell
Lumen side
Blood side
3Na
+
2K
+
ADP
ATP
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