110
Chapter 4
compartments will be equal, and thus the solute concentra-
tions must also be equal. To reach this state of equilibrium,
enough water must pass from compartment 1 to 2 to increase
the volume of compartment 2 by one-third and decrease the
volume of compartment 1 by an equal amount. Note that it is
the presence of a membrane impermeable to solute that leads
to the volume changes associated with osmosis.
The two compartments in our example were treated as if
they were infi nitely expandable, so that the net transfer of water
does not create a pressure difference across the membrane. In
contrast, if the walls of compartment 2 in Figure 4–18 had only
a limited capacity to expand, as occurs across plasma mem-
branes, the movement of water into compartment 2 would
raise the pressure in compartment 2, which would oppose fur-
ther net water entry. Thus the movement of water into com-
partment 2 can be prevented by the application of pressure to
compartment 2. This leads to an important defi nition: When
a solution containing solutes is separated from pure water by a
semipermeable membrane
(a membrane permeable to water
but not to solutes), the pressure that must be applied to the
solution to prevent the net fl
ow of water into it is termed the
osmotic pressure
of the solution. The greater the osmolarity
of a solution, the greater its osmotic pressure. It is important
to recognize that the osmotic pressure of a solution does not
push water molecules into the solution. Rather, it represents
the amount of pressure that would have to be applied to the
solution to
prevent
the net fl ow of water into the solution. Like
osmolarity, the osmotic pressure of a solution is a measure of
the solution’s water concentration—the lower the water con-
centration, the higher the osmotic pressure.
Extracellular Osmolarity and Cell Volume
We can now apply the principles learned about osmosis to cells,
which meet all the criteria necessary to produce an osmotic
fl ow of water across a membrane. Both the intracellular and
extracellular fl uids contain water, and cells are surrounded by
a membrane that is very permeable to water but impermeable
to many substances
(nonpenetrating solutes).
About 85 percent of the extracellular solute particles
is sodium and chloride ions, which can diffuse into the cell
through ion channels in the plasma membrane or enter the
cell during secondary active transport. As we have seen, how-
ever, the plasma membrane contains Na
+
/K
+
-ATPase pumps
that actively move sodium ions out of the cell. Thus, sodium
moves into cells and is pumped back out, behaving as if it never
Figure 4–18
The movement of water across a membrane that is permeable to
water but not to solute leads to an equilibrium state involving a
change in the volumes of the two compartments. In this case, a
net diffusion of water (0.33 L) occurs from compartment 1 to 2.
(We will assume that the membrane in this example stretches as the
volume of compartment 2 increases so that no signifi cant change in
compartment pressure occurs.)
12
2 Osm
53.5 M
1 L
4 Osm
51.5 M
1 L
Initial
Equilibrium
3 Osm
52.5 M
1 L
3 Osm
52.5 M
1 L
Water
Solute
Solute
Water
volume
Solute
Water
volume
Figure 4–17
Between two compartments of equal volume, the net diffusion of
water and solute across a membrane permeable to both leads to
diffusion equilibrium of both, with no change in the volume of
either compartment. (For clarity’s sake, not all water molecules are
shown in this fi
gure or in Figure 4–18.)
Solute
Water
volume
Solute
Water
volume
2 Osm
53.5 M
1 L
4 Osm
51.5 M
1 L
Initial
Equilibrium
3 Osm
52.5 M
0.67 L
3 Osm
52.5 M
1.33 L
Water
12
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