Chapter 3
Cells: The Living Units
71
3
However, such major changes in hydrostatic (and osmotic)
pressures do not occur in living animal cells, which lack rigid
cell walls. Osmotic imbalances cause animal cells to swell or
shrink (due to net water gain or loss) until either (1) the solute
concentration is the same on both sides of the plasma mem-
brane, or (2) the membrane stretches to its breaking point.
Tonicity
Such changes in animal cells lead us to the important
concept of
tonicity
(to-nis
9
ĭ-te). As noted, many solutes, par-
ticularly intracellular proteins and selected ions, cannot diffuse
through the plasma membrane. Consequently, any change in
their concentration alters the water concentration on the two
sides of the membrane and results in a net loss or gain of water
by the cell.
Tonicity
refers to the ability of a solution to change the shape
or tone of cells by altering the cells’ internal water volume (
tono
5
tension).
Isotonic
(“the same tonicity”)
solutions
have the same con-
centrations of nonpenetrating solutes as those found in cells
(0.9% saline or 5% glucose). Cells exposed to isotonic solu-
tions retain their normal shape, and exhibit no net loss or
gain of water
(Figure 3.9a)
. As you might expect, the body’s
extracellular fluids and most intravenous solutions (solu-
tions infused into the body via a vein) are isotonic.
Hypertonic solutions
have a higher concentration of non-
penetrating solutes than seen in the cell (for example, a strong
saline solution). Cells immersed in hypertonic solutions lose
water and shrink, or
crenate
(kre
9
nat) (Figure 3.9b).
and water occurs, each moving down its own concentration
gradient. Equilibrium is reached when the water (and solute)
concentration on both sides of the membrane is the same
(Figure 3.8a)
.
If we consider the same system, but make the membrane
imper-
meable to solute particles
, we see quite a different result (Figure 3.8b).
Water quickly diffuses from the le± to the right compartment and
continues to do so until its concentration is the same on the two
sides of the membrane. Notice that in this case equilibrium results
from the movement of water alone (the solutes are prevented from
moving). Notice also that the movement of water leads to dramatic
changes in the volumes of the two compartments.
Te last situation mimics osmosis across plasma membranes
of living cells, with one major difference. In our examples, the
volumes of the compartments are infinitely expandable and the
effect of pressure exerted by the added weight of the higher fluid
column is not considered. In living plant cells, which have rigid
cell walls external to their plasma membranes, this is not the
case. As water diffuses into the cell, the point is finally reached
where the
hydrostatic pressure
(the back pressure exerted by
water against the membrane) within the cell is equal to its
os-
motic pressure
(the tendency of water to move into the cell by
osmosis). At this point, there is no further (net) water entry. As a
rule, the higher the amount of nondiffusible, or
nonpenetrating
,
solutes in a cell, the higher the osmotic pressure and the greater
the hydrostatic pressure must be to resist further net water entry.
In our plant cell, hydrostatic pressure is pushing water out, and
osmotic pressure is pulling water in; therefore, you could think
of the osmotic pressure as an osmotic “suck.”
(a)
Isotonic solutions
Cells retain their normal size and
shape in isotonic solutions (same
solute/water concentration as inside
cells; water moves in and out).
(b)
Hypertonic solutions
Cells lose water by osmosis and shrink
in a hypertonic solution (contains a
higher concentration of solutes
than are present inside the cells).
(c)
Hypotonic solutions
Cells take on water by osmosis until they
become bloated and burst (lyse) in a
hypotonic solution (contains a lower
concentration of solutes than are
present inside cells).
Figure 3.9
The effect of solutions of varying tonicities on living red blood cells.
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