92
UNIT ONE
stances across the cell membrane. Consider oxygen and car-
bon dioxide. Cell membranes are permeable to both. In the
body, oxygen diffuses into cells and carbon dioxide diffuses
out of cells, but equilibrium is never reached. Intracellular
oxygen is always low because oxygen is constantly used up
in metabolic reactions. Extracellular oxygen is maintained at
a high level by homeostatic mechanisms in the respiratory
and cardiovascular systems. Thus, a concentration gradient
always allows oxygen to diffuse into cells.
The level of carbon dioxide, produced as a waste prod-
uct of metabolism, is always high inside cells. Homeostasis
maintains a lower extracellular carbon dioxide level, so a
concentration gradient always favors carbon dioxide diffus-
ing out of cells
(f
g. 3.22)
.
Diffusional equilibrium does not normally occur in
organisms. The term
physiological steady state,
where con-
centrations of diffusing substances are unequal but stable, is
more appropriate.
A number of factors influence the diffusion rate, but
those most important in the body are distance, the concen-
tration gradient, and temperature. In general, diffusion is
more rapid over shorter distances, larger concentration gra-
dients, and at higher temperatures. Homeostasis maintains
all three of these factors at optimum levels.
Facilitated Diffusion
Some of the previous examples considered hypothetical mem-
branes with specific permeabilities. For the cell membrane,
permeability is more complex because of its selective nature.
Lipid-soluble substances, such as oxygen, carbon dioxide, ste-
roids, and general anesthetics, freely cross the cell membrane
by simple diffusion. Small solutes that are not lipid-soluble,
such as ions of sodium, potassium, and chloride, may diffuse
through protein channels in the membrane, described earlier.
(Water molecules also diffuse through similar channels, called
pores.) This type of movement follows the concentration
another particle and bounces off. Then it moves in its new
direction until it collides again and changes direction once
more. Collisions are less likely if there are fewer particles,
so there is a net movement of particles from an area of
higher concentration to an area of lower concentration.
This difference in concentrations is called a
concentration
gradient,
and atoms, molecules, and ions are said to diffuse
down a concentration gradient. With time, the concentra-
tion of a given substance becomes uniform throughout
a solution. This is the condition of
diffusional equilibrium
(dı˘-fu
zhun-ul e
kwi-lib
re-um). At diffusional equilibrium,
although random movements continue, there is no further
net movement, and the concentration of a substance is uni-
form throughout the solution.
Random motion mixes molecules. At body temperature, small mol-
ecules such as water move more than a thousand miles per hour.
However, the internal environment is crowded from a molecule’s
point of view. A single molecule may collide with other molecules a
million times each second.
Sugar (a solute) put into a glass of water (a solvent), can
be used to illustrate diffusion
(f g. 3.20)
. The sugar at ± rst
remains in high concentration at the bottom of the glass. As
the sugar molecules move, they may collide or miss each
other. They are less likely to collide where there are fewer
sugar molecules, so sugar molecules gradually diffuse from
areas of higher concentration to areas of lower concentration
(
down
the concentration gradient), and eventually become
uniformly distributed in the water.
Diffusion of a substance across a membrane can occur
only if (1) the cell membrane is permeable to that substance
and (2) a concentration gradient exists such that the substance
is at a higher concentration on one side of the membrane or
the other
(f g. 3.21)
. This principle applies to diffusion of sub-
(1)
(2)
(3)
Time
(4)
FIGURE 3.20
A dissolving sugar cube illustrates diF
usion. (
1–3
) A sugar cube placed in water slowly disappears as the sugar molecules dissolve
and then diF
use from regions where they are more concentrated toward regions where they are less concentrated. (
4
) Eventually, the sugar
molecules are distributed evenly throughout the water.
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