Movement of Molecules Across Cell Membranes
97
Diffusion
The molecules of any substance, be it solid, liquid, or gas, are in
a continuous state of movement or vibration, and the warmer a
substance is, the faster its molecules move. The average speed
of this “thermal motion” also depends upon the mass of the
molecule. At body temperature, a molecule of water moves at
about 2500 km/h (1500 mi/h), whereas a molecule of glu-
cose, which is 10 times heavier, moves at about 850 km/h. In
solutions, such rapidly moving molecules cannot travel very
far before colliding with other molecules, undergoing millions
of collisions every second. Each collision alters the direction
of the molecule’s movement, so that the path of any one mol-
ecule becomes unpredictable. Because a molecule may at any
instant be moving in any direction, such movement is random,
with no preferred direction of movement.
The random thermal motion of molecules in a liquid or
gas will eventually distribute them uniformly throughout a con-
tainer. Thus, if we start with a solution in which a solute is more
concentrated in one region than another (
Figure 4–1a
), ran-
dom thermal motion will redistribute the solute from regions
of higher concentration to regions of lower concentration
until the solute reaches a uniform concentration throughout
the solution (
Figure 4–1b
). This movement of molecules
from one location to another solely as a result of their random
thermal motion is known as
diffusion.
Many processes in living organisms are closely associ-
ated with diffusion. For example, oxygen, nutrients, and other
molecules enter and leave the smallest blood vessels (capil-
laries) by diffusion, and the movement of many substances
across plasma membranes and organelle membranes occurs by
diffusion.
Magnitude and Direction of Diffusion
Figure 4–2
illustrates the diffusion of glucose between two
compartments of equal volume separated by a permeable
barrier. Initially, glucose is present in compartment 1 at a
concentration of 20 mmol/L, and there is no glucose in com-
partment 2. The random movements of the glucose molecules
in compartment 1 carry some of them into compartment 2.
The amount of material crossing a surface in a unit of time is
known as a
ux.
This one-way fl
ux of glucose from compart-
ment 1 to compartment 2 depends on the concentration of
glucose in compartment 1. If the number of molecules in a
unit of volume is doubled, the fl ux of molecules across each
surface of the unit will also be doubled because twice as many
molecules will be moving in any direction at a given time.
After a short time, some of the glucose molecules that
have entered compartment 2 will randomly move back into
compartment 1 (see Figure 4–2, time B). The magnitude
of the glucose fl ux from compartment 2 to compartment 1
depends upon the concentration of glucose in compartment 2
at any time.
The
net fl
ux
of glucose between the two compartments
at any instant is the difference between the two one-way fl
uxes.
It is the net fl ux that determines the net gain of molecules in
compartment 2 and the net loss from compartment 1.
Eventually the concentrations of glucose in the two
compartments become equal at 10 mmol/L. The two one-way
fl uxes are then equal in magnitude but opposite in direction,
and the net fl
ux of glucose is zero (see Figure 4–2, time C).
The system has now reached
diffusion equilibrium.
No fur-
ther change in the glucose concentrations of the two compart-
ments will occur because equal numbers of glucose molecules
will continue to diffuse in both directions between the two
compartments.
Several important properties of diffusion can be empha-
sized using this example. Three fl uxes can be identifi ed at any
surface—the two one-way fl
uxes occurring in opposite direc-
tions from one compartment to the other, and the net fl
ux,
which is the difference between them (
Figure 4–3
). The net
fl ux is the most important component in diffusion because it
is the net amount of material transferred from one location
to another. Although the movement of individual molecules
is random,
the net fl
ux always proceeds from regions of higher
concentration to regions of lower concentration.
For this reason,
we often say that substances move “downhill” by diffusion.
The greater the difference in concentration between any two
regions, the greater the magnitude of the net fl ux. Thus, the
concentration difference determines both the direction and
the magnitude of the net fl
ux.
At any concentration difference, however, the magni-
tude of the net fl ux depends on several additional factors: (1)
temperature—the higher the temperature, the greater the
Figure 4–1
Diffusion. (a) Molecules initially concentrated in one region of a
solution will, due to their random thermal motion, undergo a net
diffusion from the region of higher concentration to the region
of lower concentration. (b) With time, the molecules will become
uniformly distributed throughout the solution.
(a)
(b)
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