98
Chapter 4
1
2
1
2
1
2
Time A
Time B
Time C
20
10
0
AB
C
Compartment 1
Compartment 2
Glucose concentration (mmol/l)
Time
One-way flux
One-way flux
Net flux
Compartment 1
High solute concentration
Compartment 2
Low solute concentration
Figure 4–3
The two one-way fl uxes occurring during the diffusion of solute across a boundary and the net fl ux, which is the difference between the
two one-way fl uxes. The net fl ux always occurs in the direction from higher to lower concentration. The length of the arrows indicates the
magnitude of the fl
ux.
Figure 4–2
Diffusion of glucose between two compartments of equal volume separated by a barrier permeable to glucose. Initially, time A, compartment 1
contains glucose at a concentration of 20 mmol/L, and no glucose is present in compartment 2. At time B, some glucose molecules have moved
into compartment 2, and some of these are moving back into compartment 1. The length of the arrows represents the magnitudes of the one-
way movements. At time C, diffusion equilibrium has been reached, the concentrations of glucose are equal in the two compartments
(10 mmol/l), and the
net
movement is zero.
In the graph at the bottom of the fi gure, the green line represents the concentration in compartment 1, and the purple line represents the
concentration in compartment 2. Note that at time C, glucose concentration is 10 mmol/L in both compartments. At that time, diffusion
equilibrium has been reached.
Figure 4–2
physiological
inquiry
If at time C, additional glucose was added to
compartment 1 such that its concentration
was instantly increased to 15 mmol/L, what
would the graph look like following time C?
Draw the new graph on the fi gure and indicate
the glucose concentrations in compartments
1 and 2 at diffusion equilibrium. (
Note:
It
is not actually possible to instantly change
the concentration of a substance in this way
because it will immediately begin diffusing to
the other compartment as it is added.)
Answer can be found at end of chapter.
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