Glucose concentration (mmol/l)
High solute concentration
Low solute concentration
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
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
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.
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. (
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.