pieces of poster board (with a width >18 inches) will be labeled “A”;
one of these should be larger than the other, enough that the size difference can be visually appreciated.
In preparation for the activity, students should discuss the answers
to the following questions (the answers are in brackets below).
1. List two forces that influence the movement of ions. [Con-centration gradients and electrical gradients.]
2. Figure 1 illustrates the initial distribution of permeable ions
between two aqueous compartments separated by a membrane. Predict the distribution of those ions once diffusion
equilibrium is reached.
[At equilibrium, three molecules of Na+ and three molecules
of Cl− should be on each side of the membrane.]
3. In reference to the previous question, are there equal concentrations of water on either side of the membrane when diffusion equilibrium has been reached? Explain in terms of
osmosis and osmotic gradients. [There are equal amounts of
water on either side; there is no difference in the concentration
of diffusible ions at equilibrium and therefore there is no
osmotic gradient to drive the movement of water molecules.]
4. Molecules are never completely immobile. Predict the consequences of a molecule randomly crossing the membrane
after diffusion equilibrium has been reached. [Once equilibrium has been reached, the movement of a molecule across
the membrane will be counterbalanced by the movement of
another ion of the same species across the membrane in the
5. How would the presence of nondiffusible anions influence
the movement of permeable ions? [Nondiffusible anions
would attract cations and repel anions.]
6. How would the size of a nondiffusible anion influence the
movement of permeable ions? [A larger nondiffusible anion
would attract more cations and repel more anions than a
1. Generate two groups of 10 students; keep the groups separated by a “membrane” that is represented by tape on the floor.
Gaps in the tape should be wide enough (about 12–18 inches)
for students to pass though one abreast, while holding the
piece of paper that represents their ion waist-high.
2. Students on one side of the membrane will be given paper
labeled “K+”; students on the other side of the membrane will
be given paper labeled “Cl−”.
3. The instructor should advise the students that the membrane
is permeable to K+ and Cl− and ask them to predict the outcome of the demonstration. The instructor should then direct
them to follow electrical and chemical gradients until equilibrium is reached. The students must move through the gaps of
membrane holding their ion waist-high.
4. If the students have mastered the concept of diffusion, they
will arrange themselves so that five “K+” and five “Cl−” are
found on each side of the membrane (Figure 2).
a With respect to K+ and Cl−, a concentration and electrical equilibrium will be reached.
b The instructor should remind the students that biological membranes are freely permeable to water and
question the students about the movement of water
with respect to the osmotically active particles (K+
and Cl−). Equal distribution of particles on either side
of the membrane and the permeability of the membrane to water allows equilibrium to be reached.
c The students should demonstrate that after equilibrium is reached, net ion flux equals zero. This can be
done by balancing the movement of any ion across
the membrane with the movement of an ion of like
species across the membrane in the opposite direction.
1. Generate two groups of nine students each; keep the groups
separate by a permeable membrane, represented by pieces of
tape on the floor. Gaps in the tape should be wide enough
Figure 2. Diagram representing the initial (left) and final
(right) distribution of students for part 2 of the exercise.
Figure 1. Diagram representing the initial distribution of
permeable ions between two aqueous compartments
separated by a membrane.