ion type (e.g., K+ or Cl−) or size (e.g., larger “A−” or smaller
“A−”). When additional anions are incorporated into the
activity, the subsequent separation of charge will be greater
when the Gibbs-Donnan equilibrium is reached.
At the end of the exercise, the students may refer to the questions
posed in part 1 to review and discuss their answers. Alternatively,
the students could complete parts 2–4 of the activity with the background knowledge they have, and the questions in part 1 could be
used as a post-activity review or a quiz. In more advanced courses,
the instructor may have the students apply what they have learned
in this demonstration to a basic mathematical scenario (e.g., “In a
hypothetical cell, the concentration of major ions are as follows:
Na+ = 20 mM; Cl− = 5 mM; A− = 165 mM. Ignoring any osmotic
effects, what is the concentration of K+ in the cell?” [150 mM]) or
have the students make predictions about the implications of the
Gibbs-Donnan equilibrium on the regulation of cell volume.
Barrett, K.E., Barman, S.M., Boitano, S. & Brooks, H.L. (2012). Ganong’s Review
of Medical Physiology, 24th ed. (pp. 8–9). New York, NY: McGraw-Hill.
Kurbel, S. (2008). Are extracellular osmolality and sodium concentration
determined by Donnan effects of intracellular protein charges and of
pumped sodium? Journal of Theoretical Biology, 252, 769–772.
NGSS Lead States (2013). Next Generation Science Standards: For States, By
States. Washington, DC: National Academies Press.
Pitts, R.F. (1974). Volume and composition of the body fluids. In Physiology of
the Kidney and Body Fluids (pp. 11–34). Chicago, IL: Year Book Medical.
Randall, D., Burggren, W. & French, K. (2002). Eckert Animal Physiology:
Mechanisms and Adaptations, 5th ed. (pp. 87–88). New York, NY: W.H.
Widmaier, E., Raff, H. & Strang, K. (2013). Vander’s Human Physiology:
The Mechanisms of Body Function, 13th ed. New York, NY:
WINNIFRED BRYANT is an Associate Professor in the Department of
Biology, University of Wisconsin-Eau Claire, Eau Claire, WI 54701; e-mail: