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Three Cases from the Membrane Files

The Exploding Fish, the Pleasurable Poison, and the Dangerous Diet

Eric Ribbens
Department of Biological Sciences
Western Illinois University


This PowerPoint-driven case study presents three different stories, each of which explores an aspect of membranes. The first (The Exploding Fish) covers diffusion, specifically addressing the question of why animal cells explode in freshwater but fish do not, and differences between saltwater and freshwater fish. The second case (The Pleasurable Poison) is designed to show that alcohol can slip across membranes and also highlights some of the problems of ingesting this toxin. The third case (The Dangerous Diet) explores a weight-loss drug, DNP, and how it operates in mitochondrial membranes. The first of these case studies also includes a number of "clicker" questions. These cases were originally designed for a semester-long, introductory biology course for non-majors, and instructors can choose to use one or all of the cases to suit their course.


  • Explain the concept of diffusion, both generally and across a semi-permeable membrane.
  • Describe at a basic conceptual level how the mitochondria generate ATP.
  • Apply these principles to the case scenarios to indicate why water balance management is important, why drinking alcohol has such devastating effects on our bodies, and how DNP can disrupt mitochondrial activity.


Membranes; mitochondria; semi-permeable; kidneys; alcohol; dinotrophenol; DNP; ATP; diffusion; osmosis

Educational Level

High school, Undergraduate upper division


PDF, PowerPoint

Type Methods

Clicker, Interrupted



Subject Headings

Biology (General) Cell Biology

Date Posted


Teaching Notes

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Teaching notes are intended to help teachers select and adopt a case. They typically include a summary of the case, teaching objectives, information about the intended audience, details about how the case may be taught, and a list of references and resources.

Supplemental Materials

Optional PowerPoint presentation that can be used as preparatory material.



The following video(s) are recommended for use in association with this case study.

  • Diffusion Across Membranes hhmi/
    This four-second animation discusses two types of transport across cell membranes: active and passive transport. Running time: 0.04 min. Produced by HHMI BioInteractive.