Teaching diffusion with a coin

Abstract

DIFFUSION is a key subject in physiology. By means of diffusional processes, substances can be efficiently transferred at short distances. For that reason, it is a very important mechanism of exchange between the cells comprising a plant or an animal and the surrounding compartments with which they interact as well as between the compartments themselves (such as the interstitial and capillary compartments, for example). However, diffusion is too a slow process when matter has to be transported along the large distances found in the bodies of multicellular organisms. Consequently, as organisms became larger, diffusion was no longer capable of providing for all their needs, and convective transport was made necessary. Explaining to undergraduate students why diffusion works so badly at larger distances while at the same time being a process of paramount relevance at the cellular level is not a trouble-free task (4). Although students easily grasp the notion that random processes are capable of giving rise to emergent behavior, such as the net directional movement of molecules, they frequently confound a macroscopic ballistic view of movement with a microscopic diffusive one. It appears to arise from a deep-seated misconception about random processes (2). In the present article, we describe an inexpensive and simple way to make students intuitively experience the probabilistic nature and nonorientated motion of diffusing particles. This understanding allows students to realize why diffusion works so well over short distances and becomes increasingly and rapidly less effective as the distances involved become progressively larger. We believe that this activity is in agreement with the urge for the adoption of teaching approaches that more actively involve the student in the learning process, focus on problem solving, and lead