Directed or random? Student reasoning about diffusion across contexts

Diffusion is a critical component of the Physiology Core Concept of flow down gradients and is fundamental to understanding how ions, gases, or signaling molecules travel short distances in the body. When asked about diffusion, students often reason successfully using the "things move from areas of high to low concentration" heuristic but struggle to understand that random motion underlies this movement. We investigated the different knowledge resources students use when reasoning about diffusion across different contexts. Additionally, we determined if item context impacted the resources students activated and how consistent students were in their reasoning. We gave students a pair of questions from three contexts (plant, animal, and nonliving) that asked them to predict and explain where a molecule of gas would be located before and after equilibrium. Using the resources framework, we identified 14 common knowledge resources and 6 different patterns in resource activation. "High to low" and related resources were used in 73% of responses. Only 23% of responses included at least one "random motion" resource, and the vast majority of these responses described random particle motion starting only after equilibrium is reached. Item context did not significantly affect the resources students used. Students were also mostly consistent in their reasoning, with 76% using similar resources across the two items. These findings indicate that "high to low" and related resources have a high cueing priority for many students and that instructors should help students unpack random motion as the mechanism underlying diffusion instead of leaving it "black boxed."<b>NEW & NOTEWORTHY</b> We present the first physiology education study that investigates students' understanding of diffusion using a resources framework. Students frequently used "high to low" knowledge resources and rarely coordinated them with "random motion." Of the 23% that included "random motion" resources, the vast majority described random motion starting only after equilibrium. While "high to low" resources are sometimes productive, when students coordinate "random motion" resources, they have tools for a more nuanced understanding of physiological phenomena.