Diffusion and osmosis are central concepts in biology that explain mechanisms of passive transport. While biology education research has documented significant student struggles with these concepts, there are very few studies from chemistry and physics education research that explore student understanding of these ideas. Diffusion is a concept that cuts across disciplines and has important underlying physics principles that contribute to students' understanding of the process (e.g., a series of collisions resulting in a random motion process). In an introductory physics for life science majors course we documented the initial ideas students have about the topic of diffusion. We then designed a unit that leverages computational simulations to develop student understanding of how elastic collisions underlie diffusive motion. We present findings from this preliminary study on student resources for understanding diffusion, and discuss the implications for revising the curricular materials as a result of our work.