  A goal of Introductory Physics for Life Sciences (IPLS) curricula is to
prepare students to effectively use physical models and quantitative reasoning
in biological and medical settings. To assess whether this goal is being met,
we conducted a longitudinal study of the impact of IPLS on student work in
later biology and chemistry courses. We report here on one part of that study,
a comparison of written responses by students with different physics
backgrounds on a diffusion task administered in a senior biology capstone
course. We observed differences in student reasoning that were associated with
prior or concurrent enrollment in IPLS. In particular, we found that IPLS
students were more likely than non-IPLS students to reason quantitatively and
mechanistically about diffusive phenomena, and to successfully coordinate
between multiple representations of diffusive processes, even up to two years
after taking the IPLS course. Finally, we describe methodological challenges
encountered in both this task and other tasks used in our longitudinal study.
