The objective of this study is to describe the way in which two important biological phenomena, namely diffusion and osmosis, are addressed in the classroom. The study builds on extensive research conducted over the past twenty years showing that students' appropriation of these two phenomena remains partial and incomplete. To understand some of the difficulties these students face in understanding such concepts, we have geared our research toward analysis of classroom practices based on a theoretical framework involving general and specific dimensions of teaching science. Using a case study (a course made up of eight periods), we collected data in three stages: interviews with the teacher regarding his planning; a video recording of the entire course; and feedback interviews with the teacher subsequent to the course. The study's results show that the difficulties encountered by the students cannot be attributed solely to their personal characteristics (state of development of the scientific mindset, prior learning, etc.). Instead, they appear to be largely associated with teaching practices and the potential these practices hold in terms of allowing students to appropriate these concepts. The results presented in this article are significant in their contribution to improving teaching methods for diffusion and osmosis, and thus to facilitating their understanding by students. The paper also presents an example of a conceptual and methodological framework for the study of classroom practices with a view to addressing the gap between educational research and classroom practice.