This article describes an interrupted case study that intersperses information about diffusion and osmosis with content review and knowledge application questions, as well as a simple experiment that can be conducted without the use of a laboratory. The case study was developed for use in an introductory undergraduate biology course. The case would also be appropriate for use in a high school biology course and could be adapted for use with elementary or middle school students. To successfully complete this case study, students require prior knowledge and instruction about basic cell structure (including cell membrane composition and the presence of cell walls in plant cells but not animal cells), as well as access to information about how polarity affects membrane solubility.