This column provides original articles on innovations in case study teaching, assessment of the method, as well as case studies with teaching notes. In this month's issue the goal of the case study is for students to expand their understanding of membrane permeability and the proton motive force in mitochondria through the study of 2,4-dinitrophenol (DNP) action. The case is suitable for an introductory cell biology course serving life science majors and nonmajors. Students should be familiar with macromolecules and the plasma membrane and have read an introduction to mitochondria in an introductory biology textbook. The case study may be used as a stand-alone exercise in a classroom or be paired with a laboratory investigation using isolated plant mitochondria. Follow-up assignments for assessments are provided. (Contains 3 figures and 1 note.)