The idea that plants do photosynthesis but do not do cellular respiration is a common misconception among middle and high school students that often stems from an over-simplification of these processes in diagrams and formal science instruction. The activity presented here uses a conceptual change approach (Nussbaum & Novick, 1982) and an online simulation to facilitate high school students’ accurate conceptions of the relationship between photosynthesis and cellular respiration in plants.