Within the framework of variation theory, 7- to 12-year-old students’ ways of understanding cellular respiration and photosynthesis were investigated against the background of their teachers’ teaching. Eighteen students were selected by the teachers and interviewed by the researcher. Lessons were observed and video recorded, and stimulated-recall interviews with the teachers were conducted. The students’ understanding of the function of oxygen and sugar in plants was described in four categories. The study concludes that young students can develop complex understanding if opportunities to learn are presented. The opportunities depended mainly on the language used by the teachers, such as metaphors, pointing out critical aspects of the object of learning. The study has implications for the relationship between teachers’ competence and students’ learning.