This article reports on conceptual difficulties related to photosynthesis and respiratory metabolism of a Plant Physiology course for undergraduate students that could hinder their better learning of metabolic processes. A survey of results obtained in this area during the last 10 academic years was performed, as well as a specific test, aimed to find out prior conceptual schemes in the students. The test included only basic, general questions on plant metabolism, and was passed on the first day of the course. Scores obtained by different groups of students were compared. Survey of students' performance in previous years shows that questions on plant metabolism result in significantly lower number of correct answers than for the rest of questions, despite reiteration of its teaching in prior educational levels. Some areas such as respiration or light-independent reactions of photosynthesis, as well as concepts and terminology related to electrochemistry, seem to be particularly difficult. The specific test also shows differences among the different groups of students included in this investigation. Our results point out that reiteration of a particular subject in the curricula at different educational levels does not result in a coherent enough conceptual scheme in the students. To help students in this task, it is recommended to teach the general processes also in advanced courses in which these processes are taught in great detail. In this way, settlement of the appropriate conceptual scheme may be reinforced. (Contains 5 tables and 2 figures.)