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Photosynthesis is a key concept in plant biology, required for understanding food webs, plant growth and physiology, and carbon capture in climate change. Teaching photosynthesis is challenging, as students must integrate complex topics on multiple scales. At the molecular scale, students must grasp oxidation/reduction, light capture, proton gradients, ATP synthesis, and carbon fixation; at the organismal scale, it is key to link net photosynthesis to plant growth and biomass accumulation, plant nutrition and respiration; at the ecosystem level, photosynthesis must be related to global carbon capture and primary productivity. A literature review that analyzed 80 studies of education research in teaching, from elementary to post-secondary settings, reported that “All research dealing with understanding of photosynthesis points to a large amount of poor understandings or misunderstandings in the target populations, ranging from the youngest pupils to adults (university students and teachers)” (Jancarikova and Jancarik, 2022).

The most prevalent misconceptions about photosynthesis are that plants get food through their roots, or that photosynthesis replaces respiration in plants (Hazel and Prosser, 1994; Parker et al., 2017). Students demonstrate uncertainty about how oxygen is generated, confusion about how photosynthesis and respiration co-exist in plants, and are unable to link carbon fixation with the bigger picture of plant growth and biomass.

Studies have pointed out the importance of grounding photosynthesis lessons in a framework of principled reasoning. Parker et al. (2017) map students’ challenges to three guiding practices that must underlie their reasoning: practice of tracing matter (inputs and outputs); practice of tracing energy (identifying transformation of sunlight into chemical potential energy, NADPH and ATP), and the practice of organizing systems into the appropriate scales (from chloroplast to cell to tree). They used different assessment formats, such as multiple-choice with one correct answer, multiple answer (choose all that apply), essays, and interviews to see how effectively they reveal students’ misconceptions. Comparing these assessments reveal that students can choose correct answers in single answer multiple choice tests while retaining many misconceptions that only come to light in multiple-answer questions or essays/interviews. Students use a combination of formal reasoning based on principles and informal reasoning based on real world experience. These authors point out that to instructors, conservation of matter and energy are principles that are assumed to be relevant and simple ‘rules’ that are followed automatically, but this s not the case for undergraduates. Diving directly into the CSPB / SCBV Bulletin | Issue / Numero 36 | Fall 2024 chloroplast’s molecular mechanisms without activating students’ prior knowledge or connecting their learning about photosynthesis to the macroscopic world appears doomed to fail.

What instructional strategies can help overcome students’ challenges with learning about photosynthesis? Active learning strategies including polling questions or drawing activities that challenge common misconceptions can be a start (Smith et al., 2018). Parker et al. (2017) suggest starting photosynthesis instruction at the scale of plant growth and energy use, and only once the big picture inputs-outputs are mastered, should the class progress to detailed molecular mechanisms. This practically means that introductory courses can benefit from less focus on having students memorize details of light and dark reactions, if these details are disconnected from underlying principles and the importance of photosynthesis at higher scales. Providing students with a firm foundation of the role of photosynthesis and emphasizing how the principles of conservation of matter and energy apply may be more productive for all students, both those who do not continue in biology and majors who can learn detailed mechanisms in upper level courses. Connecting lessons to “place-based economically relevant organisms” is another powerful way to help students relate their photosynthesis learning to their world. An elegant example lesson plan for teaching photosynthesis using timber, potatoes, and sugar kelp examples in Maine is published (Smith et al., 2018), and was shown to improve student performance. Having students make concept maps of photosynthesis in a study of first-year students was a useful diagnostic tool for teachers and students, but pre- and post-course concept maps showed little improvement (Hazel and Prosser, 1994).

The take-away from the literature is that photosynthesis teaching as currently practiced leads to students who can learn details but not make connections or see the bigger context. Our challenge as educators is to remember that students are not automatically applying principles of conservation of energy and matter as we are, so we must be explicit in weaving reminders of basic principles into lessons. Giving students opportunities to practice organizing concepts and structures into larger biological scales before moving to practicing tracing matter and energy at the molecular scales may provide a firmer foundation on which to ground photosynthesis learning.

Lacey Samuels
Department of Botany, UBC Vancouver

References

Hazel, E. and Prosser, M. (1994) First-Year University Students' Understanding of Photosynthesis, Their Study Strategies & Learning Context. The American Biology Teacher. 56: 274- 279.

Jancarikova, K. and Jancarik, A. (2022) How to Teach Photosynthesis? A Review of Academic Research. Sustainability 14: 13529.

Parker, J.M., Anderson, C.W., Heidemann,M, Merrill, J., Merritt, B., Richmond,G. and Urban-Lurain, M. (2012) Exploring Undergraduates’ Understanding of Photosynthesis Using Diagnostic Question Clusters. CBE-Life Sciences Education 11:47-57. DOI: 10.1187/cbe.11-07-0054

Smith, M.K., Toth, E.S., Borges, K., Dastoor, F., Johnston, J., Jones, E.H., Nelson, P.R., Page, J., Pelletreau, K., Prentiss, N., Roe, J.L., Staples, J., Summers, M., Trenckmann, E., and Vinson, E. 2018. Using Place-Based Economically Relevant Organisms to Improve Student Understanding of the Roles of Carbon Dioxide, Sunlight, and Nutrients in Photosynthetic Organisms. CourseSource