Teaching the Calvin Cycle
By Elizabeth Paine, PhD
Carolina Biological Supply Company
The Calvin cycle of photosynthesis can be difficult to teach and to learn when presented only as a relentless sequence of complex reactions. Two ways to make the abstract subject easier to grasp are to help your students keep the big picture in view throughout the discussion and to have them use models for the detailed reactions. As you discuss the Calvin cycle, do not let the class forget where in the cell the cycle occurs, its initial reactants and direct products, and its main purpose.
Make sure that students keep in mind the structure of the chloroplast. The Calvin cycle occurs in the stroma rather than in and on the thylakoid membrane. Remind students of the location of the light-dependent reactions that feed the needed reactants into the Calvin cycle. Carolina carries chloroplast and plant cell models; a good drawing or illustration is also useful. You may have the students make their own drawings of chloroplast structure and label the areas where different components of photosynthesis take place.
Don't forget the reactants
Remind students of the reactants that feed into the Calvin cycle, where these reactants come from, and why they are necessary (e.g., ATP provides the energy needed by the reactions in the Calvin cycle; NADPH supplies the electrons and hydrogens needed). At least initially, do not present the reactants as chemical structures or in the context of a chemical equation; try a simple schematic with the detailed reactions depicted as a black box. Show the ATP and NADPH needed for the Calvin cycle reactions coming from the light-dependent reactions (which for this purpose may also be represented as a black box), and carbon dioxide coming from the air.
In addition, continually reinforce the key purpose of the Calvin cycle―to produce glyceraldehyde-3-phosphate (phosphoglyceraldehyde). Glyceraldehyde-3-phosphate is converted to glucose-6-phosphate, which in turn is converted to sucrose and starch (previous lessons should have explained the importance of sucrose and starch in plants). Glyceraldehyde-3-phosphate also converts into many other organic compounds needed by the plant. Although glucose-6-phosphate is often referred to as the end product of the Calvin cycle, the immediate product is glyceraldehyde-3-phosphate. If you make this distinction, students may more readily grasp the idea of why the cycle must occur 6 times, once for each glucose-6-phosphate molecule generated.
If you want your students to understand the chemical reactions in depth, have them use models. Presented in context, modeling the individual reactions may help some students make sense of the larger picture. Carolina offers a classroom kit, Modeling Photosynthesis: The Calvin Cycle, in which student groups model different reactions of the Calvin cycle. Then, in sequence, the groups present their portions of the cycle to the class.
Alternatively, you could develop your own modeling materials, e.g., toothpicks used with colored marshmallows, with each color assigned to represent a different atom. Find ways to remind students of the larger picture to ensure that they have a framework in which to place the reactions they model and so they do not get lost in the details.