Dark Reactions of Photosynthesis

This page authored by Jim Bidlack, University of Central Oklahoma, based on an original activities by Bjørn Vidnes, Norwegian Centre of Science Education, Cato Tandberg, University of Oslo, Wenche Erlien, Norwegian Centre of Science Education, Graham Kent, Grant College, Joyce Diwan, Rensselaer Polytechnic Institute, John Kyrk, Science Graphics, Rodney Boyer, John Wiley & Sons Publishers, Inc., Mike Tyree, Virginia Tech, and John Whitemarsh and Govindjee, University of Illinois at Urbana-Champaign.

This material is replicated on a number of sites as part of the SERC Pedagogic Service Project


Drawings and visualizations are used to help participants conceptualize the location and steps involved in the dark reactions of photosynthesis.

Learning Goals

  • Locate where the dark reactions of photosynthesis take place within the leaf.
  • Learn about the pathway through which carbon dioxide is initially combined with 3-phosphoglyceric acid to eventually form stored carbohydrates.
  • List the importance of the enzyme, ribulose bisphosphate carboxylase/oxygenase and its contribution towards steps in the Calvin Cycle.
  • Understand how the dark reactions facilitate energy storage in the form of fixed carbohydrates.

Context for Use

This teaching strategy for the dark reactions of photosynthesis provides a one- or two-hour presentation, with drawings and visualizations, of how carbon dioxide is "fixed" by the Calvin Cycle to facilitate energy storage in the form of carbohydrates.

Description and Teaching Materials

  1. Use an actual dicot leaf or draw a representation of this leaf on a piece of paper and cut it in half. Explain that the light and dark reactions of photosynthesis take place inside specialized cells that are found within the edge of the leaf and traverse the inside portion.
  2. Draw a picture of the edge of the leaf, showing the several layers of cells within, including top epidermis, palisade parenchyma, spongy parenchyma, midvein with vascular tissue, and bottom epidermis see for an overview of where light reactions take place in the leaf and then see for details of the dark reactions.
  3. Point out the location for photosynthetic reactions within the palisade and spongy parenchyma, and draw one cell, depicting several chloroplasts in the cytoplasm of that cell. Explain that the reactions of photosynthesis take place in the chloroplasts.
  4. Draw a chloroplast, showing the stroma and thylakoids. Explain that the dark reactions of photosynthesis take place within the stroma Some participants can better relate to the chloroplast being "like" a plate with a stack of pancakes – the pancakes representing thylakoids and the syrup representing the stroma.
  5. Explain that carbon dioxide enters through the stomatal apparatus and diffuses into parenchyma cells, where it can participate in reactions of the Calvin Cycle.
  6. Point out that the enzyme, ribulose bisphosphate carboxylase / oxygenase (RUBISCO) catalyzes the combining of carbon dioxide with ribulose bisphosphate carboxylase / oxygenase (RUBISCO) to form two molecules of 3-phosphoglyceric acid.
  7. Mention that photosynthetic organisms (both aquatic and terrestrial) are plentiful on earth and that this enzyme is very abundant in plants, making it the most abundant enzyme on earth.
  8. Show the steps and ask participants to draw the general steps of the Calvin Cycle see and, for more detail regarding the individual steps and molecular structures involved, see
  9. Explain that the RUBISCO enzyme does not always fix carbon dioxide and there is potential for oxygen to compete with carbon dioxide – leading to a process called photorespiration. See the last section for the link at
  10. Briefly explain how certain plants, called C-4 plants (as opposed to C-3 plants) have the enzyme, phosophoenolpyruvate (PEP) carboxylase, and this enzyme has a higher affinity for carbon dioxde, which is one of the major reasons why C-4 plants may be more efficient in dry, hot climates

Supporting Files:

Teaching Notes and Tips

This teaching strategy provides a more hands-on approach to the dark reactions of photosynthesis that helps students visualize how carbon dioxide enters the leaf and is converted into stored energy. It usually takes two lectures for students to gain a full understanding of when and how these reactions occur. With drawings and visualizations of leaf cells and reactions that take place, students have a more hands-on conceptualization of how these processes occur. Participants should have some familiarity of plant morphology and cell biology before approaching this topic. There is usually good understanding of where reactions take place, but a little more difficulty in understanding the specific steps. Animations and visualizations of these topics (see References and Resources below), interspersed during discussion, help participants keep actively engaged in the learning experience.


Participants may be tested on their comprehension of this learning material through multiple choice, short-answer, or essay exams. A few example questions are embedded in Activity Sheet, entitled "Questions – Dark Reactions of Photosynthesis."

References and Resources

MERLOT description and link to "C3, C4 and CAM Plants," which provides information about photorespiration and alternative pathways in C4 plants. See

MERLOT description and link to "Calvin Cycle (Photosynthesis)," which is a site that provides an animated three step version of the dark reactions of photosynthesis. See

MERLOT description and link to "Calvin Cycle – Photosynthetic Carbon Reactions," which includes details for each of the steps involved with the Calvin Cycle, molecular structures, and information about enzymes involved. See

MERLOT description and link to "Dark Reactions of Photosynthesis," which provides an animation of the Calvin Cycle, along with details of molecular structures involved. See

MERLOT description and link to "Photosynthesis Interactive Animated Tutorial," which includes an overview to light and dark reactions of photosynthesis, as well as photorespiration. See

MERLOT description and link to "Photosynthesis Video," which provides an excellent overview and guide to where photosynthesis occurs in a leaf. See

MERLOT description and link to "The Photosynthetic Process," which includes a comprehensive paper on photosynthesis and links to all figures from the original paper. See