Dark Reactions of Photosynthesis

• 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.

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.

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 http://www.merlot.org/merlot/viewMaterial.htm?id=281356 for an overview of where light reactions take place in the leaf and then see http://www.merlot.org/merlot/viewMaterial.htm?id=481172 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 http://www.merlot.org/merlot/viewMaterial.htm?id=481172. 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 http://www.merlot.org/merlot/viewMaterial.htm?id=79769 and, for more detail regarding the individual steps and molecular structures involved, see http://www.merlot.org/merlot/viewMaterial.htm?id=481167.
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 http://www.merlot.org/merlot/viewMaterial.htm?id=80033.
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 http://www.merlot.org/merlot/viewMaterial.htm?id=444380.

Supporting Files:

• Outline - Dark Reactions of Photosynthesis (Microsoft Word 32kB Aug4 10)
• Questions - Dark Reactions of Photosynthesis (Microsoft Word 34kB Aug4 10)
  
  

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."

MERLOT description and link to "C3, C4 and CAM Plants," which provides information about photorespiration and alternative pathways in C4 plants. See http://www.merlot.org/merlot/viewMaterial.htm?id=444380

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 http://www.merlot.org/merlot/viewMaterial.htm?id=79769

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 http://www.merlot.org/merlot/viewMaterial.htm?id=481172

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 http://www.merlot.org/merlot/viewMaterial.htm?id=481167

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 http://www.merlot.org/merlot/viewMaterial.htm?id=80033

MERLOT description and link to "Photosynthesis Video," which provides an excellent overview and guide to where photosynthesis occurs in a leaf. See http://www.merlot.org/merlot/viewMaterial.htm?id=281356

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 http://www.merlot.org/merlot/viewMaterial.htm?id=85664