Light Reactions of Photosynthesis

• Locate where the light reactions of photosynthesis take place within the leaf.
• Learn about the pathway through which light energy flows as it is converted into biochemical energy.
• List the basic steps for the light reactions of photosynthesis and highlight those steps that enable production of ATP, NADPH, and oxygen.
• Understand the difference between non-cyclic and cyclic photophosphorylation.

This teaching strategy for the light reactions of photosynthesis provides a one- or two-hour presentation, with drawings and visualizations, of how light energy is converted into biochemical energy.

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.
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 thylakoid membrane system and explain that the light reactions of photosynthesis take place within the thylakoid membrane.
5. Explain that specific wavelengths of light (usually blue and red, but not green) have to pass through the leaf surface, enter the parenchyma cells, and excite chlorophyll molecules within the thylakoid membrane.
6. Use a ball, piece of chalk, a pen, or similar item to represent a unit of light being emitted upon the reaction center (photosystem II in non-cyclic photophosphorylation) that starts a cascade of events through which excitation energy is passed from one molecule to another through inductive resonance tranasfer see http://www.merlot.org/merlot/viewMaterial.htm?id=80033. This can be done by passing the "photon energy" item from one participant to another, asking them to be less excited during each transfer of the item. The last participant can stand up and shout, "photosynthesis!" to indicate the point at which the right amount and wavelength of light has been received to begin the steps of non-cyclic photophosphorylation.
7. Draw the steps of non-cyclic photophosphorylation and make note of the reaction centers for photosystem II and photosystem I. Show animations of these steps using references and resources such as http://www.merlot.org/merlot/viewMaterial.htm?id=79743 – also see References and Resources below.
8. Point out three key products from the light reactions of photosynthesis, including ATP, NADPH, and oxygen.
9. Show how cyclic photophosphorylation differs from non-cyclic photophosphorylation and discuss the evolutionary impact of these different processes http://www.merlot.org/merlot/viewMaterial.htm?id=79744.
10. Explain that the energy from the light reactions of photosynthesis will be used in the dark reactions of photosynthesis to fix carbon and build molecules needed for energy storage.

Supporting Files:

• Outline - Light Reactions of Photosynthesis (Microsoft Word 30kB Aug3 10)
• Questions - Light Reactions of Photosynthesis (Microsoft Word 33kB Aug4 10)

This teaching strategy provides a more hands-on approach to the light reactions of photosynthesis that helps students visualize how light gets into a leaf and how light energy is converted to biochemical 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 this Activity Sheet in a file entitled "Questions - Light Reactions of Photosynthesis."

MERLOT description and link to "Cyclic and Noncyclic Photophosphorylation," which is a site that provides animations and tutorials for photosynthesis. See http://www.merlot.org/merlot/viewMaterial.htm?id=79744

MERLOT description and link to "Photosynthesis Animation," which includes a simple, straightforward flash animation of the light reactions. See http://www.merlot.org/merlot/viewMaterial.htm?id=79743

MERLOT description and link to "Photosynthesis Interactive Animated Tutorial," which is a site designed to accompany a textbook on Interactive Concepts in Biochemistry. This link provides a good overview of photosynthesis. 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