Key Photosynthesis Reactions to Know for Plant Physiology

Photosynthesis is crucial for plant life, converting light energy into chemical energy. It involves two main stages: light-dependent reactions that generate ATP and NADPH, and the Calvin cycle, which uses these products to turn CO2 into glucose.

1. Light-dependent reactions

   • Occur in the thylakoid membranes of chloroplasts.
   • Convert light energy into chemical energy in the form of ATP and NADPH.
   • Involve the absorption of light by chlorophyll and other pigments.

2. Calvin cycle (light-independent reactions)

   • Takes place in the stroma of chloroplasts.
   • Utilizes ATP and NADPH produced in light-dependent reactions to convert CO2 into glucose.
   • Consists of three main phases: carbon fixation, reduction, and regeneration of RuBP.

3. Carbon fixation

   • The process of converting inorganic CO2 into organic compounds.
   • Catalyzed by the enzyme RuBisCO, which combines CO2 with ribulose bisphosphate (RuBP).
   • Essential for the synthesis of carbohydrates in plants.

4. Photolysis of water

   • The splitting of water molecules into oxygen, protons, and electrons using light energy.
   • Provides the electrons needed for the electron transport chain.
   • Releases oxygen as a byproduct, which is essential for aerobic life.

5. Electron transport chain

   • A series of protein complexes located in the thylakoid membrane.
   • Transfers electrons from water to NADP+, creating NADPH.
   • Generates a proton gradient that drives ATP synthesis.

6. ATP synthesis

   • Occurs through a process called chemiosmosis, driven by the proton gradient created by the electron transport chain.
   • ATP synthase enzyme facilitates the conversion of ADP and inorganic phosphate into ATP.
   • Provides energy for various cellular processes, including the Calvin cycle.

7. NADPH production

   • Formed when electrons reduce NADP+ at the end of the electron transport chain.
   • Serves as a reducing agent in the Calvin cycle, aiding in the conversion of CO2 to glucose.
   • Plays a crucial role in various biosynthetic reactions in the plant.

8. RuBisCO activity

   • The enzyme ribulose bisphosphate carboxylase/oxygenase (RuBisCO) catalyzes the first step of the Calvin cycle.
   • Responsible for the fixation of CO2 into organic molecules.
   • Can also catalyze a reaction with oxygen, leading to photorespiration.

9. Photorespiration

   • A process that occurs when RuBisCO reacts with oxygen instead of CO2.
   • Results in the production of a two-carbon compound, which is less efficient for carbon fixation.
   • Can reduce the overall efficiency of photosynthesis, especially under high light and temperature conditions.

10. C4 carbon fixation

    • A modified pathway that allows plants to efficiently fix CO2 in hot and dry environments.
    • Involves the initial fixation of CO2 into a four-carbon compound (oxaloacetate).
    • Reduces photorespiration by concentrating CO2 around RuBisCO.

11. CAM photosynthesis

    • A strategy used by some plants (e.g., succulents) to minimize water loss.
    • CO2 is fixed at night and stored as malate, which is used during the day for photosynthesis.
    • Allows plants to open their stomata at night, reducing water loss during the day.

12. Photophosphorylation

    • The process of adding a phosphate group to ADP to form ATP using light energy.
    • Occurs during the light-dependent reactions, primarily through the action of ATP synthase.
    • Essential for providing the energy required for the Calvin cycle and other cellular processes.