Phases of Photosynthesis

Photosynthesis is a vital process that converts light energy into chemical energy, fueling life on Earth. It consists of two main phases: light-dependent reactions that generate ATP and NADPH, and light-independent reactions (Calvin cycle) that produce glucose from carbon dioxide.

1. Light-dependent reactions

   • Occur in the thylakoid membranes of chloroplasts.
   • Convert light energy into chemical energy in the form of ATP and NADPH.
   • Water molecules are split (photolysis), releasing oxygen as a byproduct.

2. Light-independent reactions (Calvin cycle)

   • Occur in the stroma of chloroplasts.
   • Use ATP and NADPH produced in light-dependent reactions to convert carbon dioxide into glucose.
   • Involves three main phases: carbon fixation, reduction, and regeneration.

3. Light absorption

   • Chlorophyll and other pigments absorb specific wavelengths of light, primarily blue and red.
   • This absorption excites electrons, initiating the light-dependent reactions.
   • Accessory pigments (like carotenoids) help capture additional light energy.

4. Electron transport chain

   • A series of proteins embedded in the thylakoid membrane that transfer excited electrons.
   • Energy released during electron transfer is used to pump protons into the thylakoid lumen, creating a proton gradient.
   • Ultimately leads to the reduction of NADP+ to NADPH.

5. ATP synthesis

   • Occurs via chemiosmosis, driven by the proton gradient established by the electron transport chain.
   • ATP synthase enzyme facilitates the conversion of ADP and inorganic phosphate into ATP.
   • Provides the energy necessary for the Calvin cycle.

6. Carbon fixation

   • The process of incorporating carbon dioxide into organic molecules.
   • Catalyzed by the enzyme RuBisCO, which combines CO2 with ribulose bisphosphate (RuBP).
   • Produces 3-phosphoglycerate (3-PGA) as the first stable product.

7. Reduction of 3-PGA

   • 3-PGA is converted into glyceraldehyde-3-phosphate (G3P) using ATP and NADPH.
   • This step is crucial for producing sugars and other carbohydrates.
   • Some G3P molecules are used to synthesize glucose and other carbohydrates.

8. Regeneration of RuBP

   • Involves the conversion of G3P back into RuBP to continue the Calvin cycle.
   • Requires ATP to facilitate the regeneration process.
   • Ensures the cycle can repeat and continue fixing carbon dioxide.