The light-dependent reaction

• The light-independent reactions occur in the thylakoid membrane of chloroplasts.
• 1. Absorption of Light Energy (photoionisation)

  • Chlorophyll molecules within photosystems (Photosystem II and Photosystem I) absorb light energy.
  • The absorbed energy excites electrons in the chlorophyll molecules to a higher energy level, causing them to leave the chlorophyll. These high-energy electrons are passed to an electron carrier.
• 2. Photolysis of Water

  • Photolysis (splitting of water using light energy) occurs in Photosystem II:

  2H2O ⟶ 4H+ + 4e- + O2

  • This reaction provides electrons to replace those lost by chlorophyll in Photosystem II.
  • The oxygen (O₂) produced as a by-product is released into the atmosphere.
  • Protons (H⁺) remain in the thylakoid space.
• 3. Electron Transport Chain (ETC)

  • The excited electrons from chlorophyll move down the electron transport chain (a series of electron carriers) embedded in the thylakoid membrane.
  • As the electrons are passed along the chain, they lose energy.
• 4. Chemiosmosis and ATP Production

  • The energy lost from the electrons as they move through the ETC is used to actively pump protons (H⁺) from the stroma into the thylakoid space, creating a proton gradient.
  • The protons diffuse back into the stroma through the enzyme ATP synthase, driven by the proton gradient. This process is called chemiosmosis.
  • As the protons flow through ATP synthase, it catalyzes the production of ATP from ADP and inorganic phosphate (Pi). This process is called photophosphorylation.
• Chemiosmosis is the process where protons (H+) diffuse through ATP synthase, driving the synthesis of ATP.
• Photophosphorylation is the process of producing ATP from ADP and Pi using light energy.
• 5. Reduction of NADP to NADPH

  • After the electrons pass through the electron transport chain via a series of oxidation-reduction reactions, the electrons reach Photosystem I, where they are re-energized by light.
  • These re-excited electrons are passed to another carrier and eventually transferred to NADP⁺, along with a proton (H⁺) from the stroma, reducing NADP⁺ to NADPH.

  NADP+ + 2e−+ 2H+ ⟶ NADPH
• 6. End Products of the Light-Dependent Reaction

  • ATP (from chemiosmosis)
  • NADPH (from the reduction of NADP⁺)
  • Oxygen (from the photolysis of water)

  ATP and NADPH are crucial for the next stage, the Calvin cycle (light-independent reaction), where they provide the energy and reducing power to convert carbon dioxide into glucose.