Light Dependent Stage of Photosynthesis

Overview

• The light-dependent stage occurs within the thylakoid membrane of the chloroplast.
• Thylakoid membranes can be folded into structures called grana.
• Proteins within these membranes facilitate the light-dependent stage.

Key Components

• Stroma: One side of the thylakoid membrane.
• Intermembrane Space: The space inside the thylakoid membrane.
• Photosystem II (PSII)
• Photosystem I (PSI)
• ATP Synthase

Photolysis of Water

• Water is split into hydrogen ions, oxygen, and electrons.
  • Oxygen: By-product, released through diffusion via the stomata.
  • Electrons: Used in Photosystem II.

Photosystem II (PSII)

• Absorbs light energy, exciting electrons to a higher energy level.
• Electrons pass down a series of electron carriers through redox reactions.
• Energy released from these reactions actively transports H⁺ ions into the intermembrane space.

Photosystem I (PSI)

• Electrons from PSII enter PSI, are re-excited by light energy.
• Electrons raise to a higher energy level.

Chemiosmosis and ATP Formation

• Proton Gradient: Established by the buildup of H⁺ ions in the intermembrane space.
• H⁺ ions flow through ATP synthase via chemiosmosis.
• ATP synthase adds inorganic phosphate to ADP, forming ATP.
• H⁺ ions and electrons form NADPH by combining with NADP⁺.

Non-Cyclic Photophosphorylation

• Involed in the creation of ATP and NADPH.
• Electron path: PSII → Electron Carriers → PSI → NADP⁺
• Electron source: Photolysis of water.

Cyclic Photophosphorylation

• Photosystem II is not involved.
• Electron path: Electron Carriers → PSI → Electron Carriers (Cycle).
• Produces ATP but not NADPH.
• Electrons are recycled, useful when water is limited.
• Maintains proton gradient for ATP synthesis.

Comparison: Non-Cyclic vs. Cyclic Photophosphorylation

• Non-Cyclic: Produces ATP and NADPH, involves both PSII and PSI, electron source is photolysis of water.
• Cyclic: Produces only ATP, involves PSI only, electrons are recycled, useful under water-limited conditions.

Important Notes for Exams

• Understand the differences and similarities between non-cyclic and cyclic photophosphorylation.
• Use accurate scientific terms in descriptions.

Conclusion

• Good luck with your exams!
• Use scientific terminology accurately.