Lecture Notes on Light Reactions

Introduction

• Light reactions are part of photosynthesis.
• Occur in the thylakoid membranes.
• Overview: light and water enter the thylakoids, releasing ATP, NADPH, and oxygen.
• Focus on understanding rather than memorizing names.

Energy Conversion

• Plants convert solar energy to chemical energy in thylakoids.
• Chemical energy is stored as NADPH and ATP.

Role of Light and Chlorophyll

• Light energy excites electrons in chlorophyll.
• Chlorophyll absorbs photons, boosting electrons to an excited state.
• Excited electrons return to ground state, releasing energy and fluorescence.

Photosystems

• Embedded in thylakoid membrane; function as light-capturing complexes.
• Photosystem 2 (PSII): Reaction center P680 (absorbs light at 680 nm).
• Photosystem 1 (PSI): Reaction center P700 (absorbs light at 700 nm).

Electron Transport and Redox Reactions

• Light energy drives electrons from ground to excited state.
• Electrons transferred from PSII to PSI via electron transport chain (ETC).
  • Involves plastoquinone, cytochrome, and plastocyanin.
  • Redox reactions occur throughout the transport.

ATP Generation

• Electron transport contributes to hydrogen ion gradient across thylakoid membrane.
• ATP Synthase: Uses gradient for ATP formation.
• Converts ADP and inorganic phosphate into ATP.

Steps in Light Reactions

1. Light enters PSII; water splits to provide electrons.
2. Electrons excited and transferred via redox reactions.
3. Hydrogen ion gradient formed in thylakoid lumen.
4. ATP generated through chemiosmosis.

Outputs of Light Reactions

• Inputs: Water, ADP, NADP+.
• Outputs: Oxygen (byproduct), ATP, NADPH.

Connection to Calvin Cycle

• ATP and NADPH used in Calvin cycle for further photosynthesis steps.
• Calvin cycle regenerates ADP and NADP+ for light reactions.

Summary

• Conversion of solar to chemical energy in ATP and NADPH.
• Process of chemiosmosis: Hydrogen ion gradient powers ATP synthesis.
• Yield: 1 NADPH and 1 ATP per pair of electrons from water.

Key Terms

• Chemiosmosis: Process of ATP generation using hydrogen ion gradient.
• Photosystems: Complexes for capturing light energy.