Overview
This lecture explains how plants create their own food through photosynthesis, focusing on the light-dependent reactions and their processes in plant cells.
Photosynthesis Overview
- Photosynthesis is the process by which plants make food using light, carbon dioxide, and water.
- Leaves act as factories, capturing energy and processing raw materials into glucose (sugar) and oxygen.
- The overall photosynthesis equation: carbon dioxide + water + light energy → glucose + oxygen.
- The balanced chemical equation: 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂.
- Photosynthesis occurs in two stages: light-dependent reactions and light-independent reactions.
Light-Dependent Reactions
- These reactions require light and occur in the chloroplast's grana.
- Sunlight is captured by chlorophyll and accessory pigments, energizing electrons.
- Photosystems are units in chloroplasts that organize chlorophyll and electron carriers.
- Key parts of a photosystem: reaction center, electron transport chain (ETC), and antenna pigments.
- Two types of photosystems: Photosystem I (PSI, absorbs 700 nm light) and Photosystem II (PSII, absorbs 680 nm light).
- Two pathways: cyclic (involves PSI only) and noncyclic (involves both PSII and PSI).
- In cyclic reactions, electrons return to PSI and ATP is produced.
- In noncyclic reactions, water is split (photolysis), producing oxygen, hydrogen ions, and electrons.
Electron Transport and ATP Production
- Excited electrons move down the electron transport chain, powering the production of ATP from ADP.
- Hydrogen ions build up in the thylakoid space, then diffuse through ATP synthase to generate ATP (process called chemiosmosis).
- Electrons eventually transfer to NADP⁺, forming NADPH (another energy carrier).
Noncyclic Light Reactions Summary
- Start at PSII with water splitting, releasing oxygen as a byproduct.
- Electron movement creates a hydrogen ion gradient for ATP synthesis.
- Electrons re-energized in PSI reduce NADP⁺ to NADPH.
- ATP and NADPH produced are used in the next stage, the Calvin cycle.
Key Terms & Definitions
- Photosynthesis — the process plants use to convert light, water, and CO₂ into food and oxygen.
- Chlorophyll — green pigment in plants that captures light energy.
- Photosystem — a complex of chlorophyll and proteins that initiate light reactions.
- Grana — stacks of thylakoids in chloroplasts where light-dependent reactions occur.
- ATP (Adenosine Triphosphate) — main energy carrier produced in photosynthesis.
- NADPH — electron and hydrogen carrier produced in the light reactions.
- Chemiosmosis — process where ion flow across a membrane produces ATP.
- Photolysis — splitting of water molecules during photosynthesis, releasing O₂.
Action Items / Next Steps
- Review the steps of light-dependent reactions and memorize key photosynthesis equations.
- Prepare for the next lesson about light-independent reactions (Calvin cycle).