Overview
This lecture covers the two main stages of photosynthesis—light-dependent and light-independent (Calvin cycle) reactions—explaining their processes, locations, importance, inputs and outputs, and the overall significance of photosynthesis for life on Earth.
Review of Photosynthesis and Its Importance
- Photosynthesis is how plants make food (glucose) from water, carbon dioxide, and sunlight, producing oxygen as a byproduct.
- The overall chemical equation: 6CO₂ + 6H₂O + sunlight → C₆H₁₂O₆ (glucose) + 6O₂.
- Photosynthesis is essential for life and ecological balance on Earth, supporting all living organisms.
Structure Involved in Photosynthesis
- Key leaf parts: cuticle, mesophyll, stomata, and epidermis are vital in photosynthesis.
- Chloroplast is the main organelle for photosynthesis; contains green pigment chlorophyll which absorbs light energy.
- Thylakoids (pancake-like structures) and stroma (fluid area) are chloroplast components involved in different stages.
Light-Dependent Reaction (Light Reaction)
- Occurs in thylakoid membranes; requires light and water as reactants.
- Chlorophyll absorbs light, exciting electrons and driving the electron transport chain.
- Water is split (photolysis) into oxygen (released) and hydrogen ions.
- ATP and NADPH (energy carriers) are produced, used in the next stage.
- Photosystem I and II absorb light and transfer electrons, replenished by splitting water.
- ATP synthase uses hydrogen ion gradient to produce ATP.
Light-Independent Reaction (Dark Reaction/Calvin Cycle)
- Occurs in the stroma; does not require light but uses ATP and NADPH from the light reaction.
- Uses carbon dioxide and hydrogen (from water) to build glucose (C₆H₁₂O₆).
- Four stages: carbon fixation (CO₂ fixed by RUBP), reduction (producing G3P using ATP and NADPH), carbohydrate formation (G3P becomes glucose), and regeneration (G3P recycled to RUBP).
- Glucose is assembled from G3P molecules produced in the cycle.
Sequence and Integration
- Light reaction inputs: light, H₂O; outputs: O₂, ATP, NADPH.
- Calvin cycle inputs: CO₂, ATP, NADPH; outputs: glucose, ADP, NADP⁺ (which recycle back).
- No light = no light reaction; no CO₂ = Calvin cycle stops; no H₂O = no oxygen produced.
Key Terms & Definitions
- Photosynthesis — process by which plants make food from CO₂, H₂O, and sunlight.
- Chloroplast — plant cell organelle where photosynthesis occurs.
- Chlorophyll — pigment that absorbs light for photosynthesis.
- Thylakoid — chloroplast membrane sacs where light-dependent reactions occur.
- Stroma — fluid area of chloroplast where Calvin cycle (dark reaction) occurs.
- ATP (Adenosine Triphosphate) — energy-carrying molecule produced in light reactions.
- NADPH — high-energy electron carrier produced in light reactions.
- Calvin Cycle — series of reactions that produce glucose from CO₂ using ATP and NADPH.
- Photolysis — splitting of water molecules using light energy.
Action Items / Next Steps
- Review the photosynthesis equation and identify reactants/products for each stage.
- Draw a basic flowchart illustrating inputs and outputs of both reactions.
- Prepare for class discussion on the effects of removing a photosynthesis reactant.