What is Photosynthesis?
Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy stored in glucose.
Learning Goals:
- Describe the light-dependent reactions and where they occur.
- Explain the Calvin cycle and carbon fixation.
- Compare photosynthesis and respiration.
Two Stages
| Stage | Location | Requires Light? | Inputs | Outputs |
|---|---|---|---|---|
| Light-dependent | Thylakoid membranes | ✅ Yes | H₂O, light, NADP⁺, ADP | O₂, ATP, NADPH |
| Light-independent (Calvin cycle) | Stroma | ❌ No | CO₂, ATP, NADPH | G3P → Glucose |
Light-Dependent Reactions
- Light hits Photosystem II → water is split (photolysis):
- Excited electrons pass through the electron transport chain → ATP is made (photophosphorylation).
- Electrons reach Photosystem I → re-energized by light → reduce NADP⁺ to NADPH.
Calvin Cycle (Light-Independent)
Three stages per turn:
- Carbon fixation: CO₂ + RuBP → 2 GP (catalyzed by RuBisCO)
- Reduction: GP → G3P (uses ATP and NADPH)
- Regeneration: G3P → RuBP (uses ATP)
3 turns of the Calvin cycle fix 3 CO₂ and produce 1 G3P (net). 6 turns → 1 glucose.
Photosynthesis vs Respiration
| Feature | Photosynthesis | Respiration |
|---|---|---|
| Energy | Absorbs light | Releases chemical energy |
| Reactants | CO₂ + H₂O | Glucose + O₂ |
| Products | Glucose + O₂ | CO₂ + H₂O |
| Location | Chloroplasts | Mitochondria |
| When | Light only | Always |
Worked Examples
Example 1: Why do plants respire at night?
Plants still need ATP for cellular processes. Without light, photosynthesis stops, so plants rely entirely on cellular respiration (consuming stored glucose + O₂).
Example 2: Compensation Point
The compensation point is when the rate of photosynthesis = rate of respiration → net gas exchange = zero. Above this light intensity, net O₂ is released.
Common Mistakes
- "Plants don't respire" — Plants respire 24/7. Photosynthesis only occurs in light.
- Saying Calvin cycle is "dark reactions" — It doesn't require darkness; it just doesn't directly use light. It needs ATP and NADPH from light reactions.
- Confusing photolysis with the Calvin cycle — Photolysis splits water in thylakoids; carbon fixation occurs in the stroma.
Exam Tips
- Always specify where each reaction occurs (thylakoid membrane vs stroma).
- RuBisCO is the world's most abundant enzyme — and the key to carbon fixation.
- The oxygen released comes from water (photolysis), not CO₂.
Related Topics
- Cellular Respiration — The reverse metabolic pathway.
- Limiting Factors — What controls the rate of photosynthesis.
- Enzyme Kinetics — RuBisCO and other photosynthetic enzymes.