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Light reactions and Calvin cycle in photosynthesis
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Photosynthesis: Process converting light energy into chemical energy
Overall equation:
6CO₂ + 6H₂O + Light → C₆H₁₂O₆ + 6O₂
Location: Chloroplasts
Two main stages:
Location: Thylakoid membrane
Key components:
Compare the light-dependent reactions and Calvin cycle: (a) where each occurs, (b) inputs and outputs of each, (c) the role of ATP and NADPH, and (d) why both are necessary for photosynthesis.
Photosynthesis Overview:
| Section | Format | Questions | Time | Weight | Calculator |
|---|---|---|---|---|---|
| Multiple Choice | MCQ | 60 | 90 min | 50% | 🚫 |
| Free Response (Long) | FRQ | 2 | 50 min | 30% | 🚫 |
| Free Response (Short) | FRQ | 4 | 40 min | 20% | 🚫 |
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Process:
PSII:
ETC between PSII and PSI:
PSI:
Chemiosmosis:
Products:
Location: Stroma
Three phases:
For one G3P (½ glucose):
For one glucose:
Problem: Hot, dry conditions cause stomata to close
Two stages: Light reactions + Calvin cycle
(a) Locations:
Light-Dependent Reactions:
Calvin Cycle (Light-Independent Reactions):
(b) Inputs and Outputs:
Light-Dependent Reactions:
Inputs:
Outputs:
Key reactions:
Calvin Cycle (1 turn makes ½ glucose):
Inputs:
Outputs:
Net for 1 glucose:
(c) Role of ATP and NADPH:
In Light Reactions:
ATP:
NADPH:
In Calvin Cycle:
ATP:
NADPH:
Ratio: 3 ATP : 2 NADPH (per CO₂)
(d) Why both are necessary:
Light Reactions provide energy:
Calvin Cycle fixes carbon:
Interdependence:
Light Reactions → ATP + NADPH → Calvin Cycle
↓
ADP + NADP⁺ ← G3P (glucose)
Cannot function independently:
Without light reactions:
Without Calvin cycle:
Both needed for complete photosynthesis:
Summary Table:
| Feature | Light Reactions | Calvin Cycle |
|---|---|---|
| Location | Thylakoid membrane | Stroma |
| Needs light? | Yes (directly) | No (indirectly) |
| Input | H₂O, ADP, NADP⁺ | CO₂, ATP, NADPH |
| Output | O₂, ATP, NADPH | G3P (glucose) |
| Purpose | Capture light energy | Fix carbon |
3 phases of Calvin Cycle:
Explain C4 and CAM photosynthesis as adaptations to hot, dry environments: (a) describe the problem with photorespiration in C3 plants, (b) explain how C4 plants avoid photorespiration (include anatomy and biochemistry), and (c) explain how CAM plants avoid water loss.
Photosynthetic Adaptations:
(a) Photorespiration problem in C3 plants:
RuBisCO's dual function:
Ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) can catalyze two reactions:
1. Carboxylation (desired): → Enters Calvin cycle → Makes glucose
2. Oxygenation (wasteful): → Photorespiration pathway → Wastes energy
Problem in hot, dry conditions:
Consequences:
Photorespiration pathway:
(b) C4 Photosynthesis - Spatial separation:
Anatomy - Kranz anatomy:
Two cell types in concentric rings:
1. Mesophyll cells (outer):
2. Bundle-sheath cells (inner):
Biochemistry:
Step 1: CO₂ fixation in mesophyll cells
PEP carboxylase (not RuBisCO!) fixes CO₂:
Key advantage:
Oxaloacetate → Malate or Aspartate (4-carbon compounds)
Step 2: Transport to bundle-sheath cells
Malate diffuses through plasmodesmata
Step 3: CO₂ release in bundle-sheath
Step 4: Calvin cycle
Released CO₂ enters Calvin cycle:
Step 5: Regeneration
Pyruvate returns to mesophyll → regenerates PEP
Energy cost:
C4 plants:
(c) CAM Photosynthesis - Temporal separation:
Crassulacean Acid Metabolism (CAM):
Temporal separation instead of spatial:
Night (stomata open):
Step 1: CO₂ uptake
Step 2: Store as malate
Step 3: Accumulate in vacuole
Day (stomata closed):
Step 4: Release malate from vacuole
Step 5: Decarboxylation
Step 6: Calvin cycle
Step 7: Regenerate PEP
Advantages:
Disadvantages:
CAM plants:
Comparison:
| Feature | C3 | C4 | CAM |
|---|---|---|---|
| CO₂ fixation | RuBisCO | PEP carboxylase | PEP carboxylase |
| Separation | None | Spatial (cells) | Temporal (day/night) |
| Photorespiration | High | Low | Low |
| Water use efficiency | Low | Medium | High |
Optimal conditions:
| Energy cost |
| 18 ATP |
| 30 ATP |
| 30 ATP |
| Growth rate | Fast | Fast | Slow |
| Examples | Rice, wheat, oak | Corn, sugarcane | Cactus, pineapple |
| % of species | 85% | 3% | 10% |