Chapter 10 — Photosynthesis
Key Concepts Quick Recap
| Term | Definition |
|---|
| Photosynthesis | Process by which chlorophyll-containing organisms use light energy to synthesise organic compounds from CO₂ and H₂O |
| Chlorophyll a | Primary photosynthetic pigment; absorbs red and blue light; releases excited electrons |
| Photosystem I (PS I) | P700; cyclic and non-cyclic phosphorylation; produces NADPH |
| Photosystem II (PS II) | P680; photolysis of water; releases O₂; passes electrons to PS I via ETC |
| Photolysis | Splitting of water by light: 2H₂O → 4H⁺ + 4e⁻ + O₂ |
| Calvin cycle | Dark reaction (light-independent); CO₂ fixation → G3P → glucose; occurs in stroma |
| RuBisCO | Ribulose bisphosphate carboxylase-oxygenase; enzyme for CO₂ fixation in Calvin cycle |
| C3 plants | First stable product = 3-PGA (3C); mesophyll chloroplasts; e.g. Rice, Wheat |
| C4 plants | First stable product = OAA (4C); bundle sheath + mesophyll; e.g. Maize, Sugarcane |
| Photorespiration | Wasteful process in C3 plants; RuBisCO fixes O₂ instead of CO₂; loses fixed carbon |
| CAM plants | Crassulacean Acid Metabolism; CO₂ fixed at night as malic acid; stomata open at night |
I. Choose the Best Answer (MCQs)
1. The site of light reaction (photochemical phase) in chloroplasts is: (a) Stroma (b) Thylakoid membrane (grana) (c) Outer membrane (d) Intermembrane space
Answer(b) Thylakoid membrane (grana) Light reactions occur on the thylakoid membranes (grana) of chloroplasts. Photosystems I and II, electron transport chain, and ATP synthase are embedded in thylakoid membranes. Dark reactions (Calvin cycle) occur in the stroma.
2. Oxygen released during photosynthesis comes from: (a) CO₂ (b) Glucose (c) H₂O (d) Chlorophyll
Answer(c) H₂O Oxygen is released by photolysis of water: 2H₂O → 4H⁺ + 4e⁻ + O₂. This was confirmed by using ¹⁸O-labelled water (H₂¹⁸O) — the ¹⁸O appeared in O₂ released, not in CO₂ used. (Van Niel's hypothesis, confirmed by Ruben and Kamen using radioactive tracers.)
3. The first stable product of CO₂ fixation in C3 plants is: (a) Oxaloacetic acid (OAA) (b) Malic acid (c) 3-Phosphoglyceric acid (3-PGA) (d) Ribulose bisphosphate (RuBP)
Answer(c) 3-Phosphoglyceric acid (3-PGA) In C3 plants, RuBisCO fixes CO₂ onto RuBP (5C) → 2 molecules of 3-PGA (3C). This is the Calvin cycle. 3-PGA is the first stable product. In C4 plants, first product is OAA (4C).
4. The number of CO₂ molecules required to produce one molecule of glucose in the Calvin cycle is: (a) 3 (b) 6 (c) 12 (d) 18
Answer(b) 6 6CO₂ + 18 ATP + 12 NADPH → 1 glucose (C₆H₁₂O₆). The Calvin cycle requires 3 turns to fix 3 CO₂ → 1 G3P; 6 turns (6 CO₂) → 2 G3P → 1 glucose.
5. Which pigment is NOT found in chloroplasts? (a) Chlorophyll a (b) Chlorophyll b (c) Carotenoids (d) Phycoerythrin
Answer(d) Phycoerythrin Phycoerythrin is a red pigment found in red algae (Rhodophyta) — NOT in chloroplasts of higher plants. Chloroplasts of higher plants contain: chlorophyll a (primary), chlorophyll b, carotene (orange), xanthophyll (yellow).
6. C4 pathway was discovered by: (a) Calvin and Benson (b) Hatch and Slack (c) Krebs and Johnson (d) Hill and Bendall
Answer(b) Hatch and Slack M.D. Hatch and C.R. Slack (1966) discovered the C4 pathway in sugarcane. It is also called the Hatch-Slack pathway. Calvin and Benson discovered the C3 (Calvin) cycle. Hill discovered the light reaction (Hill reaction).
7. The law of limiting factors in photosynthesis was given by: (a) Blackman (b) Engelman (c) Warburg (d) Emerson
Answer(a) Blackman F.F. Blackman's Law of Limiting Factors (1905): "When a process is conditioned as to its rapidity by a number of factors, the rate of the process is limited by the pace of the slowest factor." If light is abundant but CO₂ is low, the rate is limited by CO₂.
II. Short Answer Questions
Q1. Distinguish between PS I and PS II.
Answer
| Feature | Photosystem I (PS I) | Photosystem II (PS II) |
|---|
| Reaction centre | P700 (absorbs 700 nm light) | P680 (absorbs 680 nm light) |
| Location | Unstacked thylakoids (stroma lamellae) | Stacked thylakoids (grana) |
| Function | Produces NADPH (non-cyclic); produces ATP (cyclic) | Photolysis of water; releases O₂; passes e⁻ to PS I |
| Electron donor | Plastocyanin (from PS II via ETC) | Water (H₂O) |
| End product | NADPH, ATP | O₂; transfers electrons |
| Cyclic phosphorylation | Yes | No |
Q2. Describe the Calvin cycle (C3 cycle).
Answer
The Calvin cycle (dark reaction/light-independent reaction) occurs in the stroma. It was worked out by M. Calvin, A. Benson, and J. Bassham using ¹⁴C-labelled CO₂.
Three stages:
1. Carboxylation (CO₂ fixation): CO₂ + RuBP (5C) → [unstable 6C] → 2 × 3-PGA (3C) Enzyme: RuBisCO
2. Reduction: 3-PGA + ATP + NADPH → G3P (glyceraldehyde-3-phosphate) G3P = the first carbohydrate produced (3C)
3. Regeneration of RuBP: 5 G3P + 3 ATP → 3 RuBP (5C) RuBP is regenerated for the next cycle.
Net gain per 3 CO₂:
- 1 G3P produced (extra, after regeneration)
- 2 G3P → 1 glucose
Requirements per glucose:
Q3. Compare C3 and C4 plants.
Answer
| Feature | C3 Plants | C4 Plants |
|---|
| First product | 3-PGA (3C) | OAA (4C) |
| CO₂ fixation enzyme | RuBisCO (in mesophyll) | PEP carboxylase (in mesophyll) |
| Kranz anatomy | Absent | Present (bundle sheath cells) |
| Photorespiration | High (wastes 25-50% fixed C) | Negligible (CO₂ concentrated in bundle sheath) |
| Efficiency | Less efficient in high light, high temperature | More efficient in hot, sunny conditions |
| Optimum temperature | 20-25°C | 30-45°C |
| Examples | Rice, Wheat, Potato, Sunflower | Maize, Sugarcane, Sorghum, Amaranthus |
Why C4 plants lack photorespiration: CO₂ is fixed as OAA in mesophyll, transported to bundle sheath as malic acid/aspartate → released as CO₂ at high concentration → RuBisCO works in carboxylase mode (fixes CO₂, not O₂) → no photorespiration.
III. Worked Examples
Example 1 — Calculating ATP and NADPH needs
To produce 1 glucose (6C) by photosynthesis:
Light reaction produces: 18 ATP + 12 NADPH (consumed by Calvin cycle)
Calvin cycle:
6 CO₂ → fixation → 12 G3P produced → 10 G3P regenerate RuBP → 2 G3P left
2 G3P → combine → 1 Glucose
Requirements:
Carboxylation: 6 CO₂ + 6 RuBP → 12 × 3-PGA
Reduction: 12 × 3-PGA + 12 ATP + 12 NADPH → 12 G3P
Regeneration: 10 G3P + 6 ATP → 6 RuBP
Total: 18 ATP + 12 NADPH per glucose
Common Mistakes
- Saying O₂ comes from CO₂ — it comes from H₂O (photolysis).
- Confusing PS I (P700) and PS II (P680) — remember P680 photolysis water; P700 → NADPH.
- Saying RuBisCO is in PS II — RuBisCO is a stromal enzyme (Calvin cycle).
- Forgetting that C4 plants still use the Calvin cycle — they just pre-concentrate CO₂.
- Confusing photorespiration (wasteful; O₂ fixation by RuBisCO) with normal respiration.
Exam Checklist
- Photosynthesis equation: 6CO₂ + 12H₂O + light → C₆H₁₂O₆ + 6O₂ + 6H₂O.
- O₂ from photolysis of water (PS II, P680).
- Calvin cycle: CO₂ → 3-PGA → G3P → glucose; RuBisCO; 6CO₂, 18ATP, 12NADPH per glucose.
- C4: first product OAA; Hatch-Slack pathway; bundle sheath cells; no photorespiration.
- PS I vs PS II table (P700/P680, products, location).
- Blackman's Law of Limiting Factors.
