How to Use This Page
Solve each Samacheer Kalvi exercise question on your own first, then compare your answer with the solutions below. Photosynthesis covers the light reaction, Calvin cycle, C3 and C4 pathways, photorespiration, and factors affecting photosynthesis — focus on understanding each stage and comparing pathways.
Key Concepts (Quick Recap)
Overall Equation of Photosynthesis
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
(Light energy + Chlorophyll)
- The oxygen released comes from the splitting of water (photolysis), NOT from CO₂
- This was proven by Hill and later by Ruben et al. using isotopic oxygen (O¹⁸)
Light Reaction (Photochemical Phase)
Location: Thylakoid membranes (grana) of chloroplast Requirements: Light, chlorophyll, water, ADP, NADP⁺ Products: ATP, NADPH, O₂
| Step | Process | Details |
|---|
| Photolysis of water | 2H₂O → 4H⁺ + 4e⁻ + O₂ | Oxygen is released here; electrons go to PSII |
| Photosystem II (PSII) | Absorbs light at 680 nm (P680) | Excites electrons; passes them to ETC |
| Electron Transport Chain | Electrons pass from PSII → PQ → Cyt b₆f → PC → PSI | Energy released pumps H⁺ into thylakoid lumen |
| Photosystem I (PSI) | Absorbs light at 700 nm (P700) | Re-excites electrons |
| NADP⁺ reduction | NADP⁺ + 2H⁺ + 2e⁻ → NADPH + H⁺ | Final electron acceptor |
| Photophosphorylation | ADP + Pi → ATP (chemiosmosis) | H⁺ gradient drives ATP synthase |
Types of Photophosphorylation
| Feature | Non-cyclic | Cyclic |
|---|
| Electron flow | PSII → ETC → PSI → NADP⁺ | PSI → ETC → PSI (cycles back) |
| Products | ATP + NADPH + O₂ | ATP only |
| Water splitting | Yes (photolysis) | No |
| NADPH produced | Yes | No |
| Oxygen released | Yes | No |
| Occurs in | Green plants (main pathway) | When NADP⁺ is limited; bacteria |
Dark Reaction (Calvin Cycle / Biosynthetic Phase)
Location: Stroma of chloroplast Requirements: CO₂, ATP, NADPH (from light reaction), RuBisCO enzyme Product: Glucose (via G3P → C₆H₁₂O₆)
Three stages of the Calvin Cycle:
1. Carboxylation (CO₂ fixation):
Answer
- RuBisCO (RuBP carboxylase-oxygenase) catalyses CO₂ + RuBP (5C) → 2 × PGA (3C)
- First stable product: 3-PGA (3-phosphoglyceric acid) — a 3-carbon compound
2. Reduction:
Answer
- PGA is phosphorylated by ATP → 1,3-bisphosphoglycerate
- 1,3-bisphosphoglycerate is reduced by NADPH → G3P (glyceraldehyde-3-phosphate)
- G3P is the sugar product; some exits the cycle to form glucose
3. Regeneration:
Answer
- 5 out of 6 G3P molecules are used to regenerate RuBP
- RuBP is regenerated so the cycle can continue
- 3 CO₂ + 9 ATP + 6 NADPH → 1 G3P (net gain)
C3 vs C4 Plants
| Feature | C3 Plants | C4 Plants |
|---|
| First stable product | 3-PGA (3-carbon) | OAA (4-carbon oxaloacetic acid) |
| CO₂ fixation enzyme | RuBisCO | PEP carboxylase (mesophyll), then RuBisCO (bundle sheath) |
| Photorespiration | High (under hot, dry conditions) | Low/absent |
| Kranz anatomy | Absent | Present (bundle sheath cells around veins) |
| Mesophyll cells | Fix CO₂ via RuBisCO | Fix CO₂ via PEP carboxylase |
| Bundle sheath cells | Not involved in CO₂ fixation | Calvin cycle operates here |
| Optimal temperature | 20–25°C | 30–40°C |
| CO₂ compensation point | Higher (21–150 ppm) | Lower (0–10 ppm) |
| Water use efficiency | Lower | Higher |
| Examples | Rice, wheat, potato, soybean | Maize, sugarcane, sorghum, amaranth |
Kranz Anatomy (C4 Plants)
- Vascular bundles are surrounded by a ring of large bundle sheath cells
- Bundle sheath cells have large number of chloroplasts (agranelle type in mesophyll, granelle type in bundle sheath)
- Mesophyll cells fix CO₂ into OAA (via PEP carboxylase)
- OAA is converted to malate/aspartate and transported to bundle sheath cells
- CO₂ is released from malate in bundle sheath cells and enters the Calvin cycle (via RuBisCO)
- This CO₂ concentration mechanism suppresses photorespiration
Photorespiration
- Definition: RuBisCO acts as an oxygenase (binds O₂ instead of CO₂) when O₂ concentration is high
- Product: 2-phosphoglycolate (2C) + 3-PGA (3C) instead of 2 × 3-PGA
- Wasteful: No ATP or sugar is produced; energy is wasted
- Conditions: Hot, dry conditions (stomata close → O₂ builds up → CO₂ drops)
- C3 plants: High photorespiration
- C4 plants: Low/absent (CO₂ concentration mechanism prevents RuBisCO from binding O₂)
Factors Affecting Photosynthesis
External factors: 1. Light intensity: Rate increases with light up to a saturation point 2. CO₂ concentration: Rate increases with CO₂ up to a saturation point 3. Temperature: Optimum 25–35°C; decreases at very high or very low temperatures 4. Water: Essential for photolysis; water stress closes stomata
Internal factors: 1. Chlorophyll content: More chlorophyll → more photosynthesis 2. Leaf anatomy: Mesophyll cells, stomatal density 3. Age of leaf: Young and mature leaves photosynthesise more than old leaves
Blackman's Law of Limiting Factors: The rate of photosynthesis is limited by the factor that is in shortest supply (the limiting factor).
I. Choose the Best Answer (MCQs)
Q1. The oxygen released during photosynthesis comes from (a) CO₂ (b) H₂O (c) Glucose (d) ATP
Answer(b) H₂O — The oxygen released during photosynthesis comes from the photolysis of water (2H₂O → 4H⁺ + 4e⁻ + O₂), not from CO₂. This was proven using isotopic oxygen (O¹⁸) experiments.
Q2. The first stable product of the Calvin cycle is (a) Glucose (b) RuBP (c) 3-PGA (d) G3P
Answer(c) 3-PGA — The first stable product of CO₂ fixation in the Calvin cycle (C3 pathway) is 3-phosphoglyceric acid (3-PGA), a 3-carbon compound. CO₂ combines with RuBP (5C) to form an unstable 6C intermediate that splits into two 3-PGA molecules.
Q3. In C4 plants, the first CO₂ fixation product is (a) 3-PGA (b) RuBP (c) OAA (d) G3P
Answer(c) OAA — In C4 plants, CO₂ is first fixed by PEP carboxylase in mesophyll cells to form oxaloacetic acid (OAA), a 4-carbon compound. This is why they are called C4 plants.
Q4. The enzyme that catalyses CO₂ fixation in the Calvin cycle is (a) PEP carboxylase (b) RuBisCO (c) ATP synthase (d) Catalase
Answer(b) RuBisCO — RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) catalyses the fixation of CO₂ with RuBP in the Calvin cycle. It is the most abundant protein on Earth.
Q5. Kranz anatomy is found in (a) C3 plants (b) C4 plants (c) CAM plants (d) All plants
Answer(b) C4 plants — Kranz anatomy (wreath-like arrangement of bundle sheath cells around vascular bundles) is a characteristic feature of C4 plants. It allows efficient CO₂ concentration to suppress photorespiration.
Q6. Photorespiration occurs when RuBisCO binds (a) CO₂ (b) O₂ (c) NADPH (d) ATP
Answer(b) O₂ — Photorespiration occurs when RuBisCO acts as an oxygenase and binds O₂ instead of CO₂. This produces 2-phosphoglycolate (2C) instead of 2 × 3-PGA, wasting energy without producing sugar.
Q7. Non-cyclic photophosphorylation produces (a) ATP only (b) ATP and NADPH (c) NADPH only (d) ATP, NADPH, and O₂
Answer(d) ATP, NADPH, and O₂ — Non-cyclic photophosphorylation involves both PSII and PSI, and produces ATP (via chemiosmosis), NADPH (from NADP⁺ reduction), and O₂ (from photolysis of water).
II. Fill in the Blanks
1. The overall equation of photosynthesis is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. 2. The oxygen released during photosynthesis comes from water (H₂O). 3. The light reaction occurs in the thylakoid membranes (grana) of the chloroplast. 4. The dark reaction (Calvin cycle) occurs in the stroma of the chloroplast. 5. Photosystem II absorbs light at 680 nm (P680). 6. Photosystem I absorbs light at 700 nm (P700). 7. The enzyme that catalyses CO₂ fixation in the Calvin cycle is RuBisCO. 8. The first stable product of the C3 pathway is 3-PGA (3-phosphoglyceric acid). 9. The first stable product of the C4 pathway is OAA (oxaloacetic acid). 10. In C4 plants, CO₂ is first fixed in mesophyll cells by PEP carboxylase. 11. The Calvin cycle operates in bundle sheath cells of C4 plants. 12. Kranz anatomy is characteristic of C4 plants. 13. Photorespiration is a wasteful process where RuBisCO binds O₂ instead of CO₂. 14. Cyclic photophosphorylation produces only ATP (no NADPH or O₂). 15. The law of limiting factors for photosynthesis was proposed by Blackman.
III. True or False (with corrections for false statements)
Q1. The oxygen released during photosynthesis comes from CO₂.
AnswerFalse — Correction: The oxygen released during photosynthesis comes from WATER (H₂O), not from CO₂. This was proven by Hill and by Ruben et al. using O¹⁸ isotope experiments.
Q2. The dark reaction can occur in the absence of light.
AnswerTrue — The dark reaction (Calvin cycle) does not directly require light. However, it depends on ATP and NADPH produced by the light reaction, so it cannot continue indefinitely in the dark.
Q3. C4 plants show photorespiration.
AnswerFalse — Correction: C4 plants have a CO₂ concentration mechanism (via Kranz anatomy) that maintains high CO₂ levels in bundle sheath cells. This prevents RuBisCO from binding O₂, so C4 plants have very LOW or NO photorespiration. C3 plants show significant photorespiration.
Q4. RuBisCO is the most abundant protein on Earth.
AnswerTrue — RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) is the most abundant protein on Earth. It makes up about 16–25% of the soluble protein in leaf tissue.
Q5. Cyclic photophosphorylation produces NADPH.
AnswerFalse — Correction: Cyclic photophosphorylation produces only ATP. It does NOT produce NADPH or O₂. Non-cyclic photophosphorylation produces ATP, NADPH, and O₂.
IV. Match the Following
| Column A | Column B | Answer |
|---|
| 1. Light reaction | (a) CO₂ fixation via RuBisCO | (b) Occurs in thylakoids; produces ATP, NADPH |
| 2. Dark reaction (Calvin cycle) | (b) Occurs in thylakoids; produces ATP, NADPH | (a) Occurs in stroma; fixes CO₂ |
| 3. C3 pathway | (c) CO₂ fixation via PEP carboxylase; Kranz anatomy | (d) First product is 3-PGA |
| 4. C4 pathway | (d) First product is 3-PGA | (c) First product is OAA; Kranz anatomy |
| 5. Photorespiration | (e) Produces only ATP | (f) RuBisCO binds O₂; wastes energy |
V. Answer Briefly (2-mark questions)
Q1. Differentiate between light reaction and dark reaction.
Answer
| Feature | Light Reaction | Dark Reaction (Calvin Cycle) | |---|---|---| | Location | Thylakoid membranes (grana) | Stroma | | Light required | Yes (directly) | No (but depends on light reaction products) | | Products | ATP, NADPH, O₂ | Glucose (via G3P) | | Reactants | H₂O, ADP, NADP⁺ | CO₂, ATP, NADPH | | Enzymes | Photosystems, ATP synthase | RuBisCO and others |
Q2. What is photorespiration? Why is it wasteful?
Answer
Photorespiration is a process where RuBisCO acts as an oxygenase and binds O₂ instead of CO₂, producing 2-phosphoglycolate (2C) + 3-PGA (3C) instead of two molecules of 3-PGA. It is wasteful because: 1. No sugar (glucose) is produced 2. ATP is consumed but no energy is gained 3. The 2-carbon phosphoglycolate must be recycled, wasting more energy 4. It reduces the efficiency of photosynthesis by up to 25% in C3 plants
Q3. What are the differences between C3 and C4 plants?
Answer
| Feature | C3 Plants | C4 Plants | |---|---|---| | First stable product | 3-PGA (3-carbon) | OAA (4-carbon) | | CO₂ fixation enzyme | RuBisCO | PEP carboxylase (mesophyll), RuBisCO (bundle sheath) | | Photorespiration | High | Low/absent | | Kranz anatomy | Absent | Present | | Optimal temperature | 20–25°C | 30–40°C | | Water use efficiency | Lower | Higher | | Examples | Rice, wheat, potato | Maize, sugarcane, sorghum |
Q4. What is Blackman's Law of Limiting Factors?
Answer
Blackman's Law of Limiting Factors states that the rate of a physiological process (like photosynthesis) is limited by the factor that is in shortest supply (the limiting factor), regardless of how favourable other factors are. For example, if light intensity and temperature are optimal but CO₂ concentration is low, the rate of photosynthesis will be limited by CO₂ and will not increase even if light or temperature is increased further.
VI. Answer in Detail (5-mark questions)
Q1. Describe the light reaction of photosynthesis.
Answer
The light reaction (photochemical phase) occurs in the thylakoid membranes (grana) of the chloroplast. It requires light, chlorophyll, water, ADP, and NADP⁺. It produces ATP, NADPH, and O₂. Step 1: Absorption of Light - Chlorophyll and other pigments in Photosystem II (P680) and Photosystem I (P700) absorb light energy - This excites electrons in the chlorophyll molecules Step 2: Photolysis of Water - Water molecules are split in the presence of light and manganese-containing oxygen-evolving complex - 2H₂O → 4H⁺ + 4e⁻ + O₂ - The released O₂ is the oxygen we breathe - Electrons replace those lost by PSII Step 3: Electron Transport (Non-cyclic) - PSII (P680) absorbs light → electrons are excited - Electrons pass through: PSII → Plastoquinone (PQ) → Cytochrome b₆f complex → Plastocyanin (PC) → PSI - Energy released pumps H⁺ ions from stroma into thylakoid lumen (creating proton gradient) - PSI (P700) absorbs light → electrons are re-excited - Electrons are finally accepted by NADP⁺ → NADPH + H⁺ Step 4: Photophosphorylation (ATP synthesis) - The H⁺ gradient across the thylakoid membrane drives ATP synthase - H⁺ ions flow back from lumen to stroma through ATP synthase - ADP + Pi → ATP (chemiosmosis) Products of Non-cyclic Light Reaction: - 2 H₂O → O₂ + 4 H⁺ (photolysis) - NADP⁺ + 2H⁺ + 2e⁻ → NADPH + H⁺ - ADP + Pi → ATP Cyclic Photophosphorylation: - Involves only PSI (P700) - Electrons cycle back from PSI → ETC → PSI - Produces only ATP (no NADPH, no O₂) - Occurs when NADP⁺ is not available or when additional ATP is needed
Q2. Describe the Calvin cycle (dark reaction).
Answer
The Calvin cycle (C3 pathway) occurs in the stroma of the chloroplast. It uses ATP and NADPH from the light reaction to fix CO₂ into glucose. It was discovered by Melvin Calvin, Benson, and Bassham using radioactive C¹⁴. Three stages of the Calvin Cycle: 1. Carboxylation (CO₂ fixation): - RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) catalyses the reaction - CO₂ (1C) combines with RuBP (5C) → unstable 6C intermediate - The 6C intermediate immediately splits into two molecules of 3-PGA (3C) - This is the first stable product of the C3 pathway Reaction: CO₂ + RuBP (5C) → 2 × 3-PGA (3C) 2. Reduction: - Each 3-PGA is phosphorylated by ATP: 3-PGA + ATP → 1,3-bisphosphoglycerate + ADP - 1,3-bisphosphoglycerate is reduced by NADPH: 1,3-bisphosphoglycerate + NADPH → G3P (glyceraldehyde-3-phosphate) + NADP⁺ + Pi - For 3 CO₂ molecules: 6 G3P are produced - 5 G3P are used to regenerate RuBP; 1 G3P is the net gain 3. Regeneration of RuBP: - 5 G3P (15C total) are converted back to 3 RuBP (3 × 5C = 15C) - This requires 3 ATP molecules - RuBP is regenerated so the cycle can continue Net equation (for 3 CO₂): 3CO₂ + 6NADPH + 9ATP → G3P + 6NADP⁺ + 9ADP + 9Pi For 1 molecule of glucose: 6CO₂ + 12NADPH + 18ATP → C₆H₁₂O₆ + 12NADP⁺ + 18ADP + 18Pi Summary: - Input: 6 CO₂ + 18 ATP + 12 NADPH (from light reaction) - Output: 1 glucose (C₆H₁₂O₆) - Enzyme: RuBisCO (most abundant protein on Earth)
Q3. Describe the C4 pathway (Hatch-Slack pathway).
Answer
The C4 pathway (Hatch-Slack pathway) is a CO₂ concentration mechanism found in tropical plants (maize, sugarcane, sorghum) that minimises photorespiration. Kranz Anatomy (characteristic of C4 plants): - Vascular bundles are surrounded by a wreath-like arrangement of bundle sheath cells - Bundle sheath cells are large with thick walls and many chloroplasts - Mesophyll cells surround the bundle sheath cells - This dual-cell system allows spatial separation of CO₂ fixation and the Calvin cycle Steps of the C4 Pathway: Step 1: CO₂ fixation in Mesophyll Cells - CO₂ enters mesophyll cells through stomata - PEP carboxylase fixes CO₂ with PEP (3C) → OAA (Oxaloacetic acid, 4C) - PEP carboxylase has high affinity for CO₂ and no oxygenase activity Reaction: CO₂ + PEP (3C) → OAA (4C) Step 2: Conversion and Transport - OAA is converted to malate (or aspartate depending on the plant) - Malate is transported from mesophyll cells to bundle sheath cells through plasmodesmata Step 3: Decarboxylation in Bundle Sheath Cells - Malate is decarboxylated (CO₂ is released) in bundle sheath cells - This releases CO₂ right near RuBisCO, maintaining high CO₂ concentration - The 3C product (pyruvate) returns to mesophyll cells Step 4: Calvin Cycle in Bundle Sheath Cells - CO₂ is fixed by RuBisCO in the Calvin cycle (same as C3 pathway) - High CO₂ concentration in bundle sheath cells suppresses photorespiration - RuBisCO acts as a carboxylase (not oxygenase) because CO₂ is abundant Step 5: Regeneration of PEP - Pyruvate (3C) returns to mesophyll cells - Pyruvate is phosphorylated by ATP → PEP (3C) - PEP is ready to fix another CO₂ Advantages of C4 Pathway: 1. PEP carboxylase has high affinity for CO₂ (even at low CO₂) 2. CO₂ concentration mechanism suppresses photorespiration 3. Higher photosynthetic efficiency at high temperatures (30–40°C) 4. Better water use efficiency (need less water per unit CO₂ fixed) 5. Higher productivity in hot, dry climates Disadvantages: 1. Requires more ATP (2 extra ATP per CO₂ fixed) 2. Kranz anatomy is required (not found in all plants)
Common Mistakes to Avoid
- Saying O₂ comes from CO₂: The O₂ released during photosynthesis comes from WATER (photolysis of water), NOT from CO₂.
- Confusing light reaction and dark reaction products: Light reaction produces ATP, NADPH, and O₂. Dark reaction uses these to produce glucose (via G3P).
- Confusing C3 and C4 first products: C3 first product = 3-PGA (3 carbon). C4 first product = OAA (4 carbon).
- Confusing RuBisCO and PEP carboxylase: RuBisCO fixes CO₂ in the Calvin cycle (C3 and in bundle sheath cells of C4). PEP carboxylase fixes CO₂ in mesophyll cells of C4 plants.
- Saying dark reaction doesn't need light at all: Dark reaction doesn't directly need light, but it depends on ATP and NADPH from the light reaction. Without light, these are not produced, and the dark reaction stops.
- Confusing cyclic and non-cyclic photophosphorylation: Cyclic = only ATP (no NADPH, no O₂). Non-cyclic = ATP + NADPH + O₂.
- Confusing photorespiration and respiration: Photorespiration is a wasteful process where RuBisCO binds O₂ (produces no ATP). Respiration is the breakdown of glucose to produce ATP.
Exam Checklist
- [ ] Can you write the overall equation of photosynthesis?
- [ ] Can you explain the light reaction with all steps (PSII, ETC, PSI, photolysis, ATP, NADPH)?
- [ ] Can you differentiate cyclic and non-cyclic photophosphorylation?
- [ ] Can you describe the three stages of the Calvin cycle?
- [ ] Can you compare C3 and C4 plants (at least 6 points)?
- [ ] Can you explain the C4 pathway (Hatch-Slack pathway) with steps?
- [ ] Can you explain Kranz anatomy?
- [ ] Can you define photorespiration and explain why it is wasteful?
- [ ] Can you list the factors affecting photosynthesis?
- [ ] Can you state Blackman's Law of Limiting Factors?
