NEET Biology Photosynthesis in Higher Plants 2027 — Light & Dark Reactions, C3-C4-CAM Pathways and 40 Practice MCQs

Last Updated: April 2026

NEET photosynthesis 2027 aspirants take note — Plant Physiology contributes 8–10% of NEET Biology, and within it, “Photosynthesis in Higher Plants” (Class 11 NCERT, Chapter 13) alone yields 2–4 questions every year in NEET. The 2024 NEET-UG paper had 3 direct questions from this chapter (light reaction, C4 cycle, photorespiration). The 2026 NTA cutoff for unreserved candidates was 720 → 162, meaning even 1 wrong answer here can cost a 200-rank slide. This guide covers light & dark reactions, C3-C4-CAM pathways, photorespiration, and 40 NEET-grade MCQs with full solutions.

1. Site of Photosynthesis: The Chloroplast

Photosynthesis occurs in the chloroplast, a double-membrane plastid containing flattened sacs called thylakoids. Stacks of thylakoids form grana, embedded in a fluid stroma. Chlorophyll-a (P680, P700), chlorophyll-b, carotenoids and xanthophylls reside in thylakoid membranes as light-harvesting complexes.

Pigments and Absorption Spectra

• Chlorophyll-a: blue-violet (430 nm) and red (662 nm). Primary photosynthetic pigment.
• Chlorophyll-b: 453 nm and 642 nm. Accessory.
• Carotenoids: protect chlorophyll from photo-oxidation; absorb in 400–500 nm range.
• Action spectrum mirrors absorption spectrum of chlorophyll-a — proves chl-a is the chief pigment.

2. Light Reaction (Photochemical Phase)

Occurs in thylakoid membranes. Two photosystems work in tandem in the Z-scheme:

Photosystem II (P680)

Absorbs 680 nm light → excited electron travels down ETC (PQ → Cyt b6f → PC) to PS-I. Lost electron is replaced by photolysis of water: 2H₂O → 4H⁺ + O₂ + 4e⁻ (occurs on lumen side, catalysed by Mn-containing oxygen-evolving complex).

Photosystem I (P700)

Absorbs 700 nm light → excited electron passes via Fd to NADP⁺ reductase, reducing NADP⁺ to NADPH.

Outputs of Light Reaction

Per pair of water molecules split: 1 O₂ + ~3 ATP + 2 NADPH. Cyclic photophosphorylation (PS-I only, no NADPH, no O₂) makes extra ATP when needed.

3. Dark Reaction — Calvin Cycle (C3 Pathway)

Occurs in stroma. Three phases:

1. Carboxylation: CO₂ + RuBP → 2 PGA (3-C). Catalysed by RuBisCO (most abundant enzyme on Earth).
2. Reduction: PGA → G3P using ATP + NADPH (2 ATP + 2 NADPH per CO₂ reduction step).
3. Regeneration: 5 G3P → 3 RuBP using 3 ATP.

Net per CO₂ fixed: 3 ATP + 2 NADPH consumed. To make 1 glucose (6 CO₂): 18 ATP + 12 NADPH.

4. C4 Pathway (Hatch–Slack Cycle)

Tropical grasses (maize, sugarcane, sorghum) use C4. Two cell types: mesophyll (PEP-carboxylase fixes CO₂ to OAA → malate) and bundle sheath (Kranz anatomy; malate releases CO₂ for the standard Calvin cycle). PEP-case has no oxygenase activity → no photorespiration.

5. CAM Pathway (Crassulacean Acid Metabolism)

Succulents (cactus, pineapple, Bryophyllum). Stomata open at night → CO₂ fixed as malate (stored in vacuole). Day: stomata closed, malate decarboxylated, CO₂ enters Calvin cycle. Adaptation to arid environments — extreme water-use efficiency.

6. C3 vs C4 vs CAM — Master Comparison Table

7. Photorespiration (C2 Cycle)

When O₂:CO₂ ratio is high (hot day, closed stomata), RuBisCO acts as oxygenase — RuBP + O₂ → 1 PGA + 1 phosphoglycolate. Phosphoglycolate is metabolised in chloroplast → peroxisome → mitochondrion, releasing CO₂. Wasteful process (no ATP, no sugar). Absent in C4 plants.

8. Limiting Factors (Blackman’s Law)

• Light intensity: linear up to saturation point (~10% of full sunlight for most C3).
• CO₂: major limiting factor; doubling CO₂ from 360 ppm → 720 ppm raises C3 yield ~30%.
• Temperature: C4 optimum ~35 °C, C3 optimum ~25 °C.
• Water stress: closes stomata → CO₂ supply cuts off.

9. 40 NEET-Grade MCQs with Solutions

1. Q1. The site of photolysis of water is — (a) Stroma (b) Outer membrane (c) Thylakoid lumen (d) Cytosol. Ans: (c). Oxygen-evolving complex faces lumen.
2. Q2. Number of ATP molecules required to synthesise one molecule of glucose in C3 plants is — (a) 18 (b) 12 (c) 24 (d) 36. Ans: (a) 18.
3. Q3. Z-scheme of electron transport during photosynthesis was given by — (a) Hill (b) Calvin (c) Hatch & Slack (d) Robert Hill (Z scheme summarised by Hill et al.). Ans: (d).
4. Q4. Photorespiration is favoured by — (a) Low O₂ (b) High O₂ & high temp (c) Low light (d) High CO₂. Ans: (b).
5. Q5. The first stable product of C4 cycle is — (a) PGA (b) OAA (c) RuBP (d) Malate. Ans: (b).
6. Q6. Bundle sheath cells of C4 plants — (a) Lack chlorophyll (b) Lack PS-II (c) Have grana only (d) Lack chloroplasts. Ans: (b). Hence reduced O₂ generation, reduced photorespiration.
7. Q7. Quantasomes are — (a) Units in stroma (b) Units in thylakoid membrane (c) DNA loops (d) Ribosomes. Ans: (b).
8. Q8. Chlorophyll molecule contains — (a) Fe (b) Mg (c) Mn (d) Cu. Ans: (b).
9. Q9. Hill reaction proved — (a) O₂ comes from CO₂ (b) O₂ comes from H₂O (c) Photosynthesis needs Mg (d) Both A & C. Ans: (b).
10. Q10. Cyclic photophosphorylation produces — (a) ATP only (b) NADPH only (c) ATP + NADPH (d) ATP + O₂. Ans: (a).
11. Q11. Stomata of CAM plants open at — (a) Day (b) Night (c) Both (d) Neither. Ans: (b).
12. Q12. Most efficient CO₂ fixer is — (a) RuBisCO (b) PEP-carboxylase (c) Carbonic anhydrase (d) Hexokinase. Ans: (b). PEP-case has higher affinity, no oxygenase activity.
13. Q13. Action spectrum closely resembles absorption spectrum of — (a) Chl-a (b) Chl-b (c) Carotenoid (d) Xanthophyll. Ans: (a).
14. Q14. Number of CO₂ needed for one glucose in C4 plants — (a) 6 (b) 12 (c) 18 (d) 24. Ans: (a).
15. Q15. Red Drop was overcome by — (a) Hill reaction (b) Emerson’s Enhancement effect (c) Calvin cycle (d) Photolysis. Ans: (b).
16. Q16. Which is NOT a feature of light reaction — (a) ATP synthesis (b) NADPH synthesis (c) CO₂ fixation (d) O₂ evolution. Ans: (c).
17. Q17. Compensation point — (a) Net CO₂ uptake = 0 (b) Photosynthesis = 0 (c) Respiration = 0 (d) Both photosynthesis & respiration max. Ans: (a).
18. Q18. Chlorophyll-a peak absorption is at — (a) 430 & 662 nm (b) 453 & 642 nm (c) 500 nm (d) 700 nm only. Ans: (a).
19. Q19. CAM plants store CO₂ overnight as — (a) Malate (b) Pyruvate (c) Glucose (d) Citrate. Ans: (a).
20. Q20. Photosystem II is sensitive to — (a) Cu (b) Mn (c) Mg (d) Fe (Mn cluster oxidises water). Ans: (b).
21. Q21. Calvin cycle was elucidated using — (a) ³²P (b) ¹⁴C (c) ¹⁵N (d) ³H. Ans: (b). Calvin used ¹⁴CO₂ + Chlorella.
22. Q22. Chemiosmotic hypothesis was given by — (a) Calvin (b) Mitchell (c) Hill (d) Hatch. Ans: (b).
23. Q23. Quantum requirement of photosynthesis is — (a) 4 (b) 8 (c) 12 (d) 16 photons per O₂. Ans: (b).
24. Q24. Carotenoids protect chlorophyll from — (a) Heat (b) Photo-oxidation (c) Cold (d) Drought. Ans: (b).
25. Q25. ATP synthase complex is also called — (a) F0-F1 particle (b) Cyt b6f (c) PS-I (d) PQ. Ans: (a).
26. Q26. Granum is — (a) Stack of thylakoids (b) Stroma lamella (c) DNA (d) Lipid body. Ans: (a).
27. Q27. Chloroplast DNA is — (a) Linear (b) Circular (c) Branched (d) Absent. Ans: (b).
28. Q28. Which is C4 plant — (a) Wheat (b) Maize (c) Rice (d) Pea. Ans: (b).
29. Q29. Light compensation point is highest in — (a) Sun plants (b) Shade plants (c) C4 plants (d) CAM plants. Ans: (a).
30. Q30. Glycolate is metabolised in — (a) Chloroplast only (b) Peroxisome only (c) Chloroplast → peroxisome → mitochondrion (d) Mitochondrion only. Ans: (c).
31. Q31. Most NEET-favourite: which enzyme has both carboxylase and oxygenase activity — (a) PEP-case (b) RuBisCO (c) Carbonic anhydrase (d) NADP-reductase. Ans: (b).
32. Q32. Path of electron flow in non-cyclic photophosphorylation — (a) PS-I → PS-II → NADP (b) PS-II → PS-I → NADP (c) PS-II → NADP (d) PS-I → NADP. Ans: (b).
33. Q33. Which wavelength is least effective in photosynthesis — (a) Violet (b) Green (c) Red (d) Blue. Ans: (b) (chlorophyll reflects green).
34. Q34. Total ATP per CO₂ fixed in C4 cycle — (a) 3 (b) 4 (c) 5 (d) 6. Ans: (c).
35. Q35. Photophosphorylation is — (a) ATP from ADP using light (b) NADPH from NADP using light (c) Both (d) None. Ans: (a).
36. Q36. Stroma contains enzymes for — (a) Light reaction (b) Calvin cycle (c) Both (d) Glycolysis. Ans: (b).
37. Q37. Plants grown in green light show — (a) Maximum growth (b) Minimum growth (c) Equal growth (d) Etiolation. Ans: (b).
38. Q38. Emerson’s enhancement effect involves — (a) Two pigment systems (b) Single PS (c) Three PS (d) Cyt only. Ans: (a).
39. Q39. Which is NOT a CAM plant — (a) Cactus (b) Pineapple (c) Wheat (d) Bryophyllum. Ans: (c).
40. Q40. Chemiosmotic gradient drives — (a) ATP synthesis (b) NADPH synthesis (c) Both (d) CO₂ fixation. Ans: (a).

10. NEET 2027 Strategy for This Chapter

Master the Z-scheme diagram, C4 pathway diagram (Kranz anatomy), and NCERT Table 13.1 (C3 vs C4). Solve previous-year questions from NEET Gurukul Free Resources and join our structured Biology course. For your NEET 2027 timeline, this chapter alone deserves 12–15 hours of revision across 3 cycles.

FAQ

Q1. Why is RuBisCO called inefficient?

It binds both CO₂ and O₂. At low CO₂:O₂, oxygenase activity dominates → photorespiration → carbon loss.

Q2. How many ATP are needed per glucose in C4?

30 ATP (24 for Calvin cycle + 6 for the C4 shuttle, 1 ATP per CO₂ pumped into bundle sheath).

Q3. Are C4 plants more productive?

Yes in hot tropical conditions — sugarcane and maize have ~50% higher productivity than wheat per unit area.

Q4. What pigment gives autumn leaves their colour?

Carotenoids (yellow-orange) and anthocyanins (red) become visible as chlorophyll degrades.

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