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Photosynthesis in Higher Plants – Class 11 Biology notes, important questions and MCQs

NCERT Class 11 Biology, chapter 11. Notes, key terms, important questions with answers and MCQs.

Chapter overview

Green plants convert light energy into chemical energy. Light reactions in the thylakoids make ATP and NADPH, which the Calvin cycle in the stroma uses to fix CO₂.

Light reactions

Pigments: chlorophyll a (main), chlorophyll b, carotenoids; absorb mainly blue and red light.
Photosystems PS I (P700) and PS II (P680); splitting of water releases O₂ from PS II.
Non-cyclic photophosphorylation makes ATP + NADPH; cyclic makes only ATP; chemiosmosis drives ATP synthase.

Dark reactions and pathways

Calvin cycle (C₃): RuBisCO fixes CO₂ with RuBP; first stable product 3-PGA; 3 ATP + 2 NADPH per CO₂.
C₄ plants (maize, sugarcane): Kranz anatomy, PEP carboxylase, first product OAA; no photorespiration.
Photorespiration in C₃ plants wastes energy when RuBisCO binds O₂; Blackman's law of limiting factors.

Key terms

RuBisCO: enzyme that fixes CO₂ in the Calvin cycle, the most abundant protein
Kranz anatomy: bundle-sheath cells around veins in C₄ leaves
Photorespiration: light-dependent release of CO₂ in C₃ plants when RuBisCO uses O₂ instead of CO₂; wastes energy

Important questions with answers

Why is RuBisCO called the most abundant enzyme on Earth?2 marks

It is present in very large amounts in the chloroplasts of all green plants because it fixes CO₂ slowly, so plants need a lot of it.

Explain chemiosmosis in chloroplasts.3 marks

During light reactions, splitting of water and electron transport pump H⁺ into the thylakoid lumen, creating a proton gradient. H⁺ flow back into the stroma through ATP synthase (CF₀–CF₁), and this flow drives ATP formation.

Describe the Calvin cycle in three stages and explain why C₄ plants are more efficient in hot, dry conditions.5 marks

Carboxylation: RuBisCO fixes CO₂ with RuBP to form two molecules of 3-PGA. Reduction: 3-PGA is reduced to glyceraldehyde-3-phosphate using ATP and NADPH; for one glucose, 6 turns fix 6 CO₂. Regeneration: RuBP is regenerated using ATP so the cycle continues. Overall, each CO₂ needs 3 ATP and 2 NADPH (18 ATP and 12 NADPH per glucose). C₄ plants first fix CO₂ in mesophyll cells using PEP carboxylase (which has no affinity for O₂) and then release it at high concentration in bundle-sheath cells for the Calvin cycle. This avoids photorespiration, allows stomata to stay partly closed to save water, and works well at high temperature and light.

Quick revision

PS II reaction centre P680.
C₄ first product: OAA.
C₃ first product: 3-PGA.

Exam tips

O₂ released in photosynthesis comes from water, not CO₂.
Compare C₃ and C₄ in a table: first product, CO₂ acceptor, enzyme, leaf anatomy, photorespiration, optimum temperature and examples.

Practice MCQs

O₂ released in photosynthesis comes from
1. CO₂
2. glucose
3. water
4. chlorophyll
Show answer
Answer: (c) water. Photolysis.
Light reactions occur in the
1. thylakoids
2. stroma
3. cytoplasm
4. mitochondria
Show answer
Answer: (a) thylakoids. Grana.
First stable product of the Calvin cycle is
1. OAA
2. glucose
3. 3-PGA
4. RuBP
Show answer
Answer: (c) 3-PGA. C₃ pathway.
Reaction centre of PS I is
1. P680
2. P600
3. P700
4. P800
Show answer
Answer: (c) P700. P700.
The first CO₂ acceptor in C₄ plants is
1. RuBP
2. OAA
3. PEP
4. 3-PGA
Show answer
Answer: (c) PEP. PEP carboxylase.
Kranz anatomy is found in
1. wheat
2. potato
3. rice
4. maize
Show answer
Answer: (d) maize. C₄ plant.
For fixing one CO₂ in the Calvin cycle, needs are
1. 2 ATP + 2 NADPH
2. 1 ATP + 1 NADPH
3. 3 ATP + 3 NADPH
4. 3 ATP + 2 NADPH
Show answer
Answer: (d) 3 ATP + 2 NADPH. Standard.
Cyclic photophosphorylation involves
1. PS I only and produces ATP only
2. PS II only and produces NADPH
3. both PS and splits water
4. no light
Show answer
Answer: (a) PS I only and produces ATP only. No O₂ or NADPH.

All chapters of Class 11 Biology

Written by Kundali IQ from standard, well-established facts to help with revision. This is not an official NCERT publication; always read your textbook too.