Home › Free notes › Class 11 Biology › Respiration in Plants

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

NCERT Class 11 Biology, chapter 12. Notes, key terms, important questions with answers and MCQs.

Chapter overview

Respiration breaks down food to release energy as ATP. Glycolysis, the Krebs cycle and the electron transport system complete aerobic respiration.

Stages

Glycolysis (EMP pathway) in the cytoplasm: glucose → 2 pyruvate, net 2 ATP + 2 NADH.
Pyruvate → acetyl CoA in mitochondria; Krebs (TCA) cycle in the matrix produces NADH, FADH₂, GTP and CO₂.
ETS on the inner mitochondrial membrane; O₂ is the final electron acceptor; oxidative phosphorylation via ATP synthase.

Anaerobic respiration and RQ

Fermentation: yeast makes ethanol + CO₂; muscles make lactic acid.
Net gain about 38 ATP per glucose (textbook value) in aerobic respiration; only 2 in fermentation.
Respiratory quotient RQ = CO₂ evolved/O₂ consumed: carbohydrates 1, fats < 1 (≈ 0.7), organic acids > 1.

Key terms

Glycolysis: breakdown of glucose to pyruvate in the cytoplasm
Respiratory quotient: ratio of CO₂ evolved to O₂ consumed
Amphibolic pathway: a pathway used for both breakdown (catabolism) and synthesis (anabolism), like the respiratory pathway

Important questions with answers

Why is the respiratory pathway called amphibolic?2 marks

It breaks down glucose and fats to release energy (catabolism), but its intermediates such as acetyl CoA and Krebs-cycle acids are also used to make fatty acids, amino acids and other molecules (anabolism).

Differentiate between aerobic respiration and fermentation.3 marks

Aerobic respiration uses O₂, completely oxidises glucose to CO₂ and water, occurs in cytoplasm and mitochondria, and gives about 38 ATP. Fermentation occurs without O₂ in the cytoplasm, partially breaks glucose into ethanol + CO₂ or lactic acid, and gives only 2 ATP.

Describe the electron transport system and oxidative phosphorylation in mitochondria.5 marks

NADH and FADH₂ from glycolysis and the Krebs cycle give their electrons to the ETS on the inner mitochondrial membrane. Electrons from NADH pass through complex I (NADH dehydrogenase) to ubiquinone; FADH₂ feeds in at complex II. Reduced ubiquinone passes electrons to complex III (cytochrome bc₁), then cytochrome c to complex IV (cytochrome c oxidase, with cytochromes a, a₃ and copper), which finally gives them to O₂, forming water. Energy released pumps H⁺ into the intermembrane space; H⁺ flow back through complex V (ATP synthase, F₀–F₁), which makes ATP from ADP and Pi. This is oxidative phosphorylation; O₂ is the final electron acceptor.

Quick revision

Glycolysis occurs in the cytoplasm.
Krebs cycle in the mitochondrial matrix.
O₂ is the final electron acceptor.

Exam tips

RQ of tripalmitin is about 0.7.
Track ATP: glycolysis net 2 ATP + 2 NADH; link reaction 2 NADH; Krebs cycle 2 GTP/ATP + 6 NADH + 2 FADH₂; each NADH ≈ 3 ATP and FADH₂ ≈ 2 ATP (textbook values).

Practice MCQs

Glycolysis takes place in the
1. cytoplasm
2. mitochondria
3. nucleus
4. chloroplast
Show answer
Answer: (a) cytoplasm. Cytosol.
Final electron acceptor in aerobic respiration is
1. CO₂
2. NAD
3. O₂
4. water
Show answer
Answer: (c) O₂. Forms water.
Yeast fermentation produces
1. ethanol and CO₂
2. lactic acid
3. acetic acid
4. O₂
Show answer
Answer: (a) ethanol and CO₂. Alcoholic fermentation.
RQ of carbohydrates is
1. 0.7
2. 1
3. 4
4. 0
Show answer
Answer: (b) 1. Equal CO₂ and O₂.
Pyruvate is converted to acetyl CoA in the
1. mitochondrial matrix
2. cytoplasm
3. stroma
4. nucleus
Show answer
Answer: (a) mitochondrial matrix. Oxidative decarboxylation.
Krebs cycle occurs in the
1. cytoplasm
2. stroma
3. inner membrane
4. mitochondrial matrix
Show answer
Answer: (d) mitochondrial matrix. Matrix.
Net ATP from glycolysis of one glucose is
1. 2
2. 4
3. 8
4. 36
Show answer
Answer: (a) 2. 4 made − 2 used.
RQ of the fat tripalmitin is about
1. 1.0
2. 1.3
3. 0.7
4. 4.0
Show answer
Answer: (c) 0.7. Fats need more O₂.

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.