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Breathing and Exchange of Gases – Class 11 Biology notes, important questions and MCQs

NCERT Class 11 Biology, chapter 14. Notes, key terms, important questions with answers and MCQs.

Chapter overview

Breathing brings O₂ into the lungs and removes CO₂. Gases are exchanged by diffusion in the alveoli and carried by blood.

Mechanism

Inspiration: diaphragm and external intercostal muscles contract, chest volume increases, pressure falls.
Volumes: tidal volume ≈ 500 mL; vital capacity = TV + IRV + ERV; residual volume stays in lungs.
Breathing is controlled by the respiratory rhythm centre in the medulla.

Transport and disorders

About 97% of O₂ is carried by haemoglobin (oxyhaemoglobin); oxygen dissociation curve is sigmoid.
CO₂ is carried mostly as bicarbonate (≈ 70%), about 20–25% as carbaminohaemoglobin, 7% dissolved.
Disorders: asthma, emphysema, occupational lung disease.

Key terms

Tidal volume: air inspired or expired in a normal breath
Vital capacity: maximum air that can be breathed out after a maximum inhalation
Partial pressure: pressure contributed by one gas in a mixture; gases diffuse from higher to lower partial pressure

Important questions with answers

Why is the exchange of gases very efficient in the alveoli?2 marks

Alveoli have a huge surface area, a very thin diffusion membrane, a rich blood supply, and steep partial-pressure gradients for O₂ and CO₂.

Describe how breathing is regulated.3 marks

The respiratory rhythm centre in the medulla sets the basic rhythm; the pneumotaxic centre in the pons can adjust it. A chemosensitive area near the rhythm centre responds to rising CO₂ and H⁺, increasing breathing; receptors in the aortic arch and carotid arteries also detect these changes.

Explain how oxygen and carbon dioxide are transported in blood, including the role of carbonic anhydrase.5 marks

Oxygen: about 97% binds reversibly to haemoglobin in RBCs as oxyhaemoglobin (each Hb carries up to 4 O₂); binding depends on pO₂, and high pO₂ in lungs favours binding while low pO₂, high pCO₂, high H⁺ and temperature in tissues favour release. About 3% dissolves in plasma. Carbon dioxide: about 20–25% binds to haemoglobin as carbaminohaemoglobin, about 7% dissolves in plasma, and about 70% travels as bicarbonate. In tissues, carbonic anhydrase in RBCs speeds up CO₂ + H₂O → H₂CO₃ → HCO₃⁻ + H⁺; in the lungs the reaction reverses and CO₂ is breathed out.

Quick revision

Diaphragm contracts in inspiration.
Most O₂ is carried by haemoglobin.
Most CO₂ is carried as bicarbonate.

Exam tips

Diffusion of CO₂ is about 20–25 times faster than O₂ across membranes.
Remember partial pressures (mm Hg): alveoli pO₂ 104, pCO₂ 40; deoxygenated blood pO₂ 40, pCO₂ 45; tissues pO₂ 40, pCO₂ 45.

Practice MCQs

Gas exchange in lungs occurs in the
1. trachea
2. alveoli
3. bronchi
4. pharynx
Show answer
Answer: (b) alveoli. Alveoli.
During inspiration the diaphragm
1. contracts
2. relaxes
3. stays still
4. moves up
Show answer
Answer: (a) contracts. Flattens.
Normal tidal volume in humans is about
1. 500 mL
2. 50 mL
3. 5000 mL
4. 1500 mL
Show answer
Answer: (a) 500 mL. ~500 mL.
Most CO₂ is transported as
1. dissolved CO₂
2. bicarbonate
3. carbaminohaemoglobin
4. carbonic acid gas
Show answer
Answer: (b) bicarbonate. About 70%.
The volume of air left in the lungs after maximum exhalation is
1. tidal volume
2. residual volume
3. vital capacity
4. inspiratory reserve volume
Show answer
Answer: (b) residual volume. About 1100–1200 mL.
Respiratory rhythm centre is located in the
1. cerebrum
2. medulla
3. cerebellum
4. hypothalamus
Show answer
Answer: (b) medulla. Medulla oblongata.
Emphysema is a disorder in which
1. haemoglobin is low
2. bronchi are inflamed only
3. alveolar walls are damaged
4. diaphragm is paralysed
Show answer
Answer: (c) alveolar walls are damaged. Reduced surface area.
A shift of the oxygen dissociation curve to the right occurs with
1. high H⁺ (low pH) and high temperature
2. low pCO₂
3. low temperature
4. high pO₂ only
Show answer
Answer: (a) high H⁺ (low pH) and high temperature. Bohr effect; O₂ released in tissues.

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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.