Breathing and Exchange of Gases — Class 11 Biology NCERT Solutions (Free)
Free step-by-step NCERT solutions for Class 11 Biology chapter "Breathing and Exchange of Gases" — 8 important questions with detailed answers for CBSE board exam preparation.
TL;DR: Free step-by-step NCERT solutions for Class 11 Biology chapter "Breathing and Exchange of Gases" — 8 important questions with detailed answers for CBS…
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NCERT Solutions for Class 11 Biology — Breathing and Exchange of Gases. Step-by-step answers to all 8 textbook questions from this chapter, written for CBSE board preparation and free to use.
Breathing and Exchange of Gases — All 8 Questions Solved
Q1. Define breathing. How is it different from cellular respiration?
Breathing is the physical process of taking in air (oxygen) into the lungs and removing air (containing CO2) from the lungs through ventilation.
Cellular respiration is the biochemical process of oxidation of glucose in cells to release energy as ATP.
Difference: Breathing is a mechanical process (lungs, diaphragm); cellular respiration is a chemical process (mitochondria). Breathing facilitates gas exchange; respiration consumes O2 and releases CO2.
Q2. Describe the anatomy of the human respiratory system.
Key components:
1. Nasal cavity: Filters, warms, and humidifies air
2. Pharynx: Common passage for air and food
3. Larynx: Voice box; contains vocal cords
4. Trachea: Windpipe; divides into two bronchi
5. Bronchi and bronchioles: Tubes that branch and narrow
6. Lungs: Two organs containing alveoli for gas exchange; right lung has 3 lobes, left has 2 lobes
7. Diaphragm: Dome-shaped muscle below lungs; primary breathing muscle
8. Intercostal muscles: Between ribs; assist in breathing
Q3. Explain the process of inspiration and expiration.
Inspiration (inhalation):
- Diaphragm contracts and flattens (moves downward)
- External intercostal muscles contract (rib cage moves up and out)
- Thoracic cavity volume increases
- Intrapulmonary pressure decreases below atmospheric pressure
- Air flows into lungs
Expiration (exhalation):
- Diaphragm relaxes and returns to dome shape (moves upward)
- External intercostal muscles relax
- Thoracic cavity volume decreases
- Intrapulmonary pressure increases above atmospheric
- Air flows out of lungs
Q4. What are tidal volume, vital capacity, and residual volume?
Tidal volume: Volume of air breathed in or out during normal quiet breathing (~500 mL in adults).
Vital capacity: Maximum volume of air that can be exhaled after maximum inhalation (~3500-4500 mL). Calculated as inspiratory reserve + tidal volume + expiratory reserve.
Residual volume: Volume of air remaining in lungs after maximum expiration (~1100-1200 mL); cannot be expelled.
Total lung capacity = Vital capacity + Residual volume (~5500-6000 mL)
These measurements are important in assessing respiratory function and lung health.
Q5. Describe gas exchange in the alveoli. What makes it efficient?
Gas exchange occurs across the alveolar wall between alveolar air and blood in the pulmonary capillaries.
Process: O2 diffuses from alveolar air (high pO2) into blood; CO2 diffuses from blood (high pCO2) into alveolar air.
Factors making it efficient:
1. Large surface area: ~70 m² provided by millions of alveoli
2. Thin alveolar wall: Allows rapid diffusion
3. Rich blood supply: Extensive capillary network ensures constant removal of CO2 and supply of O2
4. Concentration gradient: Maintained by continuous ventilation and circulation
5. Diffusion distance: Very short (~0.5 micrometer)
Oxygen and CO2 follow partial pressure gradients.
Q6. How is oxygen transported in blood? Explain the role of hemoglobin.
Oxygen transport:
1. ~97% bound to hemoglobin: O2 + Hb ↔ HbO2 (oxyhemoglobin); transport from lungs to tissues
2. ~3% dissolved in plasma (minor)
Hemoglobin (Hb) is a protein in RBCs with 4 subunits, each containing heme with iron (Fe2+) that binds O2.
Oxygen affinity of hemoglobin is affected by:
- pO2 (higher pO2, higher affinity)
- pCO2 (higher pCO2, lower affinity - Bohr effect)
- pH (lower pH, lower affinity)
- Temperature (higher temperature, lower affinity)
At tissues (high pCO2, low pH, high temperature), O2 is released; at lungs, O2 is loaded.
Q7. Describe how carbon dioxide is transported in blood.
CO2 transport occurs in three ways:
1. Dissolved in plasma: ~7% (minor; CO2 is slightly more soluble than O2)
2. Bound to hemoglobin: ~23% as carbaminohemoglobin (HbCO2); binds to globin chain, not heme
3. As bicarbonate ions: ~70% (major form)
Bicarbonate formation (in RBCs):
CO2 + H2O ⇌ H2CO3 (carbonic acid) ⇌ HCO3- + H+
Catalyzed by carbonic anhydrase (enzyme in RBCs)
HCO3- exits RBC and enters plasma; Cl- enters RBC (chloride shift)
At lungs, reactions reverse: HCO3- → CO2 which is exhaled.
This system is efficient and maintains blood pH through buffering.
Q8. What is the role of the respiratory center in the brain?
Respiratory centers are located in the medulla oblongata and pons of the brainstem.
Medulla oblongata contains:
- Dorsal respiratory group: Controls inspiration
- Ventral respiratory group: Controls expiration and forced breathing
Pons contains:
- Pneumotaxic center: Fine-tunes breathing rhythm
- Apneustic center: Prolongs inspiration
These centers generate rhythmic breathing signals and send impulses via vagus nerve (CN X) to respiratory muscles.
Regulation factors:
- pCO2 and pO2 (chemoreceptors)
- pH (peripheral and central chemoreceptors)
- Stretch receptors in lungs (Hering-Breuer reflex)
These allow breathing to adjust to metabolic demands.
Breathing and Exchange of Gases — Practice MCQs with Answers
Attempt these 20 multiple-choice questions after working through the solutions above. Each carries the correct option and the reasoning behind it, so a wrong answer tells you which idea to revisit.
MCQ 1. Which of the following is the correct pathway for air entering the lungs?
- A. Nostrils → Larynx → Pharynx → Trachea → Bronchi → Bronchioles → Alveoli
- B. Nostrils → Pharynx → Larynx → Trachea → Bronchi → Bronchioles → Alveoli
- C. Nostrils → Pharynx → Trachea → Larynx → Bronchi → Bronchioles → Alveoli
- D. Nostrils → Larynx → Trachea → Pharynx → Bronchi → Bronchioles → Alveoli
Answer: B. Nostrils → Pharynx → Larynx → Trachea → Bronchi → Bronchioles → Alveoli — Air first enters through the nostrils, passes through the pharynx, then the larynx (voice box), followed by the trachea (windpipe), which divides into bronchi, then bronchioles, and finally reaches the alveoli.
MCQ 2. During inspiration, the diaphragm and external intercostal muscles perform which of the following actions?
- A. Diaphragm contracts, external intercostals relax
- B. Diaphragm relaxes, external intercostals contract
- C. Diaphragm contracts, external intercostals contract
- D. Diaphragm relaxes, external intercostals relax
Answer: C. Diaphragm contracts, external intercostals contract — Inspiration is an active process where the diaphragm contracts (flattens) and the external intercostal muscles contract (pulling ribs upwards and outwards). This increases the thoracic volume, reducing intrathoracic pressure and drawing air in.
MCQ 3. A person's Inspiratory Reserve Volume (IRV) is 3000 mL, Tidal Volume (TV) is 500 mL, Expiratory Reserve Volume (ERV) is 1000 mL, and Residual Volume (RV) is 1200 mL. What is their Vital Capacity (VC)?
- A. 4700 mL
- B. 3500 mL
- C. 4500 mL
- D. 5700 mL
Answer: C. 4500 mL — Vital Capacity (VC) is the maximum volume of air a person can breathe in after a forced expiration. It is the sum of Tidal Volume (TV), Inspiratory Reserve Volume (IRV), and Expiratory Reserve Volume (ERV). So, VC = 500 mL + 3000 mL + 1000 mL = 4500 mL.
MCQ 4. In the alveoli, the partial pressure of oxygen (pO2) is approximately:
- A. 40 mmHg
- B. 95 mmHg
- C. 104 mmHg
- D. 159 mmHg
Answer: C. 104 mmHg — The partial pressure of oxygen in the alveolar air is approximately 104 mmHg, which is higher than that in the deoxygenated blood, facilitating oxygen diffusion into the blood.
MCQ 5. A shift of the oxygen dissociation curve to the right indicates:
- A. Increased affinity of haemoglobin for oxygen
- B. Decreased affinity of haemoglobin for oxygen
- C. Increased binding of oxygen to haemoglobin
- D. No change in oxygen affinity
Answer: B. Decreased affinity of haemoglobin for oxygen — A rightward shift of the oxygen dissociation curve signifies a decreased affinity of haemoglobin for oxygen, meaning haemoglobin releases oxygen more readily. This occurs in conditions like increased CO2, increased H+ (lower pH), and higher temperature, which are characteristic of metabolically active tissues.
MCQ 6. The 'chloride shift' phenomenon during CO2 transport in blood primarily involves the movement of chloride ions into:
- A. Plasma
- B. Alveoli
- C. Red blood cells
- D. Tissue cells
Answer: C. Red blood cells — The chloride shift (Hamburger phenomenon) involves the influx of chloride ions (Cl-) into red blood cells from the plasma. This movement compensates for the efflux of bicarbonate ions (HCO3-) from the red blood cells into the plasma, maintaining electrochemical neutrality across the red blood cell membrane as CO2 is transported as bicarbonate.
MCQ 7. The primary respiratory rhythm centre is located in the:
- A. Cerebellum
- B. Medulla oblongata
- C. Pons
- D. Hypothalamus
Answer: B. Medulla oblongata — The primary respiratory rhythm centre, responsible for regulating breathing rhythm, is located in the medulla oblongata of the brainstem.
MCQ 8. Emphysema is a chronic disorder in which:
- A. Alveolar walls are damaged, leading to reduced respiratory surface.
- B. Bronchioles are inflamed, causing difficulty in breathing.
- C. Fluid accumulates in the lungs, making gas exchange difficult.
- D. Air passages are blocked due to excessive mucus production.
Answer: A. Alveolar walls are damaged, leading to reduced respiratory surface. — Emphysema is characterized by the damage to alveolar walls, which significantly reduces the total surface area available for gas exchange. This leads to chronic shortness of breath and impaired oxygen absorption.
MCQ 9. What is the primary function of the cartilaginous rings present in the trachea?
- A. To prevent the entry of food into the trachea.
- B. To support the vocal cords for sound production.
- C. To prevent the trachea from collapsing during exhalation.
- D. To filter dust particles from inhaled air.
Answer: C. To prevent the trachea from collapsing during exhalation. — The C-shaped cartilaginous rings in the trachea provide structural support, preventing the trachea (windpipe) from collapsing, especially during periods of decreased pressure, ensuring a continuous air passage.
MCQ 10. Which of the following factors does NOT favor the diffusion of gases across the alveolar membrane?
- A. High partial pressure gradient
- B. Thin diffusion membrane
- C. High solubility of the gas
- D. Thick diffusion membrane
Answer: D. Thick diffusion membrane — A thick diffusion membrane increases the distance gases must travel, thus impeding efficient diffusion. A thin membrane, high partial pressure gradient, and high solubility of the gas all favour diffusion.
MCQ 11. The Bohr effect describes the effect of what on oxygen binding to haemoglobin?
- A. CO2 and H+ concentration
- B. O2 partial pressure
- C. Temperature only
- D. 2,3-Bisphosphoglycerate (BPG)
Answer: A. CO2 and H+ concentration — The Bohr effect refers to the decrease in haemoglobin's oxygen affinity when the pH decreases (due to increased H+ ions) or CO2 concentration increases. This facilitates oxygen release in tissues with higher metabolic activity.
MCQ 12. What is the shape of the diaphragm when it contracts during inspiration?
- A. Dome-shaped
- B. Flattened
- C. Concave
- D. Inverted U-shaped
Answer: B. Flattened — During inspiration, the diaphragm contracts and moves downwards, becoming relatively flattened. This increases the vertical volume of the thoracic cavity.
MCQ 13. Forced expiration involves the contraction of which muscles?
- A. External intercostal muscles and diaphragm
- B. Internal intercostal muscles and abdominal muscles
- C. External intercostal muscles and abdominal muscles
- D. Diaphragm and abdominal muscles
Answer: B. Internal intercostal muscles and abdominal muscles — While normal expiration is passive, forced expiration is an active process involving the contraction of the internal intercostal muscles (pulling ribs inwards) and the abdominal muscles (pushing the diaphragm up), further reducing thoracic volume.
MCQ 14. In humans, the primary respiratory pigment responsible for oxygen transport is:
- A. Myoglobin
- B. Haemoglobin
- C. Haemocyanin
- D. Chlorophyll
Answer: B. Haemoglobin — Haemoglobin, found in red blood cells, is the principal respiratory pigment in humans, binding reversibly with oxygen to transport it from the lungs to the tissues.
MCQ 15. If Total Lung Capacity (TLC) is 5800 mL, Residual Volume (RV) is 1200 mL, and Expiratory Reserve Volume (ERV) is 1000 mL, what is the Inspiratory Capacity (IC)?
- A. 3600 mL
- B. 4600 mL
- C. 2600 mL
- D. 3100 mL
Answer: A. 3600 mL — Total Lung Capacity (TLC) = Inspiratory Capacity (IC) + Expiratory Reserve Volume (ERV) + Residual Volume (RV). Therefore, IC = TLC - (ERV + RV) = 5800 mL - (1000 mL + 1200 mL) = 5800 mL - 2200 mL = 3600 mL.
MCQ 16. The vocal cords are located in which part of the respiratory system?
- A. Pharynx
- B. Trachea
- C. Larynx
- D. Bronchi
Answer: C. Larynx — The larynx, also known as the voice box, contains the vocal cords which vibrate to produce sound when air passes over them.
MCQ 17. Anatomical dead space refers to the volume of air that:
- A. Remains in the alveoli after forced expiration.
- B. Is involved in gas exchange but not useful.
- C. Fills the respiratory passages but does not participate in gas exchange.
- D. Is exhaled after normal inspiration.
Answer: C. Fills the respiratory passages but does not participate in gas exchange. — Anatomical dead space is the volume of air that occupies the conducting zone of the respiratory system (nasal cavity, pharynx, larynx, trachea, bronchi, bronchioles) and does not participate in gas exchange with the blood.
MCQ 18. At high partial pressure of CO2 and low pH, the affinity of haemoglobin for oxygen:
- A. Increases significantly
- B. Decreases significantly
- C. Remains unchanged
- D. Initially increases, then decreases
Answer: B. Decreases significantly — High partial pressure of CO2 and low pH (high H+ concentration) are conditions found in active tissues. Under these conditions, haemoglobin's affinity for oxygen decreases, promoting the release of oxygen where it is most needed. This is part of the Bohr effect.
MCQ 19. The primary site for the exchange of gases (O2 and CO2) in the human respiratory system is the:
- A. Trachea
- B. Bronchi
- C. Bronchioles
- D. Alveoli
Answer: D. Alveoli — The alveoli are tiny air sacs in the lungs with extremely thin walls and a rich blood supply, making them the ideal and primary site for the efficient exchange of oxygen and carbon dioxide between air and blood.
MCQ 20. Peripheral chemoreceptors, sensitive to changes in pO2, pCO2, and H+ concentration, are located in the:
- A. Medulla oblongata
- B. Carotid and aortic bodies
- C. Pons
- D. Lungs themselves
Answer: B. Carotid and aortic bodies — Peripheral chemoreceptors are specialized cells located in the carotid arteries (carotid bodies) and the aorta (aortic bodies). They monitor changes in arterial blood gas levels (pO2, pCO2, and pH) and send signals to the respiratory centres in the brainstem to adjust breathing.
How to Use These Solutions
Attempt each question yourself first and only then compare with the worked answer. Marks in the CBSE board exam are awarded for the METHOD as much as the final result, so reproduce the steps rather than memorising the last line. Where a solution states a law, a formula or a definition, learn that wording — examiners look for it.
Related practice for this chapter: chapter MCQs, previous-year questions, revision notes and sample papers.