Previous year questions: Breathing and Exchange of Gases

These ten questions come from the NCERT Exemplar chapter on breathing and exchange of gases, the only official previous year source available for Class 11 Biology. The set covers breathing mechanics, lung volumes, gas transport, the dissociation curve and the neural control. Marks follow the book convention: 1 mark MCQ, 2 marks very short answer, 3 marks short answer and 5 marks long answer. Each heading quotes the Exemplar chapter and question number.

Q1 (NCERT Exemplar, 1 mark; Chapter 17, Multiple Choice 5)

With reference to normal breathing, mark the true statement.

Answer: Inspiration is the active phase because the diaphragm and external intercostal muscles contract and increase the thoracic volume. Normal expiration is passive, because it follows the relaxation of those muscles. The options that call both phases active or both passive are wrong, and the one that reverses the two is the common trap. At rest the muscles of inspiration are the diaphragm and the external intercostals, and the abdominal muscles are called in only when the breath has to be forced. The examiner also expects the word passive for normal expiration, because no muscle contracts during it.

Q2 (NCERT Exemplar, 1 mark; Chapter 17, Multiple Choice 9)

Incidence of emphysema is high in cigarette smokers. In such cases what is damaged?

Answer: The walls of the alveoli are damaged, so the surface available for gas exchange falls. The bronchioles, the plasma membrane and the respiratory muscles are not the primary site of the damage. This is why emphysema causes breathlessness on exertion. The lost surface cannot be rebuilt, so a smoker who has damaged enough alveoli has to breathe harder to move the same amount of oxygen into the blood.

Q3 (NCERT Exemplar, 1 mark; Chapter 17, Multiple Choice 10)

Which centre can reduce the inspiratory duration upon stimulation?

Answer: The pneumotaxic centre in the pons reduces the duration of inspiration when stimulated. The medullary inspiratory centre sets the rhythm, and the chemosensitive centre responds to carbon dioxide and hydrogen ions rather than controlling the duration directly. The three centres are therefore easy to keep apart: medulla for rhythm, pons for the switch that ends inspiration, and the chemosensitive area for chemical feedback.

Q4 (NCERT Exemplar, 1 mark; Chapter 17, Multiple Choice 12)

In breathing movements, air volume can be estimated by which instrument?

Answer: A spirometer measures the volume of air moved in and out of the lungs. A stethoscope listens to sounds, a hygrometer measures humidity and a sphygmomanometer measures blood pressure, so none of them is used for lung volumes. The spirometer trace is the standard way to measure tidal volume, the reserve volumes and the capacities derived from them. A plain reading of the trace gives the volumes, and the capacities are then obtained by adding the right pair of volumes.

Q5 (NCERT Exemplar, 1 mark; Chapter 17, Multiple Choice 14)

The oxygen-haemoglobin dissociation curve shows a right shift in which case?

Answer: A right shift occurs when the partial pressure of carbon dioxide is high, because the curve moves so that haemoglobin releases oxygen more readily. High pO2, low pCO2 and low hydrogen ion concentration would shift the curve the other way or increase the affinity. A right shift is the same as a fall in affinity, and it is what the tissues need because their carbon dioxide is high and their pH is low.

Q6 (NCERT Exemplar, 2 marks; Chapter 17, Very Short Answer 5)

What amount of oxygen is supplied to the tissues through every 100 mL of oxygenated blood under normal physiological conditions?

Answer: Under normal conditions, 100 mL of oxygenated blood delivers about 5 mL of oxygen to the tissues. This is the usable share that diffuses out at the tissue capillaries, while most of the oxygen remains bound to haemoglobin for the rest of the circuit. This figure is useful when a question asks for the amount delivered rather than the amount carried.

Q7 (NCERT Exemplar, 2 marks; Chapter 17, Very Short Answer 9)

Name the organs of respiration in flatworm, birds, frog and cockroach.

Answer: A flatworm exchanges gases through its body surface, which must stay moist. Birds use lungs, frog uses moist skin, the lining of the mouth and the lungs, and cockroach uses the tracheal system with spiracles on the body surface. The matching question in the chapter uses exactly these four animals. The matching question in the chapter uses exactly these four animals, and the same list appears in the exercises. When the question comes as a list of organisms with their organs, draw a quick line between each pair before writing the answer.

Q8 (NCERT Exemplar, 3 marks; Chapter 17, Short Answer 1)

State the different modes of carbon dioxide transport in the blood.

Answer: Carbon dioxide is carried in three ways. About 70 per cent travels as bicarbonate ions in the plasma after conversion inside the red blood cells by carbonic anhydrase. About 20 to 25 per cent travels as carbaminohaemoglobin, attached to the globin part of haemoglobin. The remaining 7 per cent travels simply dissolved in the plasma. The bicarbonate route uses a chloride shift to keep the charge balanced when the bicarbonate leaves the red blood cell. The overall picture is that most carbon dioxide is carried in a form that also helps to buffer the blood, while only a small share travels as a simple dissolved gas.

Q9 (NCERT Exemplar, 3 marks; Chapter 17, Short Answer 2)

Compared with oxygen, the diffusion rate of carbon dioxide through the diffusion membrane per unit pressure difference is much higher. Explain.

Answer: The diffusion rate depends on the solubility of the gas in the membrane and on the thickness and area of the membrane, as well as on the pressure difference. Carbon dioxide is about twenty times more soluble in the diffusion membrane than oxygen, so the same pressure difference drives a much larger amount of carbon dioxide across. The membrane itself is the same thin three-layered sheet for both gases, so the difference comes from solubility. The ratio is also the reason a patient with a thickened alveolar membrane has more difficulty moving oxygen than carbon dioxide, because the oxygen transfer is the more marginal of the two. That is why oxygen, and not carbon dioxide, limits gas exchange in the lungs. A useful comparison is to state the ratio and then name the limiting gas, because the numerical value of the carbon dioxide solubility is what examiners expect to see.

Q10 (NCERT Exemplar, 5 marks; Chapter 17, Long Answer 2)

Explain the mechanism of breathing with the help of labelled sketches.

Answer: Breathing works by changing the volume of the thoracic cavity. During inspiration the diaphragm contracts and flattens, and the external intercostal muscles pull the ribs and sternum up and outward. The volume of the thorax increases, the lungs expand with it, and the pressure inside the lungs falls below the atmospheric pressure, so air enters through the nose and airways. During normal expiration the diaphragm relaxes back into its dome shape, the ribs and sternum return to their resting positions and the abdominal organs push the relaxed diaphragm upward. The thoracic volume decreases, the pressure inside the lungs rises slightly above atmospheric pressure and air is pushed out. Forced expiration adds the internal intercostals and the abdominal muscles, while forced inspiration adds the muscles of the neck and back. A labelled sketch should show the two phases side by side, with the raised ribs and contracted diaphragm in inspiration and the returned ribs with the arched diaphragm in expiration. The whole cycle repeats about 12 to 16 times per minute at rest. In the labelled sketch the labels should include the ribs, sternum, diaphragm, the volume arrows and the direction of air flow, and the two panels should be drawn as a matched pair so that the examiner can compare the two phases at a glance.

Answer-writing reminders for this set

The ten items above repeat a small number of skills. When a definition is asked, write the defining sentence and then one example or one value so that the answer does not stop at the definition. When a mechanism is asked, put the steps in time order and name the muscle or the membrane involved at each step. When a numerical value is asked, write the formula, substitute the numbers and finish with the unit. When a comparison is asked, use a table with the two items as columns and compare the same property on each row. For the neural control questions, always name the medulla, the pons and the chemosensitive area and give the job of each. For the gas transport questions, always state the three routes with their shares and then explain what keeps each gradient steep. These habits are worth more than extra facts because the examiner can see the structure of the answer at a glance.