Science — CBSE Class 10 board question

(i) Alveoli are richly supplied with blood capillaries to provide a large surface area for rapid exchange of gases (oxygen and carbon dioxide) between air in the alveoli and the blood.

(ii) Haemoglobin (respiratory pigment) has a high affinity for oxygen, so it combines with oxygen to form oxyhaemoglobin and transports it to body cells. Carbon dioxide is mostly transported dissolved in plasma or as bicarbonate ions, not by haemoglobin.

(iii) During anaerobic respiration in humans, there is incomplete breakdown of glucose (pyruvate). Due to lack of oxygen, pyruvate is converted to lactic acid (3-carbon molecule) in the muscle cells instead of being fully oxidised to CO₂ and water.

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• For (i): Link alveolar structure to function — large surface + rich blood supply = efficient gas exchange.
• For (ii): Stress haemoglobin's affinity for O₂, not CO₂. Mention CO₂ transport via plasma/bicarbonate to show contrast.
• For (iii): Key point is incomplete breakdown and no oxygen → pyruvate → lactic acid (3C). Avoid confusing with yeast (which gives ethanol + CO₂). Examiners expect "lactic acid" and "muscle cells" explicitly.