CBSE Class 10 Science 2018 · Paper 31/1/C · Question 18(a)

(a) In yeast, glucose (a 6-carbon molecule) is first broken down in the cytoplasm into pyruvate (a 3-carbon molecule), releasing a little energy; in the absence of oxygen, pyruvate is then converted into ethanol and carbon dioxide (fermentation): $\text{Glucose}\stackrel{\text{in cytoplasm}}{\to }\text{Pyruvate}+\text{Energy}\underset{\text{in yeast}}{\overset{\text{absence of oxygen}}{\to }}\text{Ethanol}+\text{Carbon dioxide}+\text{Energy}$ Overall: ${\text{C}}_6{\text{H}}_{12}{\text{O}}_6\to 2{\text{C}}_2{\text{H}}_5\text{OH}+2{\text{CO}}_2+\text{Energy}$ (b) Fishes use the oxygen dissolved in water. A fish takes in water through its mouth and forces it over the gills, which are richly supplied with blood capillaries; the dissolved oxygen diffuses from the water into the blood, and carbon dioxide diffuses from the blood into the water, which passes out through the gill openings. As water contains much less dissolved oxygen than air, fishes breathe much faster than terrestrial animals. (c) The balloon-like structures are the alveoli (singular: alveolus). Functions: 1. They provide a very large surface area for the exchange of gases. 2. Their thin walls, with an extensive network of blood capillaries, allow oxygen from the inhaled air to diffuse into the blood and carbon dioxide from the blood to diffuse into the alveolar air, to be breathed out. (d) The respiratory pigment in human beings is haemoglobin, present in the red blood cells. Role: it has a very high affinity for oxygen; it takes up oxygen from the air in the lungs and carries it to the tissues, which are deficient in oxygen.