CBSE Class 10 Science 2017 · Paper 531/1 · Question 17

(a) The energy of a food web comes from the Sun and is trapped by the producers. The energy captured by the producers does not go back to the Sun, and the energy passed to the herbivores does not come back to the producers; as it moves through the trophic levels it is no longer available to the previous level. At each level much of the energy is used in life processes and lost as heat, which plants cannot use to make food, and only about 10 per cent passes to the next level. So energy moves in one direction only. (b) Decomposers (bacteria and fungi) break down the complex organic substances of dead plants, dead animals and wastes into simple inorganic substances, which go into the soil and are used again by plants. They thus recycle nutrients and replenish the soil, and they also clean the environment of dead remains and wastes.