CBSE Class 10 Science 2023 · Paper 31/C/2 · Question 33

(a) Food chain: Grass (1st level) $\to$ Grasshopper (2nd level) $\to$ Frog (3rd level) $\to$ Snake (4th level). By the ten per cent law, only about $10\%$ of the energy at one trophic level is passed on to the next, so the energy at each level is ten times that at the level above it: 3rd level $=10\times 75=750$ J, and 2nd level $=10\times 750=7500$ J. So $7500$ J was available with the organisms of the second trophic level, of which about $750$ J passes on to the third level. (b) The energy captured by the producers does not revert to the sun, and the energy that passes to the herbivores does not come back to the producers; at each level much of it is lost as heat, which cannot be used again. As it moves progressively through the trophic levels it is no longer available to the previous level, so the flow of energy is unidirectional.