(a) "In a food chain energy flow is unidirectional." Give two reasons for the given statement.

(b) If 10,000 J energy is available at the producer level, how much energy will be available to the secondary consumers? Give reason to justify your answer.

This question directly tests two core ideas stated in the chapter under 'Food Chains and Webs': (i) the unidirectional nature of energy flow, and (ii) the 10% law governing energy transfer between trophic levels.

The chapter explicitly states: 'the flow of energy is unidirectional. The energy that is captured by the autotrophs does not revert back to the solar input and the energy which passes to the herbivores does not come back to autotrophs. As it moves progressively through the various trophic levels it is no longer available to the previous level.' This gives the first reason directly. The second reason follows from the chapter's explanation that 'a great deal of energy is lost as heat to the environment, some amount goes into digestion and in doing work' at every transfer — since this heat energy is dissipated into the surroundings, it cannot be recovered or sent backward through the food chain, reinforcing the one-way nature of energy flow.

For part (b), the chapter states 'An average of 10% of the food eaten is turned into its own body and made available for the next level of consumers' and that '10% can be taken as the average value for the amount of organic matter that is present at each step and reaches the next level of consumers.' Applying this twice — once from producers to primary consumers, and again from primary consumers to secondary consumers — gives $10{,}000 \to 1{,}000 \to 100\ \text{J}$. This progressive loss also explains why the chapter notes that food chains generally consist of only three or four steps, since very little usable energy remains beyond that.