“Energy flow in food chains is always unidirectional.”

“Energy flow in food chains is always unidirectional.” Justify this statement. Explain how the pesticides enter a food chain and subsequently get into our body.

In a food chain the energy moves progressively through the various trophic levels and is no longer available to the organisms of the previous trophic level. The energy captured by the autotrophs does not revert back to the solar input. Thus, energy flow in food chains are said to be unidirectional.
Pesticides used for crop protection when washed away go down into the soil. These pesticides absorbed by plants which are the producers. On consumption of these plants, the pesticides enter our food chain and being non-biodegradable, these chemicals get accumulated progressively and enter our body.