19.(1)Energy flow in a food chain …

The flow of energy in food chains is unidirectional. The energy which passes to the herbivores does not come back to the autotrophs. The energy that is captured by the autotrophs does not revert back to the Sun. Therefore, in the food chain, the energy moves progressively through various trophic levels. This energy is no longer available to the previous trophic level. Thus, the flow of energy in a food chain is unidirectional.

The pesticides enter a food chain and subsequently get into our body by the following way: 

• Pesticides are sprayed on the crop plants to protect them from the attack of pests. These pesticides when washed away into the soil or water bodies get absorbed by the plants. 
• When plants are consumed as food by man or other animals, the pesticides enter the food chain. Since the pesticides are non-biodegradable, these chemicals get accumulated progressively and enter our body.