Energy flow in food chains is always unidirectional because energy is transfered from one organism to another and 10% of energy is loss when it reaches to next step...
  pesticides enter a food chain beacuse food chain is made up of many organisms and if organisms will have pesticides on them then it will definately enter a food chain and it get inside our body beacuse humans are ominvore and when they eat these organism , pesticides get inside the human body

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It is unidirectional beacause the energy that is caltured by herbivores from sun cannot be revert back also the 10 percent energy that is taken by carnivores cannot be given back.