Question

how energy goes to unidirectional justified the statement

Answer 1

The flow of energy in food chains is unidirectional. The energy which is passes to the herbivores does not come back to the autotrophs

Answer 2

$\huge{ANSWER--}$

Producers convert solar energy into chemical energy in the form of organic compounds. The primary consumers derive nutrients from the producers.  According to Energy Transfer Law, only 10% of energy is transferred from one trophic level to other. So the energy that is captured by autotrophs  is not  revert back to sun and the energy transferred to herbivores does not came back to autotrophs. It just keep on moving to next trophic level in unidirectional way. That is why, flow of energy in a food chain is always unidirectional.