Question

why do we say the flow of energy in the biosphere is unidirectional

Answer 1

We say the flow of energy is unidirectional and the flow of matter is continuous because once energy is transported from one tropic level to another then at that time the lost heat called as community heat can't be come back.
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Answer 2

Answer:  The energy flow through different steps in the food chain is unidirectional. This means that energy captured by autotrophs does not revert back to the solar input and it passes to the herbivores, i.e. it moves progressively through various trophic levels. Thus, energy flow from the sun through producers to consumers is in single direction only.

Explanation: