Explain why, the flow of energy in the ecosystem is said to be unidirectional.

Energy flows from plants to herbivores, then to carnivores in a food chain. Most of the energy is utilized by the organism for its activities and un-utilized energy is dissipated in the form of heat; this energy in the form of heat cannot be returned back to plant for photosynthesis. Therefore, The flow of energy in the ecosystem is said to be unidirectional.