In an ecosystem energy enters the food chain from the sun, when plants utilize sunlight to manufacture food. This energy than passes from one trophic level to the next. At each trophic level there is loss of energy. Only 10% of the energy of one trophic level passes to the subsequent trophic level. The rest of the energy is lost as heat. The energy lost as heat cannot be utilized by the plants again and do not return to the food chain. Hence, energy flow in an ecosystem is said to be unidirectional.