Solar energy that is captured by the autotrophs cannot revert back to the Sun. This is because the solar energy is converted to chemical energy by the plants & this is stored in food. This energy is then passed on to the consumers in a food chain in form of food & hence can not get reverted back to the plants.\xa0That is why it is said that flow of energy in the biosphere is unidirectional.Sun {tex} \\to {/tex} Plants{tex} \\to {/tex} Herbivores {tex} \\to {/tex} Carnivores