Direction of Energy Flow
Now let us consider the first point that is the direction of flow of energy. Energy flows from lower (producer) to higher (herbivore, carnivore, etc.) trophic level. It never flows in the reverse direction, that is, from carnivores to herbivores to green plants. Organisms at each trophic level depend on those at lower trophic levels for the energy to sustain them and reproduce. For example, we cannot convert energy directly from the sun into food.
We depend on green plants to make such transformation for us. This is in accordance with the first law of thermodynamics, that energy cannot be created nor destroyed but may be transformed from one form 'into another. For example, the energy of visible light is transformed into chemical energy of the glucose molecule synthesised by green plants through photosynthesis. The living organisms including plants utilise glucose in respiration, which releases chemical energy, and a part of which is ultimately dissipated as heat, that is, the third form of energy.