Answer:
The sun.
Step-by-step explanation:
Sun is the ultimate source of energy. All living things on the earth using the energy comes from the sunlight.
Plants using sunlight to fuel the process of photosynthesis. Photosynthesis is the process of transforming of water and carbon dioxide to oxygen and glucose. The amount of energy used to drive the photosynthesis process is stored in the glucose molecules.
Organism feed on plants and breaking down the glucose molecules to release the stored energy. The release energy is utilized by the cells present in the organisms.
Autotrophs also have the ability to make their own food, so they are also using the energy from the sun to make their food.