All living things require energy to carry out their life activities. It is needed for various chemical reactions that occur in cells, movement, transporting certain molecules across the cell membrane. Without a constant input of energy, an organism will die.
Food is the source of energy for living things.
It is produced in autotrophs and flows among the organisms in an ecosystem. The main source of energy in an ecosystem is the sun. The sun's energy is capture by producers during photosynthesis. We can think of the sun's energy being trapped in the bonds of sugar molecules. Energy flows between organisms when one organism eats another.