The process of converting stored food into usable energy is called respiration. It is similar to burning wood to release stored energy as heat and light. Water and carbon dioxide are released during plant respiration. This is the same as in animal respiration. It occurs in all cells of a plant, in light or darkness.
Respiration is essentially the opposite of photosynthesis.
It uses food plus oxygen to change chemical energy into heat and an energy form usable by the plant. Light is not needed in this process. An example of a plant using stored food can be seen when a potato sprouts in storage. The loss of water through the leaf stomata pulls water up through the plant from the roots. Transpiration is the way plants use water to carry nutrients through their structure.