The first source of energy is stored in glucose molecules that are manufactured by leaves during photosynthesis.
Plants are known as producers and the energy stored in the plants is transferred to different organisms in the food chain.
For example,
Grass --> grasshopper --> chicken --> lion
Chemical energy stored in the glucose is eventually converted into starch for storage is found in the grass. When the grasshopper eats the grass, starch is digested to form glucose by the grasshopper’s digestive system. The glucose formed when starch in the grass is digested is small and soluble. The small, soluble glucose then enters the grasshopper’s living cells and it is broken down to release energy for the grasshopper to carry out its cellular activities. When the chicken eats the grasshopper, the body parts of the grasshopper are digested by the chicken’s digestive system and some of the amino acids are converted into glucose. Glucose enters the chicken’s living cells and glucose is broken down to release energy for the cellular activities of the lion. A similar process happens when the lion eats the chicken.
The process by which living cells break down glucose to release energy is known as respiration.