Our body get the energy they need from food through metabolism, the chemical reactions in the body cellls that convert the fuel from food into the energy needed to do everything from moving to thinking to growing. Specific proteins in the body control the chemical reactions of metabolism, and each chemical reaction is coordinated with other body functions. In fact, thousands of metabolic reactions happen at the same time — all regulated by the body — to keep our cells healthy and working.

Metabolism is a constant process that begins when we are conceived and ends when we die. It is a vital process for all life forms — not just humans. If metabolism stops, a living thing dies.

Here an example of how the process of metabolism works in humans — and it begins with plants. First, a green plant takes in energy from sunlight. The plant uses this energy and the molecule cholorophyll (which gives plants their green color) to build sugars from water and carbon dioxide in a process known as photosynthesis.

When people and animals eat the plants (or, if they are carnivores, when they eat animals that have eaten the plants), they take in this energy (in the form of sugar), along with other vital cell-building chemicals. The body next step is to break the sugar down so that the energy released can be distributed to, and used as fuel by, the body cells

  source=http://kidshealth.org/