The main energy source for all organisms is glucose (also called blood sugar). Glucose is found in the foods and fluids you eat.
When you eat carbohydrates, they break down into smaller molecules of glucose, which are then absorbed by your gut cells and transported to the bloodstream. Your body uses these glucose molecules for fuel, helping to power your organs, muscles and nervous system.
Cells use the chemical energy stored in glucose bonds to perform chemical reactions and produce ATP, the main form of energy for all cells. A typical cell has 109 molecules of ATP in solution at any time, and they are turned over (used up and replaced) every 1-2 minutes.
Plants and animals make glucose from light and other chemicals in the presence of water and carbon dioxide as part of photosynthesis. This process produces a variety of sugars and starches that are eaten by the animals and humans.
In the case of plants, some of these sugars are metabolized and used for fuel, others are stored as starch in the form of polysaccharides. The most important storage form for sugars in animals is glycogen.
Glycogen is a large branched polysaccharide that is made of many glucose molecules joined together in long chains. It is very heavy and does not dissolve easily in the cellular medium, which makes it an excellent storage molecule.
The chemistry of glycolysis is complex, but it is the most efficient way to break down and convert glucose into the molecules needed by your body for energy and storage. When your body does not need glucose from the foods you eat, it stores it in your muscles and liver as glycogen. When you do need extra glucose, your body breaks down this glycogen in the liver into glucose and uses it for energy.
