Respiration

Glucose is a carbohydrate - a compound of carbon and hydrogen. The bonds between the carbon and the hydrogen atoms are very strong. In the cells, the substrate, often glucose, is broken down into carbon dioxide and water in the presence of oxygen. This process breaks the bonds between carbon and hydrogen and thus releases energy. This is called respiration. It has three main steps:

Glycolysis

This takes place in the cytoplasm of the cells. The glucose is converted to two molecules of pyruvic acid. No oxygen is required for this step.

Kreb's Cycle

It takes place in the mitochondria. The pyruvic acid formed during glycolysis is oxidised and forms carbon dioxide and water. This reaction also releases molecules called the coenzymes: NADH₂ and FADH₂. The hydrogen atoms associated with these are the hydrogen atoms released during the oxidation of pyruvic acid.