This is the process where the chemical energy of food molecules, that plants and animals digest, is released and partially captured in the form of ATP. Fats, proteins, and carbohydrates can all be used as fuels in this process, but glucose is most commonly used as an example to observe the pathways and reactions involved. It is a catabolic pathway for the production of ATP, a high energy molecule, which is expended by cells that are working. There are three main stages in this process. They are known as glycolysis, the citric acid cycle, and electron transport. The first stage, glycolysis, involves the splitting of glucose into two molecules of a three carbon sugar. The production of this stage is two molecules of ATP, pyruvic acid, and NADH. The second stage, the citric acid cycle, produces two ATP molecules and a plethora of compounds capable of storing high energy electrons through a series of intermediate steps. The final stage, electron transport, requires oxygen. The high energy electrons are moved on to oxygen and both a gradient and ATP is ultimately produced.