The body utilises ATP already present in muscles as well as ATP produced by lactic acid fermentation for brief, bursts of energy. Cellular respiration is the sole technique to continue producing ATP for exercise that lasts longer than 90 seconds. This process requires oxygen to take place in three main steps: oxidation, reduction and regeneration. During oxidation, oxygen takes part in chemical reactions that release energy used later during muscle contraction. The products of this reaction are carbon dioxide and water. In reduction, iron(II) + 2xH+ = 2Fe(III) + 2xH~2~O. Iron is reduced while hydrogen ions are released into the cell to be reused in oxidation. Regeneration involves the restoration of both oxidised molecules of DNA and proteins. The process of regeneration occurs within minutes after exercise has stopped. ATP is also needed to carry out many other cellular functions including the transmission of messages between cells. It controls the rate at which cells divide and differentiate. Energy supply pathways control how much ATP is made from glucose or fatty acids. The mitochondria are the powerhouses of cells where oxidative phosphorylation takes place. They are responsible for generating most of the cell's energy through electron transport chains located inside them. Other parts of the cell use some of this energy to build more mitochondria. There are different pathways by which nutrients can enter cells to be used for fuel for energy.