The electron transport chain, as the name suggests, is a series of passing on electrons to atoms or molecules that are more electronegative, with oxygen ultimately being the most electronegative participant. Each time electrons are passed, there is the creation of a proton gradient (H+) that is available to create a thermodynamic potential to do mechanical work. It is this mechanical work that facilitates the creation of the adenosine triphosphate molecule (ATP) in stages within the mitochondrion. The mitochondrion is the organelle within animal cells that conducts cellular respiration, which is the oxidation of glucose (C6H12O6) with oxygen. This reaction produces ATP, along with carbon dioxide (CO2) and water (H2O) as waste products. The process starts with an electron donor, such as NAD+, which will combine readily with a hydrogen atom to become NADH. The hydrogen loses its electron, becomes the aforementioned proton gradient (H+), while the NAD+ couples with another molecule, on the way to producing ATP. Oxygen is the ultimate electron receiver, being reduced to water as a waste product, while helping to serve construct adenosine triphosphate by the transfer of electrons.