Humans need oxygen to live because it is an ingredient used during respiration to convert glucose into energy. In the absence of oxygen, cells produce only very limited quantities of energy.
The human body relies on energy produced by its cells. This energy converts glucose and oxygen into ATP, which is the energy that the body uses, as well as carbon dioxide and water, which are waste products of the reaction. When glucose combines with oxygen to create energy, it is termed "oxidization."
The process of cellular respiration to create energy includes three phases: glycolysis, the Krebs cycle and electron transport. In glycolysis, each molecule of glucose is split into two smaller sugar atoms, called pyruvate. This step also results in the production of two molecules of ATP. The pyruvate is then transported to the cell's mitochondria and converted into acetyl CoA.
In the Krebs cycle, the hydrogen atoms are removed from the CoA. Eletrons from these atoms are taken for use in later steps. The byproducts of the Krebs cycle include carbon dioxide, water, four molecules of ATP and a large amount of NADH, which carries electrons.
Finally, during electron transport, the electrons stored in NADH are passed along an electron chain to produce ATP. This final stage results in the production of 32 molecules of ATP.