Cellular respiration allows organisms to use (release) energy stored in the chemical bonds of glucose (C6H12O6). The energy in glucose is used to produce ATP. Cells use ATP to supply their energy needs. Cellular respiration is therefore a process in which the energy in glucose is transferred to ATP.
In respiration, glucose is oxidized and thus releases energy. Oxygen is reduced to form water.
The carbon atoms of the sugar molecule are released as carbon dioxide (CO2).
The complete breakdown of glucose to carbon dioxide and water requires two major steps: 1) glycolysis and 2) aerobic respiration. Glycolysis produces two ATP. Thirty-four more ATP are produced by aerobic pathways if oxygen is present.
In the absence of oxygen, fermentation reactions produce alcohol or lactic acid but no additional ATP.