There are two different ways in which glucose is oxidised to provide energy in various organism. They are aerobic and anaerobic.
In all cases first step is to break the 6 carbon molecule glucose into pyruvic acid in absence of oxygen . This process take place in cytoplasm.
(i) Aerobic → food is oxidised to provide energy in presence of oxygen. In this process pyruvic acid is broken down into 3 molecules of carbon dioxide and water. This process occurs in mitochondria.
It also releases energy in the form of ATP.
(ii) Anaerobic → where oxidation of food takes place in absence of oxygen. It occurs in yeast where pyruvic acid is broken down to form 2 carbon molecules of ethanol and 2 molecule of carbon dioxide and 2 molecules of ATP (energy)
Pyruvic acid → 2C2H5OH + 2CO2 + 2ATP.