Depending on the conditions, the breakdown of the pathway of energy production varies.

i) In presence of oxygen,

In the presence of oxygen-glucose undergoes glycolysis, each glucose molecule produces 2 molecules of pyruvate and this pyruvate enters the citric acid cycle or the kerb cycle and at the end of it, 36 ATP are generated.

ii) In the absence of oxygen, (muscle cells)

In the absence of oxygen, which is the ultimate electron acceptor in the electron transport chain occurring in mitochondria, the pyruvate molecules generated are converted into lactic acid and then ATP is produced. Under this condition, 1 glucose molecule generates only 2 ATPs.

iii) In the absence of oxygen, (microorganisms)

Under this condition, the end products are not lactic acid but ethyl alcohol and this process are also called as fermentation. This property of microorganisms is exploited at an industrial level also.