Glycolysis
Glycolysis is the breakdown of glucose into two pyruvate molecules. This process does not require oxygen (it is anaerobic).
The production of pyruvate from glucose involves the production of several intermediate molecules.
Phosphorylation of glucose and these intermediates requires ATP molecules in an energy investment stage.
More ATP molecules are then regenerated than were used in the production of other intermediates.
This breakdown of glucose into pyruvate therefore results in a net gain of ATP molecules in this energy payoff stage.
Dehydrogenase enzymes remove hydrogen ions and electrons from intermediates of this cycle, which are passed to the coenzyme NAD (forming NADH).
The hydrogen ions and electrons are passed to the electron transport chain on the inner mitochondrial membrane. This occurs in both glycolysis and the citric acid cycle.
If oxygen is available (aerobic conditions), pyruvate molecules progress into the citric acid cycle.
If oxygen is not available (anaerobic conditions) then pyruvate undergoes fermentation in the cytoplasm of the cell.