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 (during the energy investment stage.) This breakdown of glucose into pyruvate therefore results in a net gain of ATP molecules.
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 then pyruvate undergoes fermentation in the cytoplasm of the cell.
There are two types of fermentation:
- Alcoholic fermentation - pyruvate is converted to ethanol and CO2. This occurs in plant cells and fungi (e.g. yeast cells) and is an irreversible reaction.
- Lactate fermentation - pyruvate is converted to lactate. This occurs in animal cells and is a reversible reaction.
Fermentation results in much less ATP being produced than in aerobic respiration.
