Glycolysis is the first step of cellular respiration, the process by which living cells break down glucose into small, energy-containing molecules called ATP. In glycolysis, a single glucose molecule breaks down into two molecules of pyruvate, which can then be broken down further to release ATP. Glycolysis itself releases two molecules of ATP per molecule of glucose.
Unlike later chemical reactions in the process of cell respiration, glycolysis does not require an oxygen source. Its production of ATP allows the cell to generate at least a small amount of energy even without oxygen. If oxygen is available, the two pyruvate molecules produced during glycolysis undergo respiration reactions to produce another 32 molecules of ATP in total. If oxygen is not available, the plant instead uses a process called fermentation to further break down the pyruvate. In fermentation, far fewer ATP molecules form, but the cell can regenerate NAD+, a compound that is necessary to continue glycolysis of more glucose molecules.
There are 10 steps in the series of chemical reactions known as glycolysis. In each step, the carbon molecules in the glucose rearrange into a lower-energy structure. The last step is the energy generating step, where two molecules of ADP convert into higher-energy ATP molecules.