Fermentation can be best defined as the production of energy by substrate-level phosphorylation. This biological process occurs in the absence of oxygen and allows cells to generate energy by converting sugars into acids, gases, or alcohol. Unlike complete catabolism, fermentation does not require the full oxidation of glucose to carbon dioxide and water. Instead, it partially breaks down glucose, resulting in various by-products depending on the type of organism and the conditions present.
To clarify further, during fermentation, glucose undergoes glycolysis to produce pyruvate, which is then converted into different metabolites, like lactic acid or ethanol, through substrate-level phosphorylation. This is distinct from oxidative phosphorylation, where energy production relies on oxygen to fully oxidize substrates, resulting in higher energy yields.