Fermentation is a process where carbohydrates are broken down into simpler molecules under low oxygen or anaerobic conditions. In fermentation, pyruvate, the end product of glycolysis, or its derivatives, act as the terminal electron acceptors. There are fewer ATP molecules generated through fermentation than through aerobic respiration, which uses oxygen as the terminal electron acceptor. Lactic acid and ethanol fermentation are the two most common types of fermentation. Lactic acid fermentation occurs in muscle cells under anaerobic conditions. During intense muscular activity, the skeletal muscle cells experience low oxygen. The enzyme lactate dehydrogenase reduces pyruvate to lactate and oxidizes NADH to NAD+. In contrast, ethanol fermentation occurs in yeast and bacteria. Pyruvate is first converted into acetaldehyde by pyruvate decarboxylase. The acetaldehyde is then further reduced to ethanol by alcohol dehydrogenase. Both types of fermentation ensure that NAD+ is regenerated so that it can be reused in glycolysis for further ATP production, in spite of low oxygen conditions.