Anaerobic glycolysis is a metabolic process in which glucose, a sugar molecule, is broken down without the use of oxygen. Like aerobic glycolysis, which metabolizes glucose in the presence of oxygen, it produces energy for the cells. Breaking down glucose without using oxygen also produces lactate, however, and when the process is prolonged, it generally leads to lactic acidosis, which is a decrease in the pH level of the blood. It is usually alleviated when normal oxygen levels return to the cell and aerobic glycolysis takes over.
Typically, anaerobic glycolysis occurs in muscle cells during vigorous physical activity. When the energy requirement for a particular action is not adequately met by aerobic means, the muscle cells process glucose without the use of oxygen in order to produce energy quickly. Eventually, the surrounding tissue is flooded with lactate, and the muscle activity generally decreases. As the lactate concentration increases in the blood, it is slowly converted back to glucose in the liver with the aid of oxygen. The conversion of glucose to lactate and lactate back to glucose is called the Cori Cycle, which was described by Carl and Gerty Cori in the 1930s and 1940s.
Certain cells and tissues convert glucose to lactate even in the presence of oxygen, including red blood cells and cells of the retina. Since the earliest cells had to thrive in conditions that were void of oxygen, metabolic pathways such as anaerobic glycolysis evolved to produce energy. Cells that lack a mitochondria also typically use this process.
Normally, glycolysis produces two molecules of pyruvate from one glucose molecule, as well as a molecule called NADH. Each pyruvate molecule is usually converted to acetate and then processed in the citric acid cycle to form carbon dioxide and water, while NADH is oxidized to NAD+ by passing its electrons to an oxygen molecule in the mitochondria. NAD+ is a required electron acceptor in the process of glycolysis and without it, glycolysis would stop.
Under anaerobic conditions, the oxygen molecule that is required to accept the electron from NADH is usually missing, which forces the cell to find another electron acceptor. The molecule that fulfills this role is typically lactate, which is the reduced form of pyruvate. An enzyme called lactate dehydrogenase catalyzes the reaction that converts pyruvate to lactate. In the process, NADH donates its electron to pyruvate and is converted to NAD+, which is then recycled for use in glycolysis.