Anaerobic glycolysis

Anaerobic glycolysis is the transformation of glucose to lactate when limited amounts of oxygen (O₂) are available. Anaerobic glycolysis is only an effective means of energy production during short, intense exercise, providing energy for a period ranging from 10 seconds to 2 minutes. The anaerobic glycolysis (lactic acid) system is dominant from about 10–30 seconds during a maximal effort. It replenishes very quickly over this period and produces 2 ATP molecules per glucose molecule, or about 5% of glucose's energy potential (38 ATP molecules). The speed at which ATP is produced is about 100 times that of oxidative phosphorylation. The pH in the cytoplasm quickly drops when hydrogen ions accumulate in the muscle, eventually inhibiting enzymes involved in glycolysis.

The burning sensation in muscles during hard exercise can be attributed to the production of hydrogen ions during a shift to anaerobic glycolysis as oxygen is converted to carbon dioxide by aerobic respiration faster than the body can replenish it. These hydrogen ions form a part of lactic acid along with lactate. The body falls back on this less efficient but faster method of producing ATP under low oxygen conditions. This is thought to have been the primary means of energy production in earlier organisms before oxygen was at high concentration in the atmosphere and thus would represent a more ancient form of energy production in cells.

The liver later gets rid of this excess lactate by transforming it back into an important glycolytic intermediate called pyruvate; see Cori cycle.

Many anaerobic microorganisms carry out anaerobic glycolysis through Fermentation.

Fates of pyruvate under anaerobic conditions:

Pyruvate is the terminal electron acceptor in lactic acid fermentation
When sufficient oxygen is not present in the muscle cells for further oxidation of pyruvate and NADH produced in glycolysis, NAD+ is regenerated from NADH by reduction of pyruvate to lactic acid. Pyruvate is converted to lactic acid by the enzyme lactate dehydrogenase. The standard free energy change of the reaction is -25.1 kJ/mol.
Ethanol fermentation
Yeast and other anaerobic microorganisms convert glucose to ethanol and CO2 rather than pyruvate. Pyruvate is first converted to acetaldehyde by enzyme pyruvate decarboxylase in the presence of Thiamine pyrophosphate and Mg++. Carbon-dioxide is released during this reaction. Acetaldehyde is then converted to ethanol by the enzyme alcohol dehydrogenase. NADH is oxidized to NAD+ during this reaction.