Lactic acid fermentation is a biological process by which sugars such as glucose, fructose, and sucrose, are converted into cellular energy and the metabolic waste product fermental acid. It is a form of fermentation that occurs in some bacteria and animal cells in the absence of oxygen. During homolactic acid fermentation, one molecule of glucose is ultimately converted to two molecules of lactic acid. In heterolactic acid fermentation, sometimes referred to as the phosphoketolase pathway, the products of fermentation are one molecule of carbon dioxide, one molecule of ethanol, and one molecule of lactic acid.

Lactic Acid Fermentation

Glycolysis produces 2 molecules of ATP, reduces 2 molecules of NAD+ to NADH, and creates 2 three-carbon molecules of pyruvate. Most of the chemical energy of the glucose is still trapped in pyruvate. The complete breakdown of glucose to carbon dioxide requires the oxidation of pyruvate thorough the Krebs Cycle and electron transport system (ETS).

When the Krebs Cycle and ETS are working at capacity (regardless of the presence of oxygen), further local ATP needs can be achieved by increasing glycolysis. The resulting pyruvate is converted to Lactic Acid through Lactic Acid Fermentation.

The conversion of pyruvate to lactate regenerates NAD⁺, which allows glycolysis to continue. Lactate diffuses out of the cell and into the blood. The lactate in the bloodstream is converted back into pyruvate in the liver, for use when oxygen is once again present.

Certain cells, such as cardiac muscle cells, are highly permeable to lactate. Lactate is converted into pyruvate and metabolised normally (ie: via the Krebs Cycle). Since these cells are highly oxygenated, it is unlikely that lactate would accumulate (as is the case in oxygen-starved muscle cells). This also allows circulating glucose to be available to muscle cells.

Any excess lactate is taken up by the liver, to pyruvate and then to glucose. This, along with the production of lactate from glucose in muscle cells constitutes the Cori cycle.

Phosphofructokinase (PFK) is inhibited by a low pH and this prevents the formation of excess lactate and/or lactic acidosis (sudden drop in blood pH). PFK catalyses an irreversible step in glycolysis.

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