Lactic acid fermentation
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2008-06-30T21:38:01Z
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[[Image:Lactic-acid-skeletal.png|thumb|[[Lactic acid]]]]
'''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 [[lactic acid]]. It is a form of [[fermentation]] that occurs in some bacteria and [[animal cell]]s 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 [[adenosine triphosphate|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 Chain|electron transport system (ETS)]].
When the Krebs Cycle and ETS are working at capacity (regardless of the presence of oxygen<ref>{{cite journal |last=Sparks |first=Steven |year=1997 |month=July |title=The Purpose of Glycolysis (''Letter'') |journal=Science |volume=277 |issue=5325 |pages=pp. 459–463 |id= |url=http://www.sciencemag.org/cgi/content/long/277/5325/459b |doi=10.1126/science.277.5325.459b |pmid=9254411 }}</ref>), further local ATP needs can be achieved by increasing glycolysis. The resulting pyruvate is converted to [[Lactate|Lactic Acid]] through Lactic Acid Fermentation.
The conversion of pyruvate to lactate regenerates NAD<sup>+</sup>, 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.{{fact|date=February 2008}}
Certain cells, such as [[cardiac muscle]] cells, are highly permeable to lactate.{{fact|date=February 2008}} 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]], converted 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]].{{fact|date=February 2008}}
==References==
{{Reflist}}
{{Carbohydrate metabolism}}
[[Category:Cellular respiration]]
[[Category:Fermentation]]
[[Category:Metabolic pathways]]
[[de:Milchsäuregärung]]
[[es:Fermentación láctica]]
[[fr:Fermentation lactique]]
[[it:Fermentazione lattica]]
[[ja:乳酸発酵]]
[[pl:Fermentacja mlekowa]]
[[pt:Fermentação láctica]]
[[ru:Молочнокислое брожение]]
[[fi:Hapattaminen]]
[[tr:Laktik asit fermantasyonu]]