The Pasteur effect describes how available

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...

inhibits

ethanol fermentation Ethanol fermentation, also called alcoholic fermentation, is a biological process which converts sugars such as glucose, fructose, and sucrose into cellular energy, producing ethanol and carbon dioxide as by-products. Because yeasts perform this ...

, driving

yeast Yeasts are eukaryotic, single-celled microorganisms classified as members of the fungus kingdom. The first yeast originated hundreds of millions of years ago, and at least 1,500 species are currently recognized. They are estimated to constit ...

to switch toward aerobic respiration for increased generation of the energy carrier adenosine triphosphate (ATP).

Discovery

The effect was described by Louis Pasteur in 1857 in experiments showing that

aeration Aeration (also called aerification or aeriation) is the process by which air is circulated through, mixed with or dissolved in a liquid or other substances that act as a fluid (such as soil). Aeration processes create additional surface area in ...

of yeasted broth causes cell growth to increase while the fermentation rate decreases, based on lowered ethanol production.

Explanation

Yeast fungi, being

facultative anaerobes A facultative anaerobic organism is an organism that makes ATP by aerobic respiration if oxygen is present, but is capable of switching to fermentation if oxygen is absent. Some examples of facultatively anaerobic bacteria are ''Staphylococcus' ...

, can either produce energy through ethanol fermentation or aerobic respiration. When the O₂ concentration is low, the two pyruvate molecules formed through glycolysis are each fermented into

ethanol Ethanol (abbr. EtOH; also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound. It is an alcohol with the chemical formula . Its formula can be also written as or (an ethyl group linked to a ...

and

carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...

. While only 2 ATP are produced per glucose, this method is utilized under anaerobic conditions because it oxidizes the electron shuttle NADH into NAD⁺ for another round of glycolysis and ethanol fermentation. If the concentration of oxygen increases, pyruvate is instead converted to

acetyl CoA Acetyl-CoA (acetyl coenzyme A) is a molecule that participates in many biochemical reactions in protein, carbohydrate and lipid metabolism. Its main function is to deliver the acetyl group to the citric acid cycle (Krebs cycle) to be oxidized for ...

, used in the

citric acid cycle The citric acid cycle (CAC)—also known as the Krebs cycle or the TCA cycle (tricarboxylic acid cycle)—is a series of chemical reactions to release stored energy through the oxidation of acetyl-CoA derived from carbohydrates, fats, and protein ...

, and undergoes oxidative phosphorylation. Per glucose, 10 NADH and 2 FADH₂ are produced in cellular respiration for a significant amount of proton pumping to produce a proton gradient utilized by ATP Synthase. While the exact ATP output ranges based on considerations like the overall electrochemical gradient, aerobic respiration produces far more ATP than the anaerobic process of ethanol fermentation. The increased ATP and

citrate Citric acid is an organic compound with the chemical formula HOC(CO2H)(CH2CO2H)2. It is a colorless weak organic acid. It occurs naturally in citrus fruits. In biochemistry, it is an intermediate in the citric acid cycle, which occurs in the ...

from aerobic respiration allosterically inhibit the glycolysis enzyme

phosphofructokinase 1 Phosphofructokinase-1 (PFK-1) is one of the most important regulatory enzymes () of glycolysis. It is an allosteric enzyme made of 4 subunits and controlled by many activators and inhibitors. PFK-1 catalyzes the important "committed" step of gl ...

because less pyruvate is needed to produce the same amount of ATP. Despite this energetic incentive, Rosario Lagunas has shown that yeast continue to partially ferment available glucose into ethanol for many reasons. First, glucose metabolism is faster through ethanol fermentation because it involves fewer enzymes and limits all reactions to the

cytoplasm In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. ...

. Second, ethanol has bactericidal activity by causing damage to the

cell membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment ( ...

and protein denaturing, allowing yeast fungus to outcompete environmental bacteria for resources. Third, partial fermentation may be a defense mechanism against environmental competitors depleting all oxygen faster than the yeast's regulatory systems could fully switch from aerobic respiration to ethanol fermentation.

Practical implications

The fermentation processes used in alcohol production is commonly maintained in low oxygen conditions, under a blanket of carbon dioxide, while growing yeast for biomass involves aerating the broth for maximized energy production. Despite the bactericidal effects of ethanol, acidifying effects of fermentation, and low oxygen conditions of industrial alcohol production, bacteria that undergo

lactic acid fermentation Lactic acid fermentation is a metabolic process by which glucose or other six-carbon sugars (also, disaccharides of six-carbon sugars, e.g. sucrose or lactose) are converted into cellular energy and the metabolite lactate, which is lactic acid i ...

can contaminate such facilities because lactic acid has a low pKa of 3.86 to avoid decoupling the pH membrane gradient that supports regulated transport.

Discovery

Explanation

Practical implications

See also

References

Further reading