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Fermentation in progress: Bubbles of CO2 form a froth on top of the fermentation mixture.
Overview of ethanol fermentation. One glucose molecule breaks down into two pyruvate molecules (1). The energy from this exothermic reaction is used to bind inorganic phosphates to ATP and convert NAD+ to NADH. The two pyruvates are then broken down into two acetaldehyde molecules and give off two CO2 molecules as a waste product (2). The acetaldehyde is then reduced into ethanol using the energy and hydrogen from NADH; in this process the NADH is oxidized into NAD+ so that the cycle may repeat (3).

Fermentation is a metabolic process that converts sugar to acids, gases or alcohol. It occurs in yeast and bacteria, but also in oxygen-starved muscle cells, as in the case of lactic acid fermentation. Fermentation is also used more broadly to refer to the bulk growth of microorganisms on a growth medium, often with the goal of producing a specific chemical product. French microbiologist Louis Pasteur is often remembered for his insights into fermentation and its microbial causes. The science of fermentation is known as zymology.

Fermentation takes place in the lack of oxygen (when the electron transport chain is unusable) and becomes the cell’s primary means of ATP (energy) production.<ref name="Prescott Microbiology">{{#invoke:citation/CS1|citation |CitationClass=book }}</ref> It turns NADH and pyruvate produced in the glycolysis step into NAD+ and various small molecules depending on the type of fermentation (see examples below). In the presence of O2, NADH and pyruvate are used to generate ATP in respiration. This is called oxidative phosphorylation, and it generates much more ATP than glycolysis alone. For that reason, cells generally benefit from avoiding fermentation when oxygen is available, the exception being obligate anaerobes which cannot tolerate oxygen.

The first step, glycolysis, is common to all fermentation pathways:

C6H12O6 + 2 NAD+ + 2 ADP + 2 Pi → 2 CH3COCOO + 2 NADH + 2 ATP + 2 H2O + 2H+

Pyruvate is CH3COCOO. Pi is phosphate. Two ADP molecules and two Pi are converted to two ATP and two water molecules via substrate-level phosphorylation. Two molecules of NAD+ are also reduced to NADH.<ref name="AP Biology. Anestis 2006. P. 61"/>

In oxidative phosphorylation the energy for ATP formation is derived from an electrochemical proton gradient generated across the inner mitochondrial membrane (or, in the case of bacteria, the plasma membrane) via the electron transport chain. Glycolysis has substrate-level phosphorylation (ATP generated directly at the point of reaction).

Humans have used fermentation to produce food and beverages since the Neolithic age. For example, fermentation is used for preservation in a process that produces lactic acid as found in such sour foods as pickled cucumbers, kimchi and yogurt (see fermentation in food processing), as well as for producing alcoholic beverages such as wine (see fermentation in winemaking) and beer. Fermentation can even occur within the stomachs of animals, such as humans. Auto-brewery syndrome is a rare medical condition where the stomach contains brewers yeast that break down starches into ethanol; which enters the blood stream.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref>


Fermentation sections
Intro  Definitions  Examples  Chemistry   Hydrogen gas production in fermentation   Methane gas production in fermentation  History  Etymology  See also  References  External links  

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{{#invoke:Hatnote|hatnote}}

Fermentation in progress: Bubbles of CO2 form a froth on top of the fermentation mixture.
Overview of ethanol fermentation. One glucose molecule breaks down into two pyruvate molecules (1). The energy from this exothermic reaction is used to bind inorganic phosphates to ATP and convert NAD+ to NADH. The two pyruvates are then broken down into two acetaldehyde molecules and give off two CO2 molecules as a waste product (2). The acetaldehyde is then reduced into ethanol using the energy and hydrogen from NADH; in this process the NADH is oxidized into NAD+ so that the cycle may repeat (3).

Fermentation is a metabolic process that converts sugar to acids, gases or alcohol. It occurs in yeast and bacteria, but also in oxygen-starved muscle cells, as in the case of lactic acid fermentation. Fermentation is also used more broadly to refer to the bulk growth of microorganisms on a growth medium, often with the goal of producing a specific chemical product. French microbiologist Louis Pasteur is often remembered for his insights into fermentation and its microbial causes. The science of fermentation is known as zymology.

Fermentation takes place in the lack of oxygen (when the electron transport chain is unusable) and becomes the cell’s primary means of ATP (energy) production.<ref name="Prescott Microbiology">{{#invoke:citation/CS1|citation |CitationClass=book }}</ref> It turns NADH and pyruvate produced in the glycolysis step into NAD+ and various small molecules depending on the type of fermentation (see examples below). In the presence of O2, NADH and pyruvate are used to generate ATP in respiration. This is called oxidative phosphorylation, and it generates much more ATP than glycolysis alone. For that reason, cells generally benefit from avoiding fermentation when oxygen is available, the exception being obligate anaerobes which cannot tolerate oxygen.

The first step, glycolysis, is common to all fermentation pathways:

C6H12O6 + 2 NAD+ + 2 ADP + 2 Pi → 2 CH3COCOO + 2 NADH + 2 ATP + 2 H2O + 2H+

Pyruvate is CH3COCOO. Pi is phosphate. Two ADP molecules and two Pi are converted to two ATP and two water molecules via substrate-level phosphorylation. Two molecules of NAD+ are also reduced to NADH.<ref name="AP Biology. Anestis 2006. P. 61"/>

In oxidative phosphorylation the energy for ATP formation is derived from an electrochemical proton gradient generated across the inner mitochondrial membrane (or, in the case of bacteria, the plasma membrane) via the electron transport chain. Glycolysis has substrate-level phosphorylation (ATP generated directly at the point of reaction).

Humans have used fermentation to produce food and beverages since the Neolithic age. For example, fermentation is used for preservation in a process that produces lactic acid as found in such sour foods as pickled cucumbers, kimchi and yogurt (see fermentation in food processing), as well as for producing alcoholic beverages such as wine (see fermentation in winemaking) and beer. Fermentation can even occur within the stomachs of animals, such as humans. Auto-brewery syndrome is a rare medical condition where the stomach contains brewers yeast that break down starches into ethanol; which enters the blood stream.<ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref>


Fermentation sections
Intro  Definitions  Examples  Chemistry   Hydrogen gas production in fermentation   Methane gas production in fermentation  History  Etymology  See also  References  External links  

PREVIOUS: IntroNEXT: Definitions
<<>>