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Fermentation and anaerobic respiration

How cells extract energy from glucose without oxygen. In yeast, the anaerobic reactions make alcohol, while in your muscles, they make lactic acid.

Introduction

Ever wonder how yeast ferment barley malt into beer? Or how your muscles keep working when you're exercising so hard that they're very low on oxygen?
Both of these processes can happen thanks to alternative glucose breakdown pathways that occur when normal, oxygen-using (aerobic) cellular respiration is not possible—that is, when oxygen isn't around to act as an acceptor at the end of the electron transport chain. These fermentation pathways consist of glycolysis with some extra reactions tacked on at the end. In yeast, the extra reactions make alcohol, while in your muscles, they make lactic acid.
Fermentation is a widespread pathway, but it is not the only way to get energy from fuels anaerobically (in the absence of oxygen). Some living systems instead use an inorganic molecule other than O2, such as sulfate, as a final electron acceptor for an electron transport chain. This process, called anaerobic cellular respiration, is performed by some bacteria and archaea.
In this article, we'll take a closer look at anaerobic cellular respiration and at the different types of fermentation.

Anaerobic cellular respiration

Anaerobic cellular respiration is similar to aerobic cellular respiration in that electrons extracted from a fuel molecule are passed through an electron transport chain, driving ATP synthesis. Some organisms use sulfate (SO42) as the final electron acceptor at the end ot the transport chain, while others use nitrate (NO3), sulfur, or one of a variety of other molecules1.
What kinds of organisms use anaerobic cellular respiration? Some prokaryotes—bacteria and archaea—that live in low-oxygen environments rely on anaerobic respiration to break down fuels. For example, some archaea called methanogens can use carbon dioxide as a terminal electron acceptor, producing methane as a by-product. Methanogens are found in soil and in the digestive systems of ruminants, a group of animals including cows and sheep.
Similarly, sulfate-reducing bacteria and Archaea use sulfate as a terminal electron acceptor, producing hydrogen sulfide (H2S) as a byproduct. The image below is an aerial photograph of coastal waters, and the green patches indicate an overgrowth of sulfate-reducing bacteria.
Aerial photograph of coastal waters with blooms of sulfate-reducing bacteria appearing as large patches of green in the water.
Image credit: "Metabolism without oxygen: Figure 1," OpenStax College, Biology, CC BY 3.0; Modification of work by NASA/Jeff Schmaltz, MODIS Land Rapid Response Team at NASA GSFC, Visible Earth Catalog of NASA images.

Fermentation

Fermentation is another anaerobic (non-oxygen-requiring) pathway for breaking down glucose, one that's performed by many types of organisms and cells. In fermentation, the only energy extraction pathway is glycolysis, with one or two extra reactions tacked on at the end.
Fermentation and cellular respiration begin the same way, with glycolysis. In fermentation, however, the pyruvate made in glycolysis does not continue through oxidation and the citric acid cycle, and the electron transport chain does not run. Because the electron transport chain isn't functional, the NADH made in glycolysis cannot drop its electrons off there to turn back into NAD+
The purpose of the extra reactions in fermentation, then, is to regenerate the electron carrier NAD+ from the NADH produced in glycolysis. The extra reactions accomplish this by letting NADH drop its electrons off with an organic molecule (such as pyruvate, the end product of glycolysis). This drop-off allows glycolysis to keep running by ensuring a steady supply of NAD+.

Lactic acid fermentation

In lactic acid fermentation, NADH transfers its electrons directly to pyruvate, generating lactate as a byproduct. Lactate, which is just the deprotonated form of lactic acid, gives the process its name. The bacteria that make yogurt carry out lactic acid fermentation, as do the red blood cells in your body, which don’t have mitochondria and thus can’t perform cellular respiration.
Diagram of lactic acid fermentation. Lactic acid fermentation has two steps: glycolysis and NADH regeneration.
During glycolysis, one glucose molecule is converted to two pyruvate molecules, producing two net ATP and two NADH.
During NADH regeneration, the two NADH donate electrons and hydrogen atoms to the two pyruvate molecules, producing two lactate molecules and regenerating NAD+.
Muscle cells also carry out lactic acid fermentation, though only when they have too little oxygen for aerobic respiration to continue—for instance, when you’ve been exercising very hard. It was once thought that the accumulation of lactate in muscles was responsible for soreness caused by exercise, but recent research suggests this is probably not the case.
Lactic acid produced in muscle cells is transported through the bloodstream to the liver, where it’s converted back to pyruvate and processed normally in the remaining reactions of cellular respiration.

Alcohol fermentation

Another familiar fermentation process is alcohol fermentation, in which NADH donates its electrons to a derivative of pyruvate, producing ethanol.
Going from pyruvate to ethanol is a two-step process. In the first step, a carboxyl group is removed from pyruvate and released in as carbon dioxide, producing a two-carbon molecule called acetaldehyde. In the second step, NADH passes its electrons to acetaldehyde, regenerating NAD+ and forming ethanol.
Diagram of alcohol fermentation. Alcohol fermentation has two steps: glycolysis and NADH regeneration.
During glycolysis, one glucose molecule is converted to two pyruvate molecules, producing two net ATP and two NADH.
During NADH regeneration, the two pyruvate molecules are first converted to two acetaldehyde molecules, releasing two carbon dioxide molecules in the process. The two NADH then donate electrons and hydrogen atoms to the two acetaldehyde molecules, producing two ethanol molecules and regenerating NAD+.
Alcohol fermentation by yeast produces the ethanol found in alcoholic drinks like beer and wine. However, alcohol is toxic to yeasts in large quantities (just as it is to humans), which puts an upper limit on the percentage alcohol in these drinks. Ethanol tolerance of yeast ranges from about 5 percent to 21 percent, depending on the yeast strain and environmental conditions.

Facultative and obligate anaerobes

Many bacteria and archaea are facultative anaerobes, meaning they can switch between aerobic respiration and anaerobic pathways (fermentation or anaerobic respiration) depending on the availability of oxygen. This approach allows lets them get more ATP out of their glucose molecules when oxygen is around—since aerobic cellular respiration makes more ATP than anaerobic pathways—but to keep metabolizing and stay alive when oxygen is scarce.
Other bacteria and archaea are obligate anaerobes, meaning they can live and grow only in the absence of oxygen. Oxygen is toxic to these microorganisms and injures or kills them on exposure. For instance, the Clostridium bacteria that are responsible for botulism (a form of food poisoning) are obligate anaerobes2. Recently, some multicellular animals have even been discovered in deep-sea sediments that are free of oxygen3,4.

Self-check

Image of tanks used for wine production by fermentation of grapes. The tanks are quipped with pressure-release valves.
Image credit: "Metabolism without oxygen: Figure 3" by OpenStax College, Biology, CC BY 3.0
  1. Inside these tanks, yeasts are busily fermenting grape juice into wine. Why do winemaking tanks like these need pressure-release valves?
    Choose 1 answer:


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  • blobby green style avatar for user Phil Rattazzi
    Is there a reason why Flourine can't be used in place of oxygen as the final acceptor in the electron transport chain? Wouldn't it produce more ATP due to its higher electronegativity?
    (35 votes)
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    • leaf green style avatar for user Stefan L.
      There are a few reasons that spring to mind. The first is simply to do with availability. Oxygen makes up 21% of our atmosphere and is stable in both air and water whereas fluorine is much rarer. In addition fluorine is very reactive so would not exist by itself for very long. Also if fluorine were used as the terminal electron acceptor it would form HF, hydrofluoric acid in solution which is hard for the cells to deal with and would affect pH in the cytosol affecting enzyme function whereas oxygen just forms water. Finally fluoride is known to be damaging to the body above certain concentrations affecting things like the nervous system and hormone secretion as well as protein synthesis.
      Please bear in mind these are just my thoughts.

      P.S remember oxygen is not producing the ATP itself it is merely keeping the transport chain unblocked so the electrons keep flowing. A more electronegative element wouldn't necessarily have any effect on the rate of electron flow down the ETC and therefore wouldn't affect the rate of ATP production.
      (77 votes)
  • blobby green style avatar for user capizzanoco
    Would Balsamic Vinegar be an example of lactic acid fermentation since the grape bypasses the alcohol?
    (12 votes)
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  • leaf green style avatar for user Rachel
    In the diagrams there write, "NADH regeneration," wouldn't it be more accurate to say "NAD+ regeneration?"
    (9 votes)
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  • mr pink red style avatar for user Revan Rangotis
    Okay, this is actually really interesting... if the lactate isn't what's causing the soreness of muscles after exercising, then what is it?
    (5 votes)
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  • blobby green style avatar for user markselden
    Is fermentation really always anaerobic?

    Every information source I look at asserts that fermentation is an anaerobic process. At the same time, every source for vinegar fermentation describes a process that requires oxygen.

    Can’t find any mention of this seeming ambiguity anywhere.
    (5 votes)
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  • leaf green style avatar for user Ashley  He
    In the last section of the article, it mentions that obligate anaerobes can be harmed by oxygen. However, I don't understand how the presence of oxygen can harm them if it isn't used in their pathways.

    Basically, how does oxygen interfere with the processes of obligate anaerobes?
    (3 votes)
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    • piceratops ultimate style avatar for user FrozenPhoenix45
      Basically, for aerobic organisms to survive in oxygen, they require "defenses". Obligate anaerobes have not developed these defenses. To make a long complicated process short (and simplified), there are certain oxygen products that are produced by processes in the cell that are extremely reactive. This extreme reactivity basically tears the cell apart if these new molecules and ions are not dealt with.

      So why can aerobes survive? They have in them specific molecules that react with the dangerous molecules to make something much less dangerous (these detoxifying molecules are superoxide dismutase, catalase, and peroxidase). Obligate anaerobes do not have these detoxifiers, and as a result, the toxic molecules interfere with cellular processes.

      This is an extremely simplified explanation (I left out the chemical equations and molecules required for the process as they're not particularly well-known), but I hope this helps!

      If you still don't understand, the National Library of Medicine has a detailed (albeit complicated) article on anaerobes. You should check that out.
      (6 votes)
  • duskpin ultimate style avatar for user Angela
    The article states that recent research suggests that soreness is not caused by the accumulation of lactate; then what is the actual cause of the soreness and cramps in muscles after rigorous exercise?
    (5 votes)
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  • leaf blue style avatar for user JirehBasingan
    why plants can not regenerate pyruvate from ethanol?
    (4 votes)
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  • aqualine ultimate style avatar for user Jea
    Where is the electron transport chain in an anaerobic respiration found?
    (3 votes)
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    • piceratops ultimate style avatar for user FrozenPhoenix45
      The electron chain is not found in anaerobic respiration. Anaerobic means "without oxygen". The electron transport chain requires oxygen, therefore it cannot be present in anaerobic respiration.

      I hope this helped! Comment if you have any questions; I'll answer to the best of my ability.
      (2 votes)
  • blobby green style avatar for user Deby Erina Parung
    Why can't human undergo ethanol fermentation? is there an enzyme that is required which we don't have?
    (3 votes)
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