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.
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 , 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 synthesis. Some organisms use sulfate as the final electron acceptor at the end ot the transport chain, while others use nitrate , sulfur, or one of a variety of other molecules.
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 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.
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 made in glycolysis cannot drop its electrons off there to turn back into
The purpose of the extra reactions in fermentation, then, is to regenerate the electron carrier from the produced in glycolysis. The extra reactions accomplish this by letting 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 .
Lactic acid fermentation
In lactic acid fermentation, 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.
Another familiar fermentation process is alcohol fermentation, in which 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, passes its electrons to acetaldehyde, regenerating 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 percent to 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 anaerobes. Recently, some multicellular animals have even been discovered in deep-sea sediments that are free of oxygen.
Image of tanks used for wine production by fermentation of grapes. The tanks are quipped with pressure-release valves.
- Inside these tanks, yeasts are busily fermenting grape juice into wine. Why do winemaking tanks like these need pressure-release valves?
Want to join the conversation?
- 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?(33 votes)
- 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.(69 votes)
- Would Balsamic Vinegar be an example of lactic acid fermentation since the grape bypasses the alcohol?(12 votes)
- To make vinegar, grapes are first made into wine via fermentation. The next step in the process is the introduction of an Acetobacter bacteria strain. Acetobacter in the presence of oxygen will feed upon ethanol and release acetic acid (vinegar) as a byproduct.(6 votes)
- In the diagrams there write, "NADH regeneration," wouldn't it be more accurate to say "NAD+ regeneration?"(9 votes)
- its kind of like regenerating nad+ so that they can accept electrons to become nadh again(2 votes)
- 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)
- It is associated with damage to the muscle fibers, but the details don't appear to be well studied.
This article seems to be the source for much of the information currently available on the internet:
- 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.(4 votes)
- Good question
Aerobic fermentation is a metabolic process by which cells metabolize sugars via fermentation in the presence of oxygen and occurs through the repression of normal respiratory metabolism (also referred to as the crabtree effect in yeast). This phenomenon is rare and observed mostly in the yeast. However it is found also in cancerous cells!
Crabtree-positive yeasts will respire when grown with very low concentrations of glucose or when grown on most other carbohydrate sources.
AS fir evolution they think that it was co-evolution with the fruit - since fruit needed bacteria an dyeasta t the sam etime (and bacteria grow faster, so while yeats tried to outcompete bacteria, it had to consume part of sugar and undergo glycolytic pathway).
- 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?(4 votes)
- why plants can not regenerate pyruvate from ethanol?(4 votes)
- Anaerobic respiration in plants involves decarboxylation , or a loss of carbon dioxide (often as a gas). Regeneration of pyruvate would require this carbon dioxide to combine with ethanol, which does not generally happen.(1 vote)
- Why can't human undergo ethanol fermentation? is there an enzyme that is required which we don't have?(3 votes)
We lack alcohol pyrivate dehydrogenase.
Also, it would be lethal for humans to produce ethanol. Ethanol is very toxic, causing: drowsiness, cognitive impairment, liver failure, liver cirrhosis, and eventually death.(2 votes)
- What are the similarities and differences in aerobic and anaerobic respiration in terms of energy transferred/ ATP produced?(3 votes)
- aerobic respiration process breaks down a single glucose molecule to yield 38 units of the energy storing ATP molecules.
For the lactate fermentation, 2 molecules of ATP are produced for every molecule of glucose used.
The process of anaerobic respiration is relatively less energy-yielding as compared to the aerobic respiration process.(1 vote)
- What effect does the lactic acid produced by lactic acid fermentation have on the whole cell? Even though it isn't causing the muscle pain, does it slow or change other metabolic pathways in the cytosol?(3 votes)
- As far as I am informed, it does not cause great damage but due to heavy accumulation inside cytosol (as a result of excessive work and anaerobic respiration), it causes muscle strain.
It activates receptors which respond - and that's the moment your body signals you to stop.
Also heavy accumulated lactic acid may create disbalance in ions, pH in a cell. Everything is connected and mixing up in the cytosol. It definitely affects cell, but temporarily.(1 vote)