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?

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.

Would Balsamic Vinegar be an example of lactic acid fermentation since the grape bypasses the alcohol?

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.

In the diagrams there write, "NADH regeneration," wouldn't it be more accurate to say "NAD+ regeneration?"

its kind of like regenerating nad+ so that they can accept electrons to *become nadh again*

Okay, this is actually really interesting... if the lactate isn't what's causing the soreness of muscles after exercising, then what is it?

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: https://www.ncbi.nlm.nih.gov/pubmed/7830383

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.

*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). https://en.wikipedia.org/wiki/Aerobic_fermentation https://www.nature.com/scitable/topicpage/yeast-fermentation-and-the-making-of-beer-14372813/

why plants can not regenerate pyruvate from ethanol?

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.

Why can't human undergo ethanol fermentation? is there an enzyme that is required which we don't have?

Exactly. 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.

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?

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.

Where is the electron transport chain in an anaerobic respiration found?

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.

Main content

Course: Biology library > Unit 12

Lesson 6: Variations on cellular respiration

Fermentation and anaerobic respiration

Google Classroom

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

O_{2}

, 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

({SO}_{4}^{2 -})

as the final electron acceptor at the end ot the transport chain, while others use nitrate

({NO}_{3}^{-})

, sulfur, or one of a variety of other molecules

^{1}

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

(H_{2} S)

as a byproduct. The image below is an aerial photograph of coastal waters, and the green patches indicate an overgrowth of sulfate-reducing bacteria.

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.

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.

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 anaerobes

^{2}

. Recently, some multicellular animals have even been discovered in deep-sea sediments that are free of oxygen

^{3, 4}

Self-check

Inside these tanks, yeasts are busily fermenting grape juice into wine. Why do winemaking tanks like these need pressure-release valves?
Choose 1 answer:
(Choice A)
The yeasts produce $O_{2}$ ‍ gas by cellular respiration.
(Choice B)
The yeasts produce ${CO}_{2}$ ‍ gas by lactic acid fermentation.
(Choice C)
The yeasts produce ${CO}_{2}$ ‍ gas by alcohol fermentation.
(Choice D)
The yeasts produce ${CO}_{2}$ ‍ gas by cellular respiration.

When yeasts ferment grapes into wine, they are carrying out alcohol fermentation. As a byproduct, they produce carbon dioxide gas, which accumulates and increases the pressure inside the tanks. The pressure-release valve allows the carbon dioxide gas to be released, preventing the pressure from getting too high.
This practice problem is adapted from "Metabolism without oxygen" by OpenStax College, Biology, CC BY 3.0

Attribution

This article is a modified derivative of the following articles:

“Metabolism without oxygen” by OpenStax Biology, CC BY 3.0. Download the original article for free at http://cnx.org/contents/185cbf87-c72e-48f5-b51e-f14f21b5eabd@9.85.
"Metabolism without oxygen," in Principles of Biology, by Robert Bear, David Rintoul, Bruce Snyder, Martha Smith-Caldas, Christopher Herren, and Eva Horne, CC BY 4.0. Download the original article for free at http://cnx.org/contents/db89c8f8-a27c-4685-ad2a-19d11a2a7e2e@24.18.

The modified article is licensed under a CC BY-NC-SA 4.0 license.

Works cited

"Anaerobic respiration." Wikipedia. September 9, 2015. Accessed September 16, 2015. https://en.wikipedia.org/wiki/Anaerobic_respiration.
"Clostridium." Wikipedia. Last modified May 29, 2016. https://en.wikipedia.org/wiki/Clostridium.
R. Danovaro, A. Dell'Anno, A. Pusceddu, C. Gambi, I. Heiner, and R. M. Kristensen. "The first metazoa living in permanently anoxic conditions." BMC Biology 8 (2010): 30. http://dx.doi.org/ 10.1186/1741-7007-8-30.
"Loricifera." Wikipeida. Last modified July 12, 2016. https://en.wikipedia.org/wiki/Loricifera#In_anoxic_environment.

Additional references

"Anaerobic respiration." Wikipedia. September 9, 2015. Accessed September 16, 2015. https://en.wikipedia.org/wiki/Anaerobic_respiration.

"Clostridium." Wikipedia. Last modified May 29, 2016. https://en.wikipedia.org/wiki/Clostridium.

Danovaro, R., A. Dell'Anno, A. Pusceddu, C. Gambi, I. Heiner, and R. M. Kristensen. "The first metazoa living in permanently anoxic conditions." BMC Biology 8 (2010): 30. http://dx.doi.org/ 10.1186/1741-7007-8-30.

Kimball, J. W. "Blood." Kimball's Biology Pages. https://www.biology-pages.info/B/Blood.html

Kimball, J. W. "Glycolysis." Kimball's Biology Pages. https://www.biology-pages.info/G/Glycolysis.html

"Lactic acid." Wikipedia. September 17, 2015. Accessed September 17, 2015. https://en.wikipedia.org/wiki/Lactic_acid

"Lactic acid fermentation." Wikipedia. August 25, 2015. Accessed September 16, 2015. https://en.wikipedia.org/wiki/Lactic_acid_fermentation.

"Methanogenesis." Wikipedia. September 1, 2015. Accessed September 16, 2015. https://en.wikipedia.org/wiki/Methanogenesis.

"Sulfate-reducing bacteria." Wikipedia. April 8, 2015. Accessed September 16, 2015. https://en.wikipedia.org/wiki/Sulfate-reducing_bacteria.

Raven, J. B., L. A. Urry, M. L. Cain, S. A. Wasserman, P. V. Minorsky, and R. B. Jackson. "Cellular Respiration and Fermentation." In Campbell Biology, 162-84. 10th ed. San Francisco, CA: Pearson, 2011.

Raven, P. H., G. B. Johnson, K. A. Mason, J. B. Losos, and S. R. Singer. "How cells harvest energy." In Biology, 122-146. 10th ed. AP ed. New York, NY: McGraw-Hill, 2014.

Terry, R. "Anaerobic respiration." CK-12. August 16, 2012. http://www.ck12.org/user:dGVycnlyQHZhbGxleTI2Mi5vcmc./section/Anaerobic-Respiration-%253A%253Aof%253A%253A-Photosynthesis-and-Cellular-Respiration/.

Want to join the conversation?

Sort by:

Phil Rattazzi
Posted 8 years ago. Direct link to Phil Rattazzi's post “Is there a reason why Flo...”
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?
Button navigates to signup pageButton navigates to signup page
(35 votes)
Answer
- Stefan L.
  Posted 7 years ago. Direct link to Stefan L.'s post “There are a few reasons t...”
  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.
  Comment on Stefan L.'s post “There are a few reasons t...”
  (80 votes)
capizzanoco
Posted 8 years ago. Direct link to capizzanoco's post “Would Balsamic Vinegar be...”
Would Balsamic Vinegar be an example of lactic acid fermentation since the grape bypasses the alcohol?
Button navigates to signup pageButton navigates to signup page
(12 votes)
Answer
- Max Spencer
  Posted 6 years ago. Direct link to Max Spencer's post “To make vinegar, grapes a...”
  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.
  Button navigates to signup page
  (8 votes)
Rachel
Posted 9 years ago. Direct link to Rachel's post “In the diagrams there wri...”
In the diagrams there write, "NADH regeneration," wouldn't it be more accurate to say "NAD+ regeneration?"
Button navigates to signup pageButton navigates to signup page
(9 votes)
Answer
- Suryatej Vakkalanka
  Posted 4 years ago. Direct link to Suryatej Vakkalanka's post “its kind of like regenera...”
  its kind of like regenerating nad+ so that they can accept electrons to become nadh again
  Button navigates to signup page
  (2 votes)
Revan Rangotis
Posted 6 years ago. Direct link to Revan Rangotis's post “Okay, this is actually re...”
Okay, this is actually really interesting... if the lactate isn't what's causing the soreness of muscles after exercising, then what is it?
Button navigates to signup pageButton navigates to signup page
(5 votes)
Answer
- tyersome
  Posted 6 years ago. Direct link to tyersome's post “It is associated with dam...”
  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:
  https://www.ncbi.nlm.nih.gov/pubmed/7830383
  Comment on tyersome's post “It is associated with dam...”
  (4 votes)
Ashley He
Posted 2 years ago. Direct link to Ashley He's post “In the last section of th...”
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?
Button navigates to signup pageButton navigates to signup page
(3 votes)
Answer
- FrozenPhoenix45
  Posted 2 years ago. Direct link to FrozenPhoenix45's post “Basically, for aerobic or...”
  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.
  Comment on FrozenPhoenix45's post “Basically, for aerobic or...”
  (7 votes)
markselden
Posted 4 years ago. Direct link to markselden's post “Is fermentation really al...”
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.
Button navigates to signup pageButton navigates to signup page
(5 votes)
Answer
- Ivana - Science trainee
  Posted 4 years ago. Direct link to Ivana - Science trainee's post “*Good question* Aerobic ...”
  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).
  
  https://en.wikipedia.org/wiki/Aerobic_fermentation
  https://www.nature.com/scitable/topicpage/yeast-fermentation-and-the-making-of-beer-14372813/
  Button navigates to signup page
  (2 votes)
JirehBasingan
Posted 7 years ago. Direct link to JirehBasingan's post “why plants can not regene...”
why plants can not regenerate pyruvate from ethanol?
Button navigates to signup pageButton navigates to signup page
(5 votes)
Answer
- LakinduD
  Posted 5 years ago. Direct link to LakinduD's post “Anaerobic respiration in ...”
  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.
  Button navigates to signup page
  (2 votes)
Deby Erina Parung
Posted 6 years ago. Direct link to Deby Erina Parung's post “Why can't human undergo e...”
Why can't human undergo ethanol fermentation? is there an enzyme that is required which we don't have?
Button navigates to signup pageButton navigates to signup page
(4 votes)
Answer
- Ivana - Science trainee
  Posted 5 years ago. Direct link to Ivana - Science trainee's post “Exactly. We lack alcohol...”
  Exactly.
  
  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.
  Comment on Ivana - Science trainee's post “Exactly. We lack alcohol...”
  (2 votes)
Angela
Posted 7 years ago. Direct link to Angela's post “The article states that r...”
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?
Button navigates to signup pageButton navigates to signup page
(4 votes)
Answer
Jea
Posted 3 years ago. Direct link to Jea's post “Where is the electron tra...”
Where is the electron transport chain in an anaerobic respiration found?
Button navigates to signup pageButton navigates to signup page
(3 votes)
Answer
- FrozenPhoenix45
  Posted 3 years ago. Direct link to FrozenPhoenix45's post “The electron chain is not...”
  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.
  Comment on FrozenPhoenix45's post “The electron chain is not...”
  (3 votes)