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6.3 Fermentation

Anaerobic respiration enables organisms to convert energy for their use without oxygen. With oxygen, organisms can use aerobic cellular respiration to produce up to 38 molecules of ATP from just one glucose molecule. Without oxygen, organisms only produce two molecules of ATP per glucose molecule. Anaerobic respiration starts with glycolysis, the first stage of cellular respiration. Glycolysis does not require oxygen but produces two molecules of ATP for each glucose molecule. For glycolysis to work, NAD+ must accept electrons. For glycolysis to continue, NADH must be converted back to NAD+ for reuse. Since no oxygen is present, the ETC is not functioning, so the NADH needs to drop its electrons elsewhere. Different organisms do this step in different ways. Fermentation is a type of anaerobic respiration that regenerates NAD+ by adding the electrons from NADH to an organic molecule. There are two main types of fermentation:

Lactic Acid Fermentation

Animals and certain bacteria, including those in yogurt, use lactic acid fermentation. During intense exercise, your muscle cells rely on this process for short periods when your muscles use ATP faster than your cardiovascular system can deliver oxygen. In this process, NADH transfers electrons back onto pyruvate (pyruvic acid), which converts it into lactic acid. The NAD+ cycles back to allow glycolysis to continue, so more ATP is made.

Lactic Acid Fermentation Formula
Figure 6.3.1 Lactic acid fermentation produces lactic acid and NAD+. The NAD+ cycles back to allow glycolysis to continue so more ATP is made. Each circle represents a carbon atom. Image by Hana Zavadska, CC BY-NC 3.0

Alcohol Fermentation

Alcohol fermentation produces ethanol, an alcohol. It is carried out by single-celled fungi (called yeasts) and some bacteria. Pyruvate is broken down to release a carbon dioxide molecule, resulting in a 2-carbon molecule (acetaldehyde) remaining. NADH drops its electrons onto the acetaldehyde, which converts it into ethanol. The resulting NAD+ is reused in glycolysis to continue making ATP. We use alcoholic fermentation to make bread, wine, beer, and even biofuels.

Alcohol fermentation formula
Figure 6.3.2 Alcohol fermentation produces ethanol. Image by Hana Zavadska, CC BY-NC 3.0
A loaf of bread cut in half.
Figure 6.3.3 CO2 from fermentation causes bubbles in bread dough and explains why the dough rises. Image by Orlova Maria, Unsplash License

 

Table 6.3.1 Aerobic vs Anaerobic Respiration

Feature

Aerobic Respiration

Anaerobic Respiration
Requires oxygen? Yes No
Glucose breakdown Complete Incomplete
End products CO₂ and H₂O Animal cells: lactic acid
Plant cells and yeast: carbon dioxide and ethanol
ATP produced About 38 2

Exercises 6.3.1

Text Description
1. What is the purpose of fermentation?
  1. To generate about 32 ATP in the presence of oxygen.
  2. To allow cells to survive without using ATP.
  3. To regenerate NAD+ so glycolysis can continue to happen.
2. The fermentation pathway starts after which part of cellular respiration?
  1. The citric acid cycle
  2. Oxidative phosphorylation
  3. Glycolysis
3. Drag the words to explain how the following fact supports or does not support the assertion that glycolysis is one of the oldest metabolic pathways. Fact: Both prokaryotic and eukaryotic organisms carry out some form of glycolysis.
If glycolysis evolved relatively _____, it likely _____ be as _____ in organisms as it is. It probably evolved in very _____ organisms and _____, with the _____ of other pathways of carbohydrate metabolism that evolved _____.
Possible answers:
  • addition
  • wouldn’t
  • later
  • persisted
  • primitive
  • late
  • universal

4. Aerobic or Anaerobic?

Draggable options: Requires oxygen, Lactic acid produced, Ethanol produced, Is less efficient, Incomplete breakdown of glucose, Makes more ATP per glucose, Does not require oxygen, Complete breakdown of glucose, 2 ATP produced, About 36 ATP produced, H2O and CO2 produced.

Answers:

1.  c. To regenerate NAD+ so glycolysis can continue to happen.
2. c. Glycolysis
3. If glycolysis evolved relatively late, it likely wouldn’t be as universal in organisms as it is. It probably evolved in very primitive organisms and persisted, with the addition of other pathways of carbohydrate metabolism that evolved later.

4. Aerobic: Requires oxygen, About 36 ATP produced, Complete breakdown of glucose, H20 and C02 produced, Makes more ATP per glucose

Anaerobic: Does not require oxygen, Incomplete breakdown of glucose, Ethanol produced, Lactic acid produced, 2 ATP produced, Is less efficient.

Anaerobic Cellular Respiration in Prokaryotes

Ancient prokaryotes likely generated ATP exclusively from glycolysis before oxygen was abundant in Earth’s atmosphere. Since nearly all organisms complete glycolysis, it suggests that it evolved in primitive organisms and persisted, with the addition of other metabolic pathways that evolved later.

Some prokaryotes use different ways to convert NADH back into NAD+. The group of Archaea called methanogens reduces carbon dioxide to methane to oxidize NADH. These microorganisms, such as cows, are found in ruminants’ soil and the digestive tracts. Similarly, sulphate-reducing bacteria and Archaea, most of which are anaerobic, reduce sulphate to hydrogen sulphide to regenerate NAD+ from NADH.

“4.11 Anaerobic Processes” from Human Biology by Christine Miller is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

“4.4 Fermentation” from Biology and the Citizen by Colleen Jones is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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Biology Essentials 1 Copyright © 2025 by Kari Moreland is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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