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Anaerobic respiration

anaerobic respiration - definition and example

Anaerobic respiration
n., plural: anaerobic respirations
[ˌænəˈɹoʊbik ɹɛspɪˈɹeɪʃən]
Definition: a respiratory process n which cells break down the sugar molecules to produce energy without oxygen use

Anaerobic Respiration Definition

What is anaerobic respiration? Anaerobic (cellular) respiration is a respiratory process that occurs in both prokaryotes and eukaryotes in which cells break down the sugar molecules to produce energy without the presence of oxygen.  While fermentation involves only the glycolysis step, certain anaerobic respiration types make use of the electron transport chain system to pass the electrons to the final electron acceptor.

lactic acid fermentation - simple diagram
lactic acid fermentation – simple diagram

Anaerobic respiration (biology definition): An anaerobic process in which organic food is converted into simpler compounds, and chemical energy (ATP) is produced. Certain types use the electron transport chain system to pass the electrons to the final electron acceptor, which may be an inorganic or an organic compound, but not oxygen. Compare: aerobic respiration

Anaerobic vs. Aerobic Respiration vs. Fermentation

Some references consider fermentation as an example or part of anaerobic respiration as both of them do not use oxygen, and therefore, are anaerobic. However, other references regard them as two different processes. In this regard, we are going to deal with the two processes here.

Anaerobic stands for “without oxygen.” This method of cellular respiration does not require oxygen to generate energy. For smaller animals to breathe, there is not enough oxygen available so they need the energy to survive in the absence of oxygen. They carry out respiration to produce the energy they need, which is referred to as anaerobic respiration. That is in contrast to aerobic respiration which requires oxygen, which serves as the final electron acceptor in the electron transport chain system.

In fermentation, this step is skipped. After glycolysis, pyruvate (in lactic acid fermentation) or acetaldehyde (in alcohol fermentation) serves as the final electron acceptor. In this process, the energy from glucose is converted into another form that can be used by the cell or stored for later use.  It produces lactic acid instead of carbon dioxide and water. This type of respiration is only used for short intervals.

The process can be written as:

Glucose → Alcohol + Carbon dioxide + Energy

In the absence of oxygen, the process does not result in the development of any more ATP molecules.

This process mainly occurs in microorganisms, but it is also used by multi-cellular organisms, such as humans, albeit not as common. It is a temporary reaction to oxygen-less conditions.

During hard or vigorous exercise, such as biking, sprinting, cycling or weightlifting, our body needs high energy. As the availability of oxygen is reduced, the muscle cells within our body use lactic acid fermentation to satisfy the energy demand.

Question: What happens during anaerobic cellular respiration?

Answer: During anaerobic cellular respiration, glucose is broken down without oxygen. The chemical reaction transfers glucose energy to the cell. In fermentation, instead of carbon dioxide and water, lactic acid is produced which can lead to painful muscle cramps.

Anaerobic cellular respiration

Anaerobic cellular respiration is fairly similar to aerobic cellular respiration in which electrons are transferred through an electron transport chain generated from a fuel molecule, accelerating ATP synthesis. Many microorganisms use sulfate (SO₄²-) reducing it to hydrogen sulfide (H2S) at the end of the transport chain as the final electron acceptor, while others use nitrate (NO3-) reducing it to nitrite (NO2-). Other nitrate reducers can reduce nitrate even further to nitrous oxide (NO) or nitrogen gas (N2).

Final electron acceptor

Some living systems use an organic molecule (e.g. dimethyl sulfoxide, fumarate, and trimethylamine N-oxide) during cellular respiration as the final electron acceptor. In other living systems, an inorganic molecule is used as a final electron acceptor. Examples of inorganic molecules are sulfate ion (SO4–2), nitrate (NO3), and ferric ion (Fe3+).

Here are some of the different types of final electron receptors used for anaerobic respiration.

  • (NO3) as the terminal electron acceptor
  • Sulfate reduction uses sulfate (SO4–2) as a final electron receptor that produces hydrogen sulfide (H2S) as a metabolic end product.
  • Ferric iron (Fe3+) is a commonly used anaerobic final electron receptor used by both autotrophic and heterotrophic organisms.
  • Other inorganic final electron acceptors include the reduction of manganic ion (Mn4+) to manganous (Mn2+), Selenate (SeO42-) to selenite (SeO32-) to selenium (Se), Arsenate (AsO43-) to arsenite (AsO33-), etc.


Fermentation (biology) is the metabolic process in which glucose molecules are converted into acids, gases, or alcohol in the absence of oxygen or other electron transport chain. For example, by converting sugar into alcohol, yeast performs fermentation to obtain energy. Fermentation is performed by bacteria, converting carbohydrates into lactic acid. The study of fermentation is called zymology.

Lactic acid fermentation

Where does lactic acid fermentation occur? The fermentation of lactic acid or lactate fermentation is an anaerobic process that takes place in the cytoplasm of the cells.

In this process, the enzyme converts the pyruvic acid produced in glycolysis into a three-carbon molecule called lactic acid. Simultaneously, one hydrogen atom is added to NADH to form NAD+, which is used to restart glycolysis.

Around the same time, a proton (H+) is added to the pyruvic acid. NADH transfers the electrons directly to pyruvate, producing lactate as a byproduct. Lactate, which is just a lactic acid-deprotonated form, gives its name to the process.

In this type of anaerobic process, glucose is broken into two molecules of lactic acid to form two ATP molecules. This occurs in certain bacteria and other animal tissues, such as muscle tissue. The bacteria that make yogurt undergo lactic acid fermentation, as do the red blood cells in your body, which do not have mitochondria and hence cannot perform cellular respiration.

Glucose + ADP + NADH → Lactic acids + ATP + NAD+

lactic acid fermentation schematic diagram
Lactic Acid Fermentation Equation

Yeast fermentation

Yeast fermentation is one of the basic processes of converting glucose into ethanol and carbon dioxide. This process is normally conducted by microscopic organisms such as bacteria and some fungi such as yeast and even in some types of fish, such as goldfish. Glucose is broken down by glycolysis, which releases two ATP molecules and produces two molecules of pyruvate acid. Pyruvic acid molecules are then removed from each other by another carbon atom, resulting in two ethanol molecules and two carbon dioxide molecules. No more ATP is produced from the glycolysis reaction after the initial two. This anaerobic fermentation is also called alcohol fermentation or ethanol fermentation and this fermentation reaction is carried out as shown below:

C6H12O6 → 2CO2 + 2C2H6O + 2ATP

[Glucose → Carbon dioxide + Ethanol + 2ATP]

Yeasts usually function under aerobic conditions or in the presence of oxygen, but they may also function under anaerobic conditions or in the absence of oxygen. When oxygen is not readily accessible, alcohol fermentation occurs in the cytosol of the yeast cells.

Question: What does anaerobic respiration produce?

Answer: Depending on the type of anaerobic respiration, the final product may be N2 (in denitrification), succinate (in fumarate respiration), HS− (in sulfate respiration and sulfur reduction), methane (in methanogenesis), acetate (in acetogenesis), halide ions, and dehalogenated compound (in dehalorespiration), Fe(II) in iron reduction, Co(II) in cobalt reduction.

In fermentation, lactic acid or ethanol and molecules of ATP are the end products.

Process of anaerobic respiration

The first step of anaerobic respiration is glycolysis, in which a glucose molecule is broken down into two pyruvate molecules, releasing electrons in the process and producing two ATP molecules, the energy of the cells. As oxygen is present during aerobic respiration, some pyruvate molecules go through two other stages that serve to release more electrons, later used to power a very high amount of ATP production. But when, as in the case of fermentation, oxygen is absent, the last two stages are bypassed. Instead, pyruvate is converted into a different byproduct, and carbon dioxide is released as well. In this process, two ATP molecules are produced.

In other scenarios, the cell enters another form of respiration wherein despite the absence of oxygen the process proceeds to certain pathways leading to the use of an electron transport chain passing the electrons down to the final electron acceptor, which may be an inorganic or an organic molecule.

Products of anaerobic respiration

Glucose breaks down without the presence of oxygen in anaerobic respiration. The chemical reaction transfers the energy from glucose to the cell. The final product varies depending on the metabolic pathway involved. For example, in denitrification, the final product is N2. In fumarate respiration, succinate is the final product. In methanogenesis, the final product is methane whereas, in acetogenesis, it is acetate. In iron reduction, Fe(II) is the final product whereas, in cobalt reduction, it is Co(II). In dehalorespiration, the final products are halide ions and dehalogenation compound. In fermentation, the final product may be lactic acid or ethanol. Apart from these substances, energy in the form of ATP molecules is also produced.


Where does anaerobic respiration occur? In the fluid part of the cytoplasm, anaerobic respiration (both glycolysis and fermentation) takes place, while the majority of the energy production in aerobic respiration takes place in the mitochondria.

Similarities with aerobic respiration

  • Both aerobic and anaerobic respiration are types of cellular respiration.
  • In both aerobic and anaerobic respiration, food is broken down into simpler substances to release energy.
  • Both processes produce some byproducts.
  • Both processes use glucose as the starting molecule.
  • To catalyze their reactions, both processes depend on enzymes.
  • Via the glycolysis process, both processes produce ATP.
  • In the cytosol, the reactions of both processes take place.

Differences – distinctive features of anaerobic respiration

Aerobic respiration occurs in the presence of oxygen, hence the name. The aerobic respiration equation is as follows:

C6H12O6 + 6O2 → 6CO2 + 6H2O + energy

During aerobic respiration, there is an exchange of gases where oxygen is absorbed and carbon dioxide is released. It can be found in the mitochondria of the eukaryotes and the cytoplasm of the prokaryotes. The end products of aerobic respiration are water, carbon dioxide, and energy. During aerobic respiration, a total of 38 ATPs are produced, some of which are lost during the process. Also, during aerobic respiration, complete oxidation of carbohydrates takes place. Aerobic respiration is relatively slower than anaerobic respiration. Aerobic respiration occurs in most of the higher species including plants and animals. Cellular respiration in humans is an example.

In anaerobic respiration, the process occurs in the absence of oxygen. Examples of an anaerobic respiration equation are the following:

Denitrification: NO3 → NO2→ NO + N2O → N2 

Methanogenesis: (1) CO2 + 4 H2 → CH4 + 2 H2O, (2)

Fermentation: C6H12O6 → C2H5OH + CO2 + energy

During anaerobic respiration exchange of gases does not take place. However, some organisms release some gases, such as sulfur and nitrogen gases. Anaerobic respiration can be found only in the cytoplasm of a cell. The end products of anaerobic respiration vary, such as gases, alcohol, acids, and energy. In fermentation, only 2 ATPs are produced. Also, there is incomplete oxidation of carbohydrates. It occurs in simple prokaryotes, yeasts, and the muscle cells of humans during intense exercise. Anaerobic respiration is shorter than aerobic respiration.

Anaerobic Respiration Equations

To summarize what has been described so far, here are the equations of various cellular respirations:

Fermentation: C6H12O6 → C2H5OH + CO2 + energy

Lactic acid fermentation equation: C6H12O→ 2CO2 + 2C2H6O + 2ATP

Anaerobic respiration formulae:

Denitrification: NO3 → NO2→ NO + N2O → N2 

Methanogenesis: (1) CO2 + 4 H2 → CH4 + 2 H2O, (2)

Anaerobic Respiration Functions

All living organisms undergo cellular respiration. In certain types of bacteria and yeast, anaerobic respiration is preferred. It gives them the advantage of surviving or thriving in an anoxic environment that would be lethal to aerobic organisms.

Anaerobic respiration also has a very high speed. It produces ATP very rapidly. Aerobic respiration, on the other hand, produces ATP rather slowly.

Examples of Anaerobic Respiration


One of the most significant functions of fermentation is that it protects the cells from dying in the small amount of time between each breath and during intense activity when the red blood cells fail to provide adequate oxygen to the body cells due to under-oxygenation. Fermentation takes over as this happens and releases a substance called lactic acid which keeps the cells of the body intact during the above-mentioned cycles of under-oxygenation. Although this is quite useful for the time being, yet unfortunately, a build-up of lactic acid may cause discomfort in the muscles later.

Lactic acid production in muscles. During vigorous exercise, our muscles use oxygen to generate more ATP as compared to the supply. When this happens, the muscle cells undergo glycolysis faster than they can supply oxygen to the mitochondrial electron transport chain. As a result, anaerobic respiration and lactic acid fermentation occur within the cells, and during extended activity, the built-up lactic acid will keep our muscles painful.

Lactic acid fermentation equation:

C6H12O6 (glucose) + 2 ADP + 2 pi → 2 lactic acid + 2 ATP

Alcoholic fermentation by yeasts. Fermentation is another category of anaerobic respiration that occurs in anaerobic organisms such as yeast. When carbohydrate-rich substances are bottled with yeasts to ensure a minimal oxygen level in the container, yeasts undergo the process of anaerobic respiration. As a process, fermentation occurs where the yeast converts sugars into ethyl alcohol.

Ethanol fermentation equation:

C6H12O6 (glucose) + 2 ADP + 2 pi → 2 C2H5OH (ethanol) + 2CO2 + 2 ATP


Methanogens are prokaryotes that belong to the Archaea. These species are considered methanogens because they produce methane as a by-product by oxidizing carbohydrates in the absence of oxygen. This process is called methanogenesis. It is also a type of fermentation that results in the production of methanol, specific alcohol. This process is also referred to as methanol poisoning. Methanol poisoning can lead to nerve injury or even death in some cases.

Propionic acid fermentation in cheese

Propionic acid fermentation occurs when certain bacteria use carbohydrates such as lactose and glucose to create propionic acid and carbon dioxide. In Swiss cheese, the most common use of this method can be observed. During this process, the carbon dioxide gas produced results in the formation of bubbles in the cheese along with the distinct flavor due to carboxylic acid.


Try to answer the quiz below and find out what you have learned so far about anaerobic respiration.


Choose the best answer. 

1. What happens during anaerobic respiration?

2. All of the following may be a final electron acceptor in anaerobic respiration except for ....

3. All of these are features shared by aerobic and anaerobic type of respiration except for ...

4. An anaerobic process wherein glucose is converted into lactic acid or ethanol, with the release of energy

5. An anaerobic process wherein methane is a by product

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  • Anaerobic Respiration – Process – Regulation – TeachMePhysiology. (2020, June 22). TeachMePhysiology. https://teachmephysiology.com/biochemistry/atp-production/anaerobic-respiration/
  • Anaerobic Respiration: Definition, Equation & Examples Video. (2020). Anaerobic Respiration: Definition, Equation & Examples – Video & Lesson Transcript | Study.com. Study.Com. https://study.com/academy/lesson/anaerobic-respiration-definition-equation-examples.html
  • 7.5A: Anaerobic Cellular Respiration. (2018, July 11). Biology LibreTexts. https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/7%3A_Cellular_Respiration/7.5%3A_Metabolism_without_Oxygen/7.5A%3A_Anaerobic_Cellular_Respiration
  • Anaerobic respiration – Aerobic and anaerobic respiration – GCSE Biology (Single Science) Revision – BBC Bitesize. (2020). BBC Bitesize. https://www.bbc.co.uk/bitesize/guides/zm6rd2p/revision/2#:~:text=Anaerobic%20respiration%20in%20muscles&text=There%20is%20a%20build%2Dup,repaid’%20after%20the%20exercise%20stops.
  • 2.31: Anaerobic and Aerobic Respiration. (2016, September 21). Biology LibreTexts. https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Book%3A_Introductory_Biology_(CK-12)/02%3A_Cell_Biology/2.31%3A_Anaerobic_and_Aerobic_Respiration

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