Anaerobic Respiration: Definition, Equation, and Examples

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What is Anaerobic Respiration?

Anaerobic (cellular) respiration is a type of respiration that occurs in both prokaryotes and eukaryotes, in which cells break down sugar molecules to generate energy without using oxygen. While fermentation simply requires the glycolysis stage, certain types of anaerobic respiration rely on the electron transport chain system to deliver electrons to the ultimate electron acceptor.

Anaerobic Respiration Definition

Anaerobic respiration is an anaerobic process that converts organic food into simpler chemicals and produces chemical energy (ATP). The electron transport chain mechanism is used by some kinds to convey electrons to the final electron acceptor, which can be inorganic or organic, but not oxygen.

Anaerobic vs Aerobic Respiration

Fermentation is sometimes considered an example or element of anaerobic respiration because they both do not consume oxygen and are anaerobic. Other sources, on the other hand, see them as two distinct processes. In this case, we’ll look at the two processes separately.

The term “anaerobic” means “without oxygen.” This technique of cellular respiration generates energy without the use of oxygen. Because there isn’t enough oxygen for smaller animals to breathe, they must rely on their energy to survive the lack of oxygen. Anaerobic respiration is the process by which they produce the energy they require. In contrast, aerobic respiration necessitates the use of oxygen, which serves as the electron transport chain’s ultimate electron acceptor.

This stage is skipped during fermentation. Pyruvate (in lactic acid fermentation) or acetaldehyde are the final electron acceptors following glycolysis (in alcohol fermentation). The energy from glucose is transformed into a form that can be utilised by the cell or saved for later use in this process. Instead of carbon dioxide and water, it creates lactic acid. This form of respiration is only employed for brief periods of time.

Anaerobic Respiration Equation

Glucose ⇒ Alcohol + CO2 + Energy

The procedure does not result in the formation of any more ATP molecules in the absence of oxygen.

This mechanism is mostly seen in microbes, but it is also found in multicellular organisms like humans, though not as frequently. It’s a transitory response to a lack of oxygen.

Our bodies require a lot of energy during hard or severe exercise like biking, sprinting, cycling, or weightlifting. As oxygen becomes scarcer, muscle cells in our bodies turn to lactic acid fermentation to meet their energy needs.

Anaerobic Cellular Respiration

Anaerobic cellular respiration is similar to aerobic cellular respiration in that electrons are moved along an electron transport chain formed from a fuel molecule, which speeds up the synthesis of ATP. Many microbes employ sulphate (SO42-) as the last electron acceptor, reducing it to hydrogen sulphide (H2S) at the conclusion of the transport chain, while others use nitrate (NO3) to reduce it to nitrite (NO2). Other nitrate decomposers can further convert nitrate to nitrous oxide (NO) or nitrogen gas (N2).

Final Electron Acceptor

During cellular respiration, certain living systems utilise an organic molecule (e.g., dimethyl sulfoxide, fumarate, and trimethylamine N-oxide) as the final electron acceptor. An inorganic molecule is utilised as a final electron acceptor in other living systems. Sulphate ion (SO4–2), nitrate (NO3), and ferric ion (Fe3+) are examples of inorganic compounds.

Here are some examples of anaerobic respiration’s several types of final electron receptors.

• As the final electron acceptor (NO3)

• Sulphate reduction employs sulphate (SO4–2) as a final electron receptor, resulting in the metabolic end product hydrogen sulphide (H2S).

• Both autotrophic and heterotrophic organisms utilise ferric iron (Fe3+) as an anaerobic final electron receptor.

• Other inorganic final electron acceptors include manganic (Mn4+) to manganous (Mn2+) transformations, selenite (SeO32-) to selenium (Se), arsenate (AsO43-) to arenite (AsO33-), and so on.


Fermentation is a metabolic process that converts glucose molecules into acids, gases, or alcohol in the absence of oxygen or another electron transport chain. For example, yeast undertakes fermentation to obtain energy by turning sugar into alcohol. Bacteria perform fermentation, turning carbohydrates into lactic acid. The discipline of fermentation is termed as zymology.

Lactic Acid Fermentation

Lactic acid fermentation, also known as lactate fermentation, is an anaerobic process that occurs in the cytoplasm of cells.

In this step, the enzyme transforms the pyruvic acid generated during glycolysis into lactic acid, a three-carbon molecule. A hydrogen atom is added to NADH at the same time, resulting in NAD+, which is required to restart glycolysis.

A proton (H+) is added to the pyruvic acid at the same moment. NADH transfers electrons to pyruvate directly, resulting in lactate as a by-product. The process is named after lactate, which is simply lactic acid in its deprotonated form.

Glucose is broken down into two molecules of lactic acid and two ATP molecules in this type of anaerobic action. This can happen in bacteria as well as other animal tissues, like muscle tissue. Lactic acid fermentation occurs in the bacteria that make yoghurt, as well as in red blood cells, which lack mitochondria and hence cannot undertake cellular respiration.

Glucose + ADP + NADH ⇒ Lactic acid + ATP + NAD+

Yeast Fermentation

One of the most fundamental techniques for converting glucose to ethanol and carbon dioxide is yeast fermentation. Microscopic creatures, such as bacteria and some fungi, such as yeast, and even some fish, such as goldfish, carry out this activity.

The process of breaking down glucose into two ATP molecules and two pyruvate acid molecules is described as glycolysis. Another carbon atom separates the pyruvic acid molecules, resulting in two ethanol molecules and two carbon dioxide molecules. After the first two, the glycolysis reaction produces no more ATP. This anaerobic fermentation is also known as alcohol fermentation or ethanol fermentation, and it occurs in the following manner:

C6H12O6 → 2CO2 + 2C2H6O + 2ATP

[Glucose → Carbon dioxide + Ethanol + 2ATP]

Yeasts normally act aerobically, or in the presence of oxygen, but they can also function anaerobically, or in the lack of oxygen. Alcohol fermentation happens in the cytoplasm of yeast cells when oxygen is not easily available.

Process of Anaerobic Respiration

The first stage in anaerobic respiration is glycolysis, which involves breaking down a glucose molecule into two pyruvate molecules, releasing electrons, and creating two ATP molecules, which provide energy to the cells. When oxygen is available during aerobic respiration, some pyruvate molecules go through two additional phases that allow more electrons to be released, which are then used to fuel a large quantity of ATP generation.

When oxygen is not present, as it is in the case of fermentation, the last two stages are skipped. Rather, pyruvate is transformed into a distinct by-product, as well as carbon dioxide. In this procedure, two ATP molecules are generated.

In other cases, the cell enters a different mode of respiration in which, despite the lack of oxygen, certain pathways lead to the employment of an electron transport chain, which passes the electrons down to the final electron acceptor, which might be an inorganic or organic molecule.

Products of Anaerobic Respiration

In anaerobic respiration, glucose is broken down without the presence of oxygen. Energy is transferred from glucose to the cell through a chemical process. Depending on the metabolic route involved, the final product differs. The ultimate product of denitrification, for example, is N2. The end result of fumarate respiration is succinate.

Methane is the end result of methanogenesis, while acetate is the end product of acetogenesis. The final result of iron reduction is Fe (II), whereas the final product of cobalt reduction is Co. (II). The end products of dehalorespiration are halide ions and dehalogenation compounds. The ultimate product of fermentation can be lactic acid or ethanol. Aside from these chemicals, energy is created in the form of ATP molecules.

Anaerobic Respiration Location

Anaerobic respiration (both glycolysis and fermentation) takes place in the cytoplasm’s fluid component, whereas aerobic respiration uses the mitochondria to create the majority of its energy.

Similarities Between Aerobic and non-Aerobic Respiration.

There are some similarities between aerobic and non-aerobic respiration.

• Both aerobic and anaerobic mechanisms are involved in cellular respiration.

• Food is broken down into simpler components in both aerobic and anaerobic respiration to release energy.

• By-products are produced in both procedures.

• The initial molecule in both reactions is glucose.

• Both processes rely on enzymes to catalyse their reactions.

• Both methods produce ATP through the glycolysis process.

• The reactions of both mechanisms take place in the cytosol.


The name comes from the fact that aerobic respiration takes place in the presence of oxygen.

C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy

There occurs a gas exchange during aerobic respiration, where oxygen is received and carbon dioxide is exhaled. It can be found in both the mitochondria and the cytoplasm of eukaryotes and prokaryotes.

Water, carbon dioxide, and energy are the end products of aerobic respiration. A total of 38 ATPs is produced during aerobic respiration, a number of which are lost within the process. Additionally, carbohydrates are completely oxidised during aerobic respiration. The rate of aerobic respiration is slower than that of anaerobic respiration. Most higher organisms, including plants and animals, use aerobic respiration. Human cellular respiration is an example.

The process of anaerobic respiration takes place within the absence of oxygen. The following are some examples of an anaerobic respiration equation:

Denitrification: NO3 → NO2→ NO + N2O → N2

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

(2) CH3COOH → CH4 + CO2

Fermentation: C6H12O6 → C2H5OH + CO2 + Energy

Gas exchange does not occur during anaerobic respiration. Some species, however, release gases such as sulphur and nitrogen. Only the cytoplasm of a cell is capable of anaerobic respiration. Anaerobic respiration produces a variety of end products, including gases, alcohols, acids, and energy. Only two ATPs are created during fermentation. Carbohydrates are also not completely oxidised. During extreme exercise, it can be found in simple prokaryotes, yeasts, and human muscle cells. Anaerobic respiration lasts for a shorter period of time than aerobic respiration.

Anaerobic Respiration Equation

Here are the equations for various cellular respirations to summarise what has been stated thus far:

Fermentation: C6H12O6 → C2H5OH + CO2 + Energy

Lactic Acid Fermentation Equation: C6H12O6 → 2CO2 + 2C2H6O + 2ATP

Denitrification: NO3 → NO2→ NO + N2O → N2

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

(2) CH3COOH → CH4 + CO2

Anaerobic Respiration Function

All living creatures go through a process called cellular respiration. Anaerobic respiration is favoured by certain bacteria and yeast. It provides them with the benefit of living or thriving in an anoxic environment where aerobic species would perish.

Anaerobic respiration is likewise quite fast. It generates ATP at a breakneck speed. Aerobic respiration, on the other hand, takes a long time to generate ATP.

Anaerobic Respiration Examples
i. Fermentation

Fermentation prevents cells from dying in the short period between each breath and during intensive exercise when red blood cells fail to supply sufficient oxygen to the body cells due to under-oxygenation. As a result, fermentation takes over and releases lactic acid, which maintains the body’s cells intact during the above-mentioned periods of under-oxygenation. Although this is beneficial for the time being, a build-up of lactic acid in the muscles may cause discomfort in the future.

In muscles, lactic acid is produced. Our muscles require more oxygen to produce ATP during severe activity than the supply can provide. When this happens, muscle cells undergo glycolysis quicker than they can provide oxygen to the mitochondrial electron transport chain, resulting in muscle cell death. As a result, anaerobic respiration and lactic acid fermentation occur within the cells, and the lactic acid build-up keeps our muscles uncomfortable throughout prolonged exercise.

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

Yeast fermentation produces alcohol. Another type of anaerobic respiration that happens in anaerobic organisms like yeast is fermentation. Anaerobic respiration occurs when carbohydrate-rich compounds are sealed with yeast to ensure a minimum oxygen content in the container. Fermentation is a process in which yeast transforms carbohydrates into ethyl alcohol.

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

ii. Methanogenesis

Methanogens are archaea prokaryotes that produce methane. These organisms are classified as methanogens because they generate methane as a by-product of carbohydrate oxidation in the absence of oxygen. Methanogenesis is the name of this process. It’s also a form of fermentation that produces methanol, which is a particular alcohol. Methanol poisoning is another name for this phenomenon. In rare circumstances, methanol intoxication can result in nerve damage or even death.

iii. Propionic Acid Fermentation in Cheese

Propionic acid fermentation happens when bacteria ferment carbohydrates like lactose and glucose to produce propionic acid and carbon dioxide. The most prevalent application of this approach may be seen in Swiss cheese. The carbon dioxide gas created during this process causes bubbles to develop in the cheese, as well as a unique taste owing to carboxylic acid.

Anaerobic Respiration Citations


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