ANAEROBIC RESPIRATION: CHARACTERISTICS, TYPES AND ORGANISMS - BIOLOGY - 2025

The anaerobic respiration or anaerobic metabolic mode which is a chemical energy based on organic molecules is released. The final electron acceptor in this entire process is a molecule other than oxygen, such as the nitrate ion or sulfates.

Organisms that present this type of metabolism are prokaryotes and are called anaerobic organisms. Prokaryotes that are strictly anaerobic can only live in environments where oxygen is not present, as it is highly toxic and even lethal.

Anaerobic respiration is present in prokaryotes.

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Certain microorganisms - bacteria and yeasts - obtain their energy through the fermentation process. In this case, the process does not require oxygen or an electron transport chain. After glycolysis, a couple of extra reactions are added and the end product can be ethyl alcohol.

For years, the industry has taken advantage of this process to produce products of interest for human consumption, such as bread, wine, beer, among others.

Our muscles are also capable of anaerobic respiration. When these cells are subjected to intense effort, the lactic fermentation process begins, which results in the accumulation of this product in the muscles, creating fatigue.

characteristics

Respiration is the phenomenon by which energy is obtained in the form of ATP, starting from various organic molecules - mainly carbohydrates. This process takes place thanks to various chemical reactions that take place inside cells.

Although the main source of energy in most organisms is glucose, other molecules can be used for energy extraction, such as other sugars, fatty acids or in cases of extreme need, amino acids - the building blocks of proteins.

The energy that each molecule is capable of releasing is quantified in joules. The biochemical pathways or pathways of organisms for the degradation of said molecules depend mainly on the presence or absence of oxygen. In this way, we can classify respiration into two large groups: anaerobic and aerobic.

In anaerobic respiration, there is an electron transport chain that generates ATP, and the final acceptor of electrons is an organic substance such as nitrate ion, sulfates, among others.

It is important not to confuse this type of anaerobic respiration with fermentation. Both processes are independent of oxygen, but in the latter there is no electron transport chain.

Types

There are multiple routes by which an organism can breathe without oxygen. If there is no electron transport chain, the oxidation of organic matter will be coupled with the reduction of other atoms from the energy source in the fermentation process (see below).

In the case of a transport chain, the role of final electron acceptor can be taken by various ions, including nitrate, iron, manganese, sulfates, and carbon dioxide, among others.

The electron transport chain is an oxide reduction reaction system that leads to the production of energy in the form of ATP, by a modality called oxidative phosphorylation.

The enzymes involved in the process are found inside the bacteria, anchored to the membrane. Prokaryotes have these invaginations or vesicles that resemble the mitochondria of eukaryotic organisms. This system varies widely among bacteria. The most commons are:

Use of nitrates as an electron acceptor

A large group of bacteria with anaerobic respiration are classified as nitrate reducing bacteria. In this group, the final acceptor of the electron transport chain is the NO ₃ ^- ion.

Within this group there are different physiological modalities. Nitrate reducers can be of the respiratory type where the ion NO ₃ ^- becomes NO ₂ ^-; They can be denitrifying, where said ion passes to N ₂, or of the assimilating type where the ion in question is transformed into NH ₃.

Electron donors can be pyruvate, succinate, lactate, glycerol, NADH, among others. The representative organism of this metabolism is the well-known Escherichia coli bacteria.

Use of sulfates as an electron acceptor

Only a few species of strict anaerobic bacteria are capable of taking the sulfate ion and converting it to S ^2- and water. A few substrates are used for the reaction, among the most common are lactic acid and the four-carbon dicarboxylic acids.

Use of carbon dioxide as an electron acceptor

Archaea are prokaryotic organisms that usually inhabit extreme regions, and are characterized by exhibiting very particular metabolic pathways.

One of these are the archaea capable of producing methane and to achieve this they use carbon dioxide as the final acceptor. The end product of the reaction is methane gas (CH ₄).

These organisms only live in very specific areas of ecosystems, where the concentration of hydrogen is high, since it is one of the elements necessary for the reaction - such as the bottom of lakes or the digestive tract of certain mammals.

Fermentation

Wine fermentation

As we mentioned, fermentation is a metabolic process that does not require the presence of oxygen to take place. Note that it differs from anaerobic respiration mentioned in the previous section by the absence of an electron transport chain.

Fermentation is characterized by being a process that releases energy starting from sugars or other organic molecules, does not require oxygen, does not need a Krebs cycle or electron transport chain, its final acceptor is an organic molecule and produces small amounts of ATP - one or two.

Once the cell has completed the glycolysis process, it obtains two molecules of pyruvic acid for each molecule of glucose.

In the absence of oxygen availability, the cell can resort to the generation of some organic molecule to achieve the generation of NAD ⁺ or NADP ⁺ that can enter another cycle of glycolysis again.

Depending on the organism that carries out the fermentation, the final product can be lactic acid, ethanol, propionic acid, acetic acid, butyric acid, butanol, acetone, isopropyl alcohol, succinic acid, formic acid, butanediol, among others.

These reactions are also often associated with the excretion of carbon dioxide or dihydrogen molecules.

Organisms with anaerobic respiration

The anaerobic respiration process is typical of prokaryotes. This group of organisms is characterized by lacking a true nucleus (delimited by a biological membrane) and subcellular compartments, such as mitochondria or chloroplasts. Within this group are bacteria and archaea.

Strict anaerobes

Microorganisms that are lethally affected by the presence of oxygen are called strictly anaerobic, such as the genus Clostridium.

Possessing an anaerobic metabolism allows these microorganisms to colonize extreme environments lacking oxygen, where aerobic organisms could not inhabit, such as very deep waters, soils or the digestive tract of some animals.

Facultative anaerobes

Additionally, there are some microorganisms capable of alternating between aerobic and anaerobic metabolism, depending on their needs and environmental conditions.

However, there are bacteria with strict aerobic respiration that can only grow and develop in oxygen-rich environments.

In the microbiological sciences, knowledge of the type of metabolism is a character that helps to identify microorganisms.

Organisms with the ability to ferment

In addition, there are other organisms capable of creating airways without the need for oxygen or a transport chain, that is, they ferment.

Among them we find some types of yeast (Saccharomyces), bacteria (Streptococcus, Lactobacillus, Bacillus, Propionibacterium, Escherichia, Salmonella, Enterobacter) and even our own muscle cells. During the process, each species is characterized by excreting a different product.

Ecological relevance

From the point of view of ecology, anaerobic respiration fulfills transcendental functions within ecosystems. This process takes place in different habitats, such as marine sediments or freshwater bodies, deep soil environments, among others.

Some bacteria take sulfates to form hydrogen sulfide and use carbonate to form methane. Other species are capable of using the nitrate ion and reducing it to nitrite ion, nitrous oxide, or nitrogen gas.

These processes are vital in natural cycles, both for nitrogen and sulfur. For example, the anaerobic pathway is the main route by which nitrogen is fixed and is able to return to the atmosphere as a gas.

Differences from aerobic respiration

The most obvious difference between these two metabolic processes is the utilization of oxygen. In aerobics, this molecule acts as a final electron acceptor.

Energetically, aerobic respiration is much more beneficial, releasing significant amounts of energy - about 38 ATP molecules. In contrast, respiration in the absence of oxygen is characterized by a much lower number of ATP, which varies widely depending on the organism.

Products of excretion also vary. Aerobic respiration ends with the production of carbon dioxide and water, while in aerobic respiration the intermediates are varied - such as lactic acid, alcohol, or other organic acids, for example.

In terms of speed, aerobic respiration takes much longer. Thus, the anaerobic process represents a rapid source of energy for organisms.

References

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