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Living beings, from the simplest like bacteria to the most complex like vertebrates, depend on endless chemical reactions that require energy. This energy is obtained from the environment. Almost invariably, that energy comes from a molecule called adenosine triphosphate, or ATP. However, ATP is not found in the environment, so living things have evolved to convert other sources of energy (such as sunlight, heat, and nutrients) into ATP. The two most common ways to make such a transformation are cellular respiration and fermentation.
The first living things evolved to produce ATP by fermenting different types of carbohydrates. Later, eukaryotes developed the ability to harness more of the energy stored in carbohydrates through anaerobic respiration. Finally, other more advanced organisms began to take advantage of one of the waste products of photosynthesis, oxygen, giving rise to aerobic cellular respiration.
Because they are two anaerobic processes, many people confuse anaerobic respiration with fermentation. However, they are two very different processes in terms of their mechanism, their final products, and their energy production.
In the following sections, we’ll cover what anaerobic respiration and fermentation are, and then compare them to highlight the most important differences between one and the other.
anaerobic respiration
Anaerobic respiration is a type of cellular respiration that occurs in the absence of oxygen, or when the oxygen concentration is very low (hence the term anaerobic, which literally means in the absence of air). This type of cellular respiration is only carried out by some species of bacteria and other prokaryotes.
Being a type of cellular respiration, the process begins with glycolysis, during which a glucose molecule is transformed into two pyruvic acid molecules, producing two net ATP molecules. Pyruvic acid then enters the Krebs cycle, also called the citric acid cycle or tricarboxylic acid cycle, in which a series of chemical reactions oxidize pyruvic acid to carbon dioxide.
In the next stage of the process, molecules called electron carriers carry them into the electron transport chain where the potential energy stored in these carriers is transformed into a proton concentration gradient that moves an ATP-producing enzyme called ATP. -synth.
During this stage of the process is where most of the chemical energy is generated in the form of ATP molecules; It is common to all respiration processes, whether aerobic or anaerobic. What differentiates one from the other is which molecule is responsible for receiving and carrying the electrons so that they do not accumulate at the end of the electron transport chain.
In the presence of oxygen, this molecule is the final acceptor of the electrons, and its reduction produces water molecules. In anaerobic respiration, on the other hand, the final electron acceptor is a molecule other than oxygen and depends on the particular microorganism in question.
Final electron acceptors in anaerobic respiration
The following table shows three examples of different final electron acceptors in anaerobic respiration together with the product of their reduction and some microorganisms that use it as an energy source:
| acceptor | Final product | Microorganism |
| Sulfur | sulphides | thermoplasma |
| Nitrate | Nitrites, nitrogen oxides and N2 | Pseudomonas , Bacillus |
| Sulfate | sulphides | Desulfovibrio, Clostridium |
Energy production in anaerobic respiration
Anaerobic respiration uses the same ATP production mechanisms as aerobic respiration, ie glycolysis, the Krebs cycle, and the electron transport chain. For this reason, the energy production is the same in both types of respiration, which means that between 36 and 38 ATP molecules are produced in total. After discounting those that are consumed, the net production is between 30 and 32 molecules of ATP for each molecule of glucose that is oxidized.
Fermentation
Fermentation, like cellular respiration, is also a process designed to use the energy contained in nutrients such as carbohydrates and transform it into chemical energy usable by the cell in the form of ATP molecules. It is a purely anaerobic process, that is, it does not require oxygen and can occur in the absence of air. In fact, in most basic biology courses, fermentation is cited as the anaerobic alternative to cellular respiration, thus obviating the existence of anaerobic respiration.
However, there is a fundamental difference between fermentation and anaerobic respiration and that is that the former does not use the citric acid cycle, much less the electron transport chain, so it cannot be considered a type of respiration. cell phone.
Fermentation begins in the same way as respiration, that is, with the glycolysis of different types of six-carbon sugars called hexoses, among which glucose is the most common. However, after glycolysis, pyruvate is transformed into other end products depending on the organism carrying out the fermentation.
types of fermentation
Depending on the final product of fermentation, this can be of different types:
Alcoholic fermentation: In some cases, such as yeast, the fermentation that follows glycolysis produces ethyl alcohol or ethanol. This type of fermentation is called alcoholic fermentation. This is the type of fermentation used in the manufacture of alcoholic beverages.
Acetic fermentation: Other cells further oxidize ethanol to acetic acid, as occurs in the manufacture of vinegar.
Lactic fermentation: is one that gives lactic acid as the final product. The bacteria that ferment milk to produce yogurt ferment lactose (the sugar in milk) to lactic acid, which causes curdling of the milk proteins. In the case of vertebrate muscle tissues, they are capable of fermenting glucose to lactic acid when the oxygen concentration is low.
Energy production
Fermentation is an inefficient process in terms of energy production. The first stage, glycolysis, produces just 2 net ATP molecules (it produces 4 total but also consumes 2). The subsequent fermentation properly produces two net molecules of NADH, which is also a high-energy molecule, though not as high-energy as ATP.
Differences between fermentation and anaerobic respiration
As can be seen, there are differences and similarities between fermentation and anaerobic respiration. The main similarities are that both begin with glycolysis, both occur in the absence of oxygen, and some species of prokaryotes can carry out both. However, the similarities end there. The following table summarizes the main differences between these two ways of obtaining ATP:
| Fermentation | anaerobic respiration |
| It can be carried out by both prokaryotic and eukaryotic organisms, including multicellular organisms such as vertebrates. | Only some species of prokaryotes can carry it out. |
| Different types of fermentation give different end products of glucose oxidation, including lactic acid, acetic acid, and ethane, among others. | It oxidizes glucose completely to carbon dioxide and transfers the electrons to different types of final electron acceptors, such as elemental sulfur, sulfates, or nitrates. |
| It produces relatively little usable energy for the cell. Just two net molecules of ATP and two molecules of NADH. | It produces large amounts of ATP, making the most of the energy contained in glucose. For every glucose molecule, more than 30 ATP molecules are produced. |
| It occurs exclusively in the cytoplasm. | It starts in the cytoplasm and ends inside the mitochondria. |
| It is a relatively simple process that consists of a small number of enzymatic reactions. | It is a very complex process that requires the intervention of numerous different enzymes both in the cytosol and in the matrix, the intermembrane space, and the inner membrane of the mitochondria. |
| It can be carried out in vitro . Only the enzymes that are responsible for fermentation are required, which can function in a suitable extracellular environment. | It depends on the presence of mitochondria, so it cannot be carried out in vitro . |
References
- environment. (2018, April 11). Fermentation types . Retrieved from https://www.ambientum.com/enciclopedia_medioambiental/suelos/tipos_de_fermentacion.asp
- Clark, MA (2018, March 5). Metabolism without Oxygen–Biology 2e . Retrieved from https://opentextbc.ca/biology2eopenstax/chapter/metabolism-without-oxygen/
- Lakna, B. (2017, December 26). Difference Between Fermentation and Anaerobic Respiration | Definition, Process, Application . Retrieved from https://pediaa.com/difference-between-fermentation-and-anaerobic-respiration/
- Kaur, J. (nd). How do fermentation and anaerobic respiration differ? | Socratic . Retrieved from https://socratic.org/questions/how-do-fermentation-and-anaerobic-respiration-differ
