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Aerobic and anaerobic processes are two different types of processes that cells use to obtain energy from the food they eat, depending on the surrounding conditions. The main difference between the two is that the first is carried out by cells when they are in a medium rich in oxygen, while the second is carried out when it is absent or when the concentration of this gas is not high enough.
In addition to this fundamental difference, the biochemical reactions that occur in the presence or absence of oxygen are also different, so aerobic and anaerobic processes generally lead to different intermediate and end products, as well as a different level of energy utilization. energy stored in nutrients. On the other hand, there are also differences regarding the type of organism capable of using each process and the part of the cell in which they occur.
Differences between aerobic and anaerobic cellular processes
The following table summarizes the most important differences between these two metabolic processes. They are explained in more depth later.
| Aerobic processes | Anaerobic Processes | |
| When they occur: | They occur in the presence of oxygen. | They occur in the absence of oxygen or when the oxygen concentration is low. |
| Initial substrate: | glucose and oxygen. | Just glucose. |
| Final product: | CO 2 , water and energy in the form of ATP | Energy in the form of ATP and, depending on the particular type of process, the final product may be lactic acid or ethanol and CO 2 . |
| Stages involved: | • Glycolysis • Pyruvate oxidation • Citric acid cycle or Krebs cycle. • Oxidative phosphorylation. |
• Glycolysis • Oxidation of pyruvate • Most do not involve the Krebs cycle. • Most do not involve oxidative phosphorylation. |
| It involves the electron transport chain. | In the case of fermentation, it does not involve the electron transport chain. | |
| Power Production Efficiency: | It produces large amounts of energy in the form of ATP. For each glucose molecule, a total of 30-32 net ATP molecules are produced. | It produces little energy in the form of ATP. For every glucose molecule fermented, only 2 net ATP molecules are produced. |
| Part of the cell where it occurs: | One part occurs in the cytoplasm and another within the mitochondria. | It occurs in the cytoplasm and, in some cases, on the cell membrane. |
| Type of organization that uses it: | It occurs in aerobic organisms and in facultative anaerobes. It does not occur in strict anaerobes or in tolerant anaerobes. |
It occurs in strict, facultative and tolerant anaerobes. |
| Differences in evolution: | It is a more recent metabolic process. | It is supposed to be the oldest carbohydrate metabolic process. |
ATP: Cellular fuel
Even after digestion, the cells cannot use the substances that the food we eat is converted into directly as a source of energy. This must process them and convert them into a special molecule called adenosine triphosphate, adenosine triphosphate or ATP, for its acronym in English.
This is where aerobic and anaerobic metabolic processes come into play, as both represent different ways of turning glucose and other nutrients into ATP. Put another way, aerobic and anaerobic processes can be seen as different ways of refining food to produce the fuel cells actually need.
Aerobic processes
Aerobic processes refer to cellular respiration in the presence of oxygen. They are a series of biochemical reactions that have oxygen as the final acceptor of the electrons generated by the oxidation of glucose. The net reaction of aerobic respiration is:
C 6 H 12 O 6 (glucose) + 6O 2 + 32ADP + 32Pi → 6CO 2 + 6H 2 O + 32ATP
In this chemical equation, ADP represents adenosine monophosphate, Pi refers to inorganic phosphate, and ATP is adenosine triphosphate.
Electrons from the oxidation of glucose are transported up the electron transport chain through a series of oxidation-reduction reactions known collectively as oxidative phosphorylation. This process occurs in the mitochondria and produces large amounts of energy in the form of ATP.
Aerobic respiration begins with a stage that does not require oxygen called glycolysis . During this first phase, which occurs in the cell’s cytoplasm, the glucose molecule is split in two through various reactions to produce two molecules of a compound called pyruvate, generating two net ATP molecules.
The pyruvate formed during glycolysis is oxidized and then enters the mitochondria where it enters the Krebs cycle, also known as the tricarboxylic acid cycle or the citric acid cycle. This cycle is coupled with oxidative phosphorylation , and these two processes together with glycolysis produce a total of 32 net ATP molecules for every glucose molecule metabolized.
Anaerobic Processes
Unlike aerobic processes, anaerobic processes do not use oxygen in any of their stages. In fact, the term encompasses the processes of metabolism of glucose and other nutrients in the absence of oxygen.
The most common anaerobic processes are anaerobic respiration and the different types of fermentation.
anaerobic respiration
It refers to the way in which some anaerobic microorganisms carry out the oxidation of glucose. In these cases, instead of oxygen being the final acceptor of the electrons from glucose, other inorganic compounds such as nitrate ions, sulfate, carbon dioxide and even, in some cases, some metallic cations such as iron (III), manganese (IV) or uranium (VI).
Anaerobic respiration is very similar to aerobic respiration in that it also involves an initial stage of glycolysis and a series of oxidation reactions coupled to an electron transport chain, but it produces less energy than aerobic respiration.
fermentation
Fermentation is another type of anaerobic process. Although it also begins with the formation of pyruvate through glycolysis, it does not follow a chain of reactions that leads to its total oxidation as occurs during respiration (whether anaerobic or not).
Depending on the type of final product in which the pyruvate is transformed, different types of fermentation can be carried out. For example, muscle cells can ferment pyruvate to lactic acid if there is not enough oxygen or if there is more pyruvate than the mitochondria can handle through aerobic respiration. This can happen when we do sustained, high-intensity exercise.
Many microorganisms can also carry out other types of fermentation. Some, like yeast for example, ferment carbohydrates to ethyl alcohol . This process is used for the production of alcoholic beverages. Still other bacteria can produce methane by fermentation.
Because fermentation siphons off pyruvate before it reaches the electron transport chain, it is not considered a type of respiration, but it is a type of anaerobic process.
Difference in energy production in aerobic and anaerobic processes
One of the most important differences between aerobic and anaerobic processes is their ability to harness the chemical energy contained in glucose and other cellular foods. Aerobic respiration is much more efficient at producing energy than any of the anaerobic processes.
Both aerobic and anaerobic processes start with the same initial stage, which is glycolysis. This process has a net production of only 2 ATP molecules.
However, the similarities end here. In anaerobic processes, since there is no oxygen, pyruvate does not enter the Krebs cycle that couples with the ATP production machinery formed by the electron transport chain, so it is not possible to produce more ATP than the two molecules They come from glycolysis.
For this reason, aerobic processes are much more energy efficient than anaerobic ones.
Differences in their evolution
Anaerobic processes are believed to be older than aerobic ones, since the primordial atmosphere did not contain oxygen. It was not formed until photosynthetic organisms, primarily green plants, evolved, long after life on land arose.
Even the first single-celled eukaryotic organisms are supposed to have been anaerobic. However, by evolving through endosymbiosis, at some point they incorporated photosynthetic cells that produced oxygen as a by-product, and later evolved to be able to take advantage of this compound by virtue of its high reduction potential.
As multicellular eukaryotic organisms began to appear on Earth, larger and more complex organisms needed to produce more energy, so aerobic processes were a great evolutionary advantage. Through natural selection, organisms with the most mitochondria that could undergo aerobic respiration survived and reproduced massively, passing these favorable adaptations on to their offspring. The older versions could no longer meet the demand for ATP in the more complex organism and died out.
