Difference between cellular respiration and fermentation

All cells use the coenzyme adenosine triphosphate , also known by the initials ATP , as main direct source of energy for its metabolic processes. This molecule is a nucleotide formed by adenine (nitrogenous base), ribose (a carbohydrate) and a triphosphate group.

Molecular Structure of ATP
The use of ATP is practically universal among cellular living beings , there are some who may also use GTP (guanosine triphosphate), but no one who does not use ATP is known . Energy nutrients can be very diverse (carbohydrates, fatty acids, amino acids, minerals, metals) but all are transformed into ATP in the cellular interior.

The synthesis of ATP is performed through a series of reactions that can be classified into two major types, respiration and fermentation . Both terms are frequently used to associate respiration with oxygen (aerobic conditions) and fermentation with the absence of oxygen (anaerobic conditions), however, from a biochemical point of view, this differentiation is erroneous, since there are organisms capable of performing anaerobic respiration and organisms capable of fermentation under aerobic environmental conditions.

What is fermentation?

Fermentation is an oxidative catabolic process that starts from an organic substrate , in which oxygen does not participate (an anaerobic process ) and whose final product is another organic substance. From the fermentation it must be clear that:

Although it does not use oxygen in the anaerobic process, it is an oxidative process. Oxidation is not defined by the presence of oxygen but by electronic changes at the atomic or molecular level.
Although an anaerobic process, it is a process that can occur in the presence of oxygen . There are organisms that do not require anaerobic external conditions to perform the fermentation inside.
Among the final products are organic molecules , but there are also inorganic molecules such as carbon dioxide or water. That is, it is an incomplete oxidation or combustion ,
The substrate for fermentation is usually a simple carbohydrate, mainly glucose. Fermentation is classified according to the final product. The most common are lactic fermentation (lactic acid) and ethanolic fermentation (ethanol, also called alcoholic fermentation). Others are acetic, butyric, propionic and butyric fermentation.

Fermentation, being an incomplete oxidation, does not take advantage of all the chemical energy stored in the substrate. For example, lactic fermentation of glucose produces only two molecules of ATP per molecule of glucose.


What is Cellular respiration?

Cellular respiration is defined as the catabolic process in which a substrate is completely oxidized to generate inorganic compounds . The process is carried out through an electron transport chain, or respiratory chain, present in the cytoplasmic membrane of many types of bacteria, in the inner membrane of animal mitochondria or in plant thylakoids .

Of the bat, it must be clear that:

Although it is an oxidation, even complete, it does not have to use oxygen .
It can occur in aerobic or anaerobic external conditions .
It participates in an electron transport chain .
The substrate does not have to be organic but the end products are always inorganic .
In order for cell respiration, to completely oxidize an organic substrate, requires an electron acceptor with high electronegativity at the end of the electron transport chain. Depending on this acceptor, respiration is classified into (regardless of external environmental conditions):

Aerobic respiration : molecular oxygen is the ultimate electron acceptor.
Anaerobic Respiration : The ultimate electron acceptor is a substance other than oxygen. For example, bacteria using nitrate, sulfate, sulfur, iron III (Fe 3+ ), and others are known. There are even known bacteria which use organic molecules as the ultimate acceptor, for example fumarate.
Oxidation of a substrate has a higher yield than fermentation . For example, aerobic respiration of glucose generates 36 molecules of ATP compared to the two that are obtained from its fermentation.

Use of proton-motive force on the mitochondrial respiratory chain
Examples of cellular respiration with a starting inorganic substrate include iron bacteria which are capable of oxidizing ferrous oxide to ferric oxide and obtaining energy from this reaction.

Key Conclusion and Differences

At the cellular level, respiration and fermentation are two types of catabolic processes , a chain of reactions in which a molecule is transformed into one or more simpler molecules and the chemical energy that is released is stored in molecules of ATP.

The two are also oxidative processes, but fermentation never uses oxygen , whereas in the most common type of respiration, aerobic, molecular oxygen (O 2 ) is essential as the ultimate acceptor of electrons in the oxidative process. In the fermentation also does not intervene an electron transport chain.

This implies that aerobic respiration always needs oxygen in the environment external to the organism, that is, it needs aerobic environmental conditions . However, there are many organisms that perform fermentation under aerobic environmental conditions, for example many yeasts. That is, internally, fermentation is an anaerobic process but can occur in aerobic environmental conditions.

Another type of respiration, anaerobic respiration, does not use oxygen but continues to intervene an electron transport chain and combustion remains complete , the difference with aerobic respiration is that at the end of the electron transport chain there is another substance different from molecular oxygen. Anaerobic respiration, like fermentation, can occur in aerobic environmental conditions depending on the organism, even if it is an internal anaerobic process.

Many organisms can perform fermentation or breathing according to the conditions to which the cell is subjected. For example, in humans, aerobic respiration degrades glucose to pyruvate, and pyruvate passes into the mitochondria where it is oxidized to carbon dioxide (CO 2 ) and water (H 2 O), but if the oxygen supply is not sufficient, for example in muscle cells before intense or sudden exercise, pyruvate can follow the path of lactic fermentation.

Routes of fermentation and respiration
With all this, one could conclude that the essential difference between respiration and fermentation is the participation of an electron transport chain and the consequent complete combustion , not the presence or not of oxygen.

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