CELLULAR RESPIRATION: PROCESS, TYPES AND FUNCTIONS - BIOLOGY - 2025

The cellular respiration is a process that generates energy in the form of ATP (adenosine triphosphate). Later, this energy is directed to other cellular processes. During this phenomenon, the molecules undergo oxidation and the final acceptor of the electrons is, in most cases, an inorganic molecule.

The nature of the final electron acceptor depends on the type of respiration of the studied organism. In aerobes - like Homo sapiens - the final electron acceptor is oxygen. In contrast, for anaerobic respirators oxygen can be toxic. In the latter case, the final acceptor is an inorganic molecule other than oxygen.

Source: By Darekk2, from Wikimedia Commons

Aerobic respiration has been extensively studied by biochemists and consists of two stages: the Krebs cycle and the electron transport chain.

In eukaryotic organisms, all the machinery necessary for respiration to take place is inside the mitochondria, both in the mitochondrial matrix and in the membrane system of this organelle.

The machinery consists of enzymes that catalyze the reactions of the process. The prokaryotic lineage is characterized by the absence of organelles; For this reason, respiration occurs in specific regions of the plasma membrane that simulate an environment very similar to that of the mitochondria.

Terminology

In the field of physiology, the term "respiration" has two definitions: pulmonary respiration and cellular respiration. When we use the word breath in everyday life, we are referring to the first type.

Pulmonary respiration comprises the action of breathing in and out, this process results in the exchange of gases: oxygen and carbon dioxide. The correct term for this phenomenon is "ventilation."

In contrast, cellular respiration occurs - as its name implies - inside cells and is the process in charge of generating energy through an electron transport chain. This last process is the one that will be discussed in this article.

Where does cellular respiration occur?

Location of respiration in eukaryotes

Mitochondria

Cellular respiration takes place in a complex organelle called the mitochondria. Structurally, mitochondria are 1.5 microns wide and 2 to 8 microns long. They are characterized by having their own genetic material and by dividing by binary fission - vestigial characteristics of their endosymbiotic origin.

They have two membranes, a smooth and an internal one with folds that form the ridges. The more active the mitochondria, the more ridges it has.

The interior of the mitochondrion is called the mitochondrial matrix. In this compartment are the enzymes, coenzymes, water and phosphates necessary for respiratory reactions.

The outer membrane allows the passage of most small molecules. However, it is the inner membrane that actually restricts passage through very specific transporters. The permeability of this structure plays a fundamental role in the production of ATP.

Number of mitochondria

The enzymes and other components necessary for cellular respiration are found anchored in the membranes and free in the mitochondrial matrix.

Therefore, cells that require a greater amount of energy are characterized by having a high number of mitochondria, in contrast to cells whose energy requirement is lower.

For example, liver cells have, on average, 2,500 mitochondria, while a muscle cell (very metabolically active) contains a much higher number, and the mitochondria of this cell type are larger.

In addition, these are located in specific regions where energy is required, for example surrounding the flagellum of sperm.

Location of prokaryotic respiration

Logically, prokaryotic organisms need to breathe and they do not have mitochondria - nor complex organelles characteristic of eukaryotes. For this reason, the respiratory process takes place in small invaginations of the plasma membrane, analogously to how it occurs in mitochondria.

Types

There are two fundamental types of respiration, depending on the molecule that acted as the final acceptor of the electrons. In aerobic respiration the acceptor is oxygen, while in anaerobic it is an inorganic molecule - although in a few specific cases the acceptor is an organic molecule. We will describe each one in detail below:

Aerobic respiration

In aerobic respiration organisms, the final acceptor for electrons is oxygen. The steps that occur are divided into the Krebs cycle and the electron transport chain.

The detailed explanation of the reactions that take place in these biochemical pathways will be developed in the next section.

Anerobic respiration

The final acceptor consists of a molecule other than oxygen. The amount of ATP generated by anaerobic respiration depends on several factors, including the organism under study and the route used.

However, energy production is always higher in aerobic respiration, since the Krebs cycle only works partially and not all transporter molecules in the chain participate in respiration.

For this reason, the growth and development of anaerobic individuals is significantly less than aerobic ones.

Examples of anaerobic organisms

In some organisms oxygen is toxic and they are called strict anaerobes. The best known example is that of the bacterium that causes tetanus and botulism: Clostridium.

In addition, there are other organisms that can alternate between aerobic and anaerobic respiration, being called facultative anaerobes. In other words, they use oxygen when it suits them and in the absence of it they resort to anaerobic respiration. For example, the well-known bacterium Escherichia coli possesses this metabolism.

Certain bacteria can use the nitrate ion (NO ₃ ^-) as the final electron acceptor, such as the genera Pseudomonas and Bacillus. Said ion can be reduced to nitrite ion, nitrous oxide or nitrogen gas.

In other cases, the final acceptor consists of the sulfate ion (SO ₄ ^2-) that gives rise to hydrogen sulfide and uses the carbonate to form methane. The Desulfovibrio genus of bacteria is an example of this type of acceptor.

This reception of electrons in nitrate and sulfate molecules is crucial in the biogeochemical cycles of these compounds - nitrogen and sulfur.

Process

Glycolysis is a pathway prior to cellular respiration. It starts with a glucose molecule and the end product is pyruvate, a three-carbon molecule. Glycolysis takes place in the cytoplasm of the cell. This molecule must be able to enter the mitochondria to continue its degradation.

Pyruvate can diffuse through concentration gradients into the organelle, through the pores of the membrane. The final destination will be the matrix of the mitochondria.

Before entering the first step of cellular respiration, the pyruvate molecule undergoes certain modifications.

First, it reacts with a molecule called coenzyme A. Each pyruvate cleaves into carbon dioxide and the acetyl group, which binds to coenzyme A, giving rise to the aceyl coenzyme A complex.

In this reaction, two electrons and a hydrogen ion are transferred to NADP ⁺, yielding NADH and it is catalyzed by the pyruvate dehydrogenase enzyme complex. The reaction requires a series of cofactors.

After this modification, the two stages within respiration begin: the Krebs cycle and the electron transport chain.

The Krebs cycle

The Krebs cycle is one of the most important cyclical reactions in biochemistry. It is also known in the literature as the citric acid cycle or tricarboxylic acid cycle (TCA).

It is named after its discoverer: the German biochemist Hans Krebs. In 1953, Krebs was awarded the Nobel Prize for this discovery that marked the field of biochemistry.

The objective of the cycle is the gradual release of the energy contained in acetyl coenzyme A. It consists of a series of oxidation and reduction reactions that transfer energy to different molecules, mainly NAD ⁺.

For every two acetyl coenzyme A molecules that enter the cycle, four molecules of carbon dioxide are released, six molecules of NADH and two of FADH ₂ are generated. CO ₂ is released into the atmosphere as a waste substance from the process. GTP is also generated.

As this pathway participates in both anabolic (molecule synthesis) and catabolic (molecule degradation) processes, it is called “amphibolic”.

Krebs cycle reactions

The cycle begins with the fusion of an acetyl coenzyme A molecule with an oxaloacetate molecule. This union gives rise to a six-carbon molecule: citrate. Thus, coenzyme A is released. In fact, it is reused many times. If there is too much ATP in the cell, this step is inhibited.

The above reaction requires energy and obtains it from breaking the high-energy bond between the acetyl group and coenzyme A.

Citrate is converted to cis aconitate, and is converted to isocitrate by the enzyme aconitase. The next step is the conversion of isocitrate to alpha ketoglutarate by dehydrogenated isocitrate. This stage is relevant because it leads to the reduction of NADH and releases carbon dioxide.

Alpha ketoglutarate is converted to succinyl coenzyme A by alpha ketoglutarate dehydrogenase, which uses the same cofactors as pyruvate kinase. NADH is also generated in this step and, as the initial step, is inhibited by excess ATP.

The next product is succinate. In its production, the formation of GTP occurs. The succinate changes to fumarate. This reaction yields FADH. The fumarate, in turn, becomes malate and finally oxaloacetate.

The electron transport chain

The aim of the electron transport chain is to take the electrons from the compounds generated in previous steps, such as NADH and FADH ₂, which are at a high energy level, and drive them to a lower energy level.

This decrease in energy takes place step by step, that is, it does not occur abruptly. It consists of a series of steps where redox reactions occur.

The main components of the chain are complexes formed by proteins and enzymes coupled to cytochromes: heme-type metalloporphyrins.

Cytochromes are quite similar in terms of their structure, although each one has a particularity that allows it to perform its specific function within the chain, singing electrons at different energy levels.

The movement of electrons through the respiratory chain to lower levels, produces the release of energy. This energy can be used in the mitochondria to synthesize ATP, in a process known as oxidative phosphorylation.

Chemosmotic coupling

For a long time the mechanism of ATP formation in the chain was an enigma, until the biochemist Peter Mitchell proposed chemosmotic coupling.

In this phenomenon, a proton gradient is established across the inner mitochondrial membrane. The energy contained in this system is released and used to synthesize ATP.

Amount of ATP formed

As we saw, ATP is not forming directly in the Krebs cycle, but in the electron transport chain. For every two electrons that pass from NADH to oxygen, the synthesis of three ATP molecules occurs. This estimate may vary somewhat depending on the literature consulted.

Similarly, for every two electrons that pass from FADH ₂, two ATP molecules are formed.

Features

The main function of cellular respiration is the generation of energy in the form of ATP to be able to direct it to the functions of the cell.

Both animals and plants need to extract the chemical energy contained in the organic molecules they use for food. In the case of vegetables, these molecules are the sugars that the same plant synthesizes with the use of solar energy in the famous photosynthetic process.

Animals, on the other hand, are not capable of synthesizing their own food. Thus, heterotrophs consume food in the diet - like us, for example. The oxidation process is in charge of extracting energy from food.

We should not confuse the functions of photosynthesis with those of respiration. Plants, like animals, also breathe. Both processes are complementary and maintain the dynamics of the living world.

References

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