AEROBIC RESPIRATION: CHARACTERISTICS, STAGES AND ORGANISMS - BIOLOGY - 2025

The aerobic respiration or aerobic is a biological process that involves obtaining energy of organic molecules - mainly glucose - by a series of oxidation reactions, where the final electron acceptor is oxygen.

This process is present in the vast majority of organic beings, specifically eukaryotes. All animals, plants, and fungi breathe aerobically. Additionally, some bacteria also exhibit aerobic metabolism.

In eukaryotes, the machinery for cellular respiration is located in the mitochondria.

Source: National Human Genome Research Institute (NHGRI) from Bethesda, MD, USA, via Wikimedia Commons

In general, the process of obtaining energy from the glucose molecule is divided into glycolysis (this step is common in both the aerobic and anaerobic pathways), the Krebs cycle and the electron transport chain.

The concept of aerobic respiration is opposed to anaerobic respiration. In the latter, the final acceptor of the electrons is another inorganic substance, different from oxygen. It is typical of some prokaryotes.

What is oxygen?

Before discussing the process of aerobic respiration, it is necessary to know certain aspects of the oxygen molecule.

It is a chemical element represented in the periodic table with the letter O, and the atomic number 8. Under standard conditions of temperature and pressure, oxygen tends to bind in pairs, giving rise to the dioxygen molecule.

This gas, made up of two oxygen atoms, has no color, odor or taste, and is represented by the formula O ₂. In the atmosphere, it is a prominent component, and is necessary to sustain most life forms on earth.

Thanks to the gaseous nature of oxygen, the molecule is able to freely cross cell membranes - both the outer membrane that separates the cell from the extracellular environment, and the membranes of the subcellular compartments, including mitochondria.

Respiration characteristics

Cells use the molecules that we ingest through our diet as a kind of respiratory “fuel”.

Cellular respiration is the energy-generating process, in the form of ATP molecules, where the molecules to be degraded undergo oxidation and the final acceptor of the electrons is, in most cases, an inorganic molecule.

An essential feature that allows breathing processes to take place is the presence of an electron transport chain. In aerobic respiration, the final acceptor for electrons is the oxygen molecule.

Under normal conditions, these "fuels" are carbohydrates or carbohydrates and fats or lipids. As the body goes into precarious conditions due to lack of food, it resorts to the use of proteins to try to satisfy its energy demands.

The word respiration is part of our vocabulary in everyday life. The act of taking air into our lungs, in continuous cycles of exhalations and inhalations, we call breathing.

However, in the formal context of life sciences, such action is designated by the term ventilation. Thus, the term respiration is used to refer to processes that take place at the cellular level.

Processes (stages)

The stages of aerobic respiration involve the necessary steps to extract energy from organic molecules - in this case we will describe the case of the glucose molecule as respiratory fuel - until it reaches the oxygen acceptor.

This complex metabolic pathway is divided into glycolysis, the Krebs cycle, and the electron transport chain:

Glycolysis

Figure 1: glycolysis vs gluconeogenesis. Reactions and enzymes involved.

The first step in the breakdown of glucose monomer is glycolysis, also called glycolysis. This step does not require oxygen directly, and it is present in virtually all living things.

The objective of this metabolic pathway is the cleavage of glucose into two molecules of pyruvic acid, obtaining two net energy molecules (ATP) and reducing two molecules of NAD ⁺.

In the presence of oxygen, the pathway can continue to the Krebs cycle and the electron transport chain. In case oxygen is absent, the molecules would follow the fermentation route. In other words, glycolysis is a common metabolic pathway for aerobic and anaerobic respiration.

Prior to the Krebs cycle, oxidative decarboxylation of pyruvic acid must occur. This step is mediated by a very important enzyme complex, called pyruvate dehydrogenase, which carries out the aforementioned reaction.

Thus, pyruvate becomes an acetyl radical that is subsequently captured by coenzyme A, which is responsible for transporting it to the Krebs cycle.

Krebs cycle

The Krebs cycle, also known as the citric acid cycle or tricarboxylic acid cycle, consists of a series of biochemical reactions catalyzed by specific enzymes that seek to gradually release the chemical energy stored in acetyl coenzyme A.

It is a pathway that completely oxidizes the pyruvate molecule and occurs in the matrix of the mitochondria.

This cycle is based on a series of oxidation and reduction reactions that transfer potential energy in the form of electrons to elements that accept them, particularly the NAD ⁺ molecule.

Summary of the Krebs cycle

Each molecule of pyruvic acid breaks down into carbon dioxide and a two-carbon molecule, known as an acetyl group. With the union to coenzyme A (mentioned in the previous section) the acetyl coenzyme A complex is formed.

The two carbons of pyruvic acid enter the cycle, condense with oxaloacetate, and form a six-carbon citrate molecule. Thus, oxidative step reactions occur. The citrate returns to oxaloacetate with a theoretical production of 2 moles of carbon dioxide, 3 moles of NADH, 1 of FADH ₂ and 1 mole of GTP.

Since two pyruvate molecules are formed in glycolysis, one glucose molecule involves two revolutions of the Krebs cycle.

Electron transport chain

An electron transport chain consists of a sequence of proteins that have the ability to carry out oxidation and reduction reactions.

The passage of electrons through these protein complexes results in a gradual release of energy that is subsequently used in the generation of ATP by chemoosmotics. Importantly, the last chain reaction is of the irreversible type.

In eukaryotic organisms, which have subcellular compartments, the elements of the transporter chain are anchored to the membrane of the mitochondria. In prokaryotes, which lack these compartments, the elements of the chain are located in the plasma membrane of the cell.

The reactions of this chain lead to the formation of ATP, through the energy obtained by the displacement of hydrogen through the transporters, until it reaches the final acceptor: oxygen, a reaction that produces water.

Classes of carrier molecules

The chain is made up of three variants of conveyors. The first class are flavoproteins, characterized by the presence of flavin. This type of transporter can perform two types of reactions, both reduction and oxidation, alternatively.

The second type is made up of cytochromes. These proteins have a heme group (like that of hemoglobin), which can present different oxidation states.

The last class of transporter is ubiquinone, also known as coenzyme Q. These molecules are not protein in nature.

Organisms with aerobic respiration

Most living organisms have aerobic-type respiration. It is typical of eukaryotic organisms (beings with a true nucleus in their cells, delimited by a membrane). All animals, plants, and fungi breathe aerobically.

Animals and fungi are heterotrophic organisms, which means that the "fuel" that will be used in the metabolic pathway of respiration must be actively consumed in the diet. In contrast to plants, which have the ability to produce their own food via photosynthesis.

Some genera of prokaryotes also need oxygen for their respiration. Specifically, there are strict aerobic bacteria - that is, they only grow in oxygen-rich environments, such as pseudomonas.

Other genera of bacteria have the ability to change their metabolism from aerobic to anaerobic based on environmental conditions, such as salmonellae. In prokaryotes, being aerobic or anaerobic is an important characteristic for their classification.

Differences from anaerobic respiration

The opposite process to aerobic respiration is the anaerobic mode. The most obvious difference between the two is the use of oxygen as the final electron acceptor. Anaerobic respiration uses other inorganic molecules as acceptors.

Furthermore, in anaerobic respiration the end product of the reactions is a molecule that still has the potential to continue to oxidize. For example, lactic acid formed in the muscles during fermentation. In contrast, the end products of aerobic respiration are carbon dioxide and water.

There are also differences from an energy point of view. In the anaerobic pathway, only two ATP molecules are produced (corresponding to the glycolytic pathway), while in aerobic respiration the end product is generally about 38 ATP molecules - which is a significant difference.

References

1. Campbell, MK, & Farrell, SO (2011). Biochemistry. Sixth edition. Thomson. Brooks / Cole.
2. Curtis, H. (2006). Invitation to Biology. Sixth edition. Buenos Aires: Pan-American Medical.
3. Estrada, E & Aranzábal, M. (2002). Atlas of Vertebrate Histology. National Autonomous University of Mexico. Page 173.
4. Hall, J. (2011). Treaty of Medical Physiology. New York: Elsevier Health Sciences.
5. Harisha, S. (2005). An Introduction to Practical Biotechnology. New Delhi: Firewall Media.
6. Hill, R. (2006). Animal Physiology. Madrid: Pan-American Medical.
7. Iglesias, B., Martín, M. & Prieto, J. (2007). Bases of Physiology. Madrid: Tebar.
8. Koolman, J., & Röhm, KH (2005). Biochemistry: text and atlas. Panamerican Medical Ed.
9. Vasudevan, D. & Sreekumari S. (2012). Biochemistry Text for Medical Students. Sixth edition. Mexico: JP Medical Ltd.