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Redox Reaction: Definition, Mechanism and Significance

A redox reaction is a chemical reaction in which electrons are transferred between substances. It plays a central role in biological processes such as metabolism and cellular respiration.

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Things worth knowing about "Redox reaction"

A redox reaction is a chemical reaction in which electrons are transferred between substances. It plays a central role in biological processes such as metabolism and cellular respiration.

What is a Redox Reaction?

A redox reaction (short for reduction-oxidation reaction) is a chemical reaction in which electrons are transferred from one substance to another. The term combines reduction and oxidation, two processes that always occur simultaneously and inseparably: the substance that is oxidised loses electrons, while the substance that is reduced gains electrons.

Redox reactions are among the most fundamental chemical processes in nature and are essential to countless biological, industrial, and everyday phenomena.

Key Concepts of Redox Reactions

Oxidation

Oxidation is the process by which an atom, ion, or molecule loses electrons. As a result, the oxidation state of the particle increases. The substance that donates electrons is called the reducing agent, because it causes another substance to be reduced.

Reduction

Reduction is the reverse process, in which a particle gains electrons. The oxidation state decreases. The substance that accepts electrons is called the oxidising agent, because it causes another substance to be oxidised.

Oxidation State

The oxidation state (or oxidation number) is a formal measure that indicates how many electrons an atom has gained or lost within a compound. It is used to identify and balance redox reactions.

How Does a Redox Reaction Work?

Every redox reaction consists of two simultaneous half-reactions:

Oxidation (loss of electrons): One substance loses electrons and is oxidised.
Reduction (gain of electrons): Another substance accepts those electrons and is reduced.

A classic example is the reaction of zinc with copper sulphate solution: zinc loses electrons (is oxidised), while copper ions gain electrons (are reduced), resulting in metallic copper and zinc sulphate.

Redox Reactions in Biology and Medicine

In living organisms, redox reactions are of fundamental importance. Many vital biochemical processes rely on the controlled transfer of electrons:

Cellular respiration: In the mitochondria, nutrients such as glucose are broken down through a chain of redox reactions. Energy is released in the form of ATP (adenosine triphosphate). Oxygen serves as the final electron acceptor (oxidising agent).
Photosynthesis: Plants use light energy to oxidise water and reduce carbon dioxide, producing glucose and oxygen.
Free radicals and oxidative stress: When redox balance in the body is disrupted, reactive oxygen species (free radicals) are produced, which can damage cells and DNA. This condition is known as oxidative stress and has been linked to ageing and diseases such as cardiovascular disease, diabetes, and cancer.
Antioxidants: Compounds such as vitamin C, vitamin E, and glutathione act as antioxidants by neutralising free radicals and preventing oxidative damage.
Immune system: Immune cells use targeted redox reactions (known as the oxidative burst) to destroy pathogens.

Redox Reactions in Everyday Life and Technology

Redox reactions are also present in many everyday contexts outside the human body:

Combustion: The burning of wood, petrol, or gas is an oxidation reaction that releases energy in the form of heat and light.
Rusting: The corrosion of iron (rusting) is a slow oxidation process in which iron reacts with oxygen and water.
Batteries and accumulators: Electrochemical cells such as batteries are based on redox reactions that generate electrical energy.
Bleaching and disinfection: Bleaching agents such as hydrogen peroxide work through oxidation, destroying pigments or pathogens.

Redox Reactions in Pharmacology

In medicine and pharmacy, redox reactions also play an important role:

Drug metabolism: Many pharmaceutical compounds are transformed and broken down in the body via redox reactions, for example by enzymes of the cytochrome P450 family.
Mechanism of action of certain drugs: Some medications, such as certain antibiotics or anticancer agents, act specifically via redox mechanisms by inducing oxidative stress in target cells.
N-acetylcysteine (NAC): This medication acts as an antioxidant and is used to reduce oxidative stress, for example in cases of paracetamol (acetaminophen) overdose.

References

Stryer, L., Berg, J. M., Tymoczko, J. L.: Biochemistry. 9th Edition. W. H. Freeman and Company, New York, 2019.
Atkins, P., de Paula, J.: Physical Chemistry. 10th Edition. Oxford University Press, Oxford, 2014.
Halliwell, B., Gutteridge, J. M. C.: Free Radicals in Biology and Medicine. 5th Edition. Oxford University Press, Oxford, 2015.

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