**2. Redox reactions in biological processes**

An understanding of the redox reactions of inorganic and organic compounds is central to understanding the metabolism of living things. One of the most important processes that occurs in living organisms is photosynthesis, which consists of a series of oxidation reduction reac‐ tions; the series begins when the chlorophyll in barks or leaves of plant cells absorbs sunlight with certain wavelengths and converts carbon dioxide into carbon and oxygen in a reduction process and ends the series with the production of glucose molecules. In other organisms, glu‐ cose is being consumed to generate energy in a long series of enzyme‐catalyzed reactions; in simple words, electrons can be transferred from glucose to molecular oxygen, oxidizing the carbon molecules to carbon dioxide and reducing O<sup>2</sup> to water.

however, the theory of phlogiston was not widely accepted in the scientific media. Seventy‐five years later, Antoine‐Laurent Lavoisier (1743–1794) came with solid explanation of combustion [3]. In 1772, Lavoisier discovered that when phosphorus or sulfur is burned in air, the products are acidic in nature, and the products also weigh more than the original phosphorus or sulfur, and he came to the conclusion that the elements combine with something in the air to produce acids [4], but he could not recognize what was in the air that combined with phosphorus or sul‐ fur. In 1974, he met Joseph Priestley (the father of oxygen discovery, 1733–1804) during his visit to Paris; he told Lavoisier about the gas produced when he decomposed the compound which we now call mercury oxide. This gas supported combustion much more powerfully than normal air. Priestley believed the gas was a particularly pure version of air; he started calling it dephlo‐ gisticated air, believing its unusual properties were caused by the absence of phlogiston. In 1779, Lavoisier coined the name oxygen for the element released by decomposition of mercury oxide, and from here, explanation of certain reactions as oxidation reduction officially started [5].

Coming back to the explosive materials, the year 1964 was the year that explosives, nitrocel‐ lulose and nitroglycerin, were both discovered, and later on trinitrotoluene (TNT), involved in weapon production and widely used in the First World War (1914–1919). The cheap mixture of ammonium nitrate and fuel oil was recognized as a powerful explosive in 1955, and this was used to bomb the Federal Building in Oklahoma City in 1995; finally, the explosives that were used in the fireworks are believed to be used for the first time in China in the sixth century. Now, the five main types of redox reactions are combinations, decompositions, displace‐ ments, combustions, and disproportionations. In combination redox reactions, two elements are combined whereas one element becomes oxidant and the other reluctant; in decomposi‐ tion redox reactions, a compound is broken down into its constituent parts; in displacement redox reactions, one or more atoms is swapped out for another; in combustion reactions, a compound reacts with oxygen to produce carbon dioxide, water, and heat; and in dispropor‐ tionation redox reactions, a molecule is both reduced and oxidized. These types of reactions

are rare, and many reactions are considered in the interface between these areas.

reaction could refer also to a transfer of electrons.

4 Redox - Principles and Advanced Applications

**2. Redox reactions in biological processes**

Concluding this historical background that chemists worldwide later recognized that other ele‐ ments reacted in the same general manner as oxygen, the concepts of oxidation and reduction were extended to include other elements; electrochemistry as a new field is further broadening the definition of the oxidation reduction reaction. Investigators observed that the ferric ions could be formed from the ferrous ions by the action of oxygen gas. This consumption of oxy‐ gen, oxidation, involved a loss of electrons by the ferrous ions species, and hence, an oxidation

An understanding of the redox reactions of inorganic and organic compounds is central to understanding the metabolism of living things. One of the most important processes that occurs in living organisms is photosynthesis, which consists of a series of oxidation reduction reac‐ tions; the series begins when the chlorophyll in barks or leaves of plant cells absorbs sunlight with certain wavelengths and converts carbon dioxide into carbon and oxygen in a reduction

This aspect of redox reactions in living organisms is called cellular respiration by which cells break down molecules of food (glucose) in a series of chemical reactions to produce energy, carbon dioxide, and water; the process depends heavily on the reduction of NAD<sup>+</sup> to NADH and the reverse oxidation reaction of NADH to NAD<sup>+</sup> as intermediate steps [6]. The oxidation of glucose is a thermodynamically favored process, meaning the transfer of electrons from

**Figure 1.** An illustrated diagram for Krebs cycle, copied from the website, https://wikispaces.psu.edu.

glucose to O<sup>2</sup> is thermodynamically downhill, and cells use this released energy to carry out a wide variety of energy‐requiring activities. **Figure 1** illustrates how glucose is burned in a series of redox reactions and ends up in the formation of carbon dioxide and energy that is stored as adenosine triphosphate (ATP); in the diagram called Krebs Cycle which describes cell burning of glucose, enzymes are used in each step to lower the activation energy for each step and aid in breaking and formation of bonds; the overall reaction is a redox reaction, that is, electrons are lost or gained in each step.

Other biological processes that involve the redox reaction is the production of free radicals, which can be produced by detaching electrons from certain type of molecules and reattaching to another type of molecule instantaneously; free radicals play an important role for the pro‐ grammed cell death (apoptosis), and any uncontrolled production of free radicals may lead to cause cancer [7].
