Plant Physiology - Musiyenko M.M. 2001

Respiration
Development of concepts regarding the nature of mechanisms and pathways of redox transformations in the cell

The Development of The Doctrine of Respiration can be traced back to the last quarter of the 18th century, when the works of J. Priestley and A. Lavoisier established the gaseous COMPOSITION OF THE air. In 1780, J. Ingenhousz first demonstrated that green plants also absorb O₂ and release CO₂. Later, N. de Saussure, regarded as a founder of the doctrine of respiration, showed that the release of CO₂ and absorption of O₂ occur in green plant parts exclusively in the dark (a debatable point), whereas in non-green parts, they take place both in the dark and in the light. Throughout the 19th and early 20th centuries, significant contributions to the general theoretical understanding of respiration were made by I.P. Borodin, O.M. Bach, V.I. Palladin, S.P. Kostychev, and among foreign scientists, O. Warburg, H. Wieland, and others.

The joint efforts of 19th-century scientists made it possible to propose the following general equation for this process:

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If Proteins or fats, rather than CARBOHYDRATES, serve as the initial respiratory substrate, the energy yield will differ. It is generally accepted that The oxidation of 1 g of carbohydrates releases an average of 17 kJ, whereas 1 g of fats yields 39 kJ. However, this equation fails to reflect the full complexity of the chemistry of respiration and its intermediate stages.

Beginning in the 1920s, substantial contributions to understanding The Nature and enzymatic mechanisms of respiration were made by D. Keilin, G. Embden, H. Krebs, A. Szent-Györgyi, S. Ochoa, V.L. Kretovich, and others.

They proved that the transition from anaerobic processes to oxygen-dependent respiration allowed plant organisms to utilize the energy stored in respiratory substrate molecules more fully and efficiently.

This evolutionary transition itself was tied to The Emergence of novel enzyme systems that enabled the gradual transformation of respiratory substrates within The Cell. The Nature of respiration-related oxidation systems reflects the historically shaped ADAPTATION OF ORGANISMS to specific environmental conditions.

Theories of the Mechanisms of Biological Oxidation. Respiration is an oxidative process in which complex molecules (such as carbohydrates) are broken down into CO₂, and a significant portion of The energy released in the process is stored as ATP.

The foundational scientific concepts concerning The Role of oxygen in respiration were formulated by A. Lavoisier, who demonstrated the affinity between combustion and respiration, noting that both involve the uptake of O₂ and the release of CO₂ and heat. In his view, combustion is The addition of oxygen to a substrate, whereas respiration is nothing more than the slow combustion of nutrients within a living Organism. This raised the question of how such combustion could occur in an aqueous environment, given that Living organisms are 79–90% Water. In 1845, C.F. Schönbein proposed a theory of oxidative processes suggesting that living Cells contain compounds that are readily oxidized by oxygen and are thus capable of activating molecular oxygen. O.M. Bach continued research in this direction and, in 1897, published the work "On the Role of Peroxides in The process of Slow Oxidation," outlining his peroxide theory of biological oxidation.

As is well known, oxygen exhibits low reactivity; therefore, to oxidize a substrate, oxygen must first be activated. O.M. Bach believed that molecular oxygen can act as an oxidizing agent only when external energy is supplied to break the interatomic bonds holding the molecule together. He termed oxidation processes requiring supplementary energy forced oxidation, whereas oxidation occurring without additional Energy Expenditure was called spontaneous oxidation or combustion. In these processes, oxygen activation is driven by energy supplied by the substance being oxidized itself. Upon interacting with atmospheric oxygen, such compounds facilitate the Cleavage of one of the bonds between the oxygen atoms in the molecule:

Activated oxygen, by oxidizing another substance, yields a peroxide that can drive the further oxidation of Other Compounds. Thus, according to O.M. Bach, oxygen activation during respiration occurs via the "direct" formation of peroxides resulting from the addition of oxygen to the molecule of the substance being oxidized. He defined peroxides as compounds resembling hydrogen peroxides in which at least two oxygen atoms are linked to each other. They have the general formula:

He named compound A, which activates oxygen, an "oxygenase."

In addition to direct oxidation via peroxide formation, catalytic oxidation through these same compounds acting as intermediate products is also possible:

Thus, the oxidation theory developed by O.M. Bach is rooted in METABOLISM/2.html">THE CONCEPT OF oxygen activation. Although it is now known that the incorporation of O₂ into Organic compounds is not directly tied to respiration, these studies laid the groundwork for Modern views on The Mechanism of oxygen activation.

Following D. Keilin's discovery of cytochrome oxidase and other Cytochromes in cells in 1925, it was established that these very components drive The transfer of electrons and protons to O₂, resulting in The formation of water or peroxides.

Building upon the peroxide theory, O.M. Bach put forward another hypothesis linking biological oxidation to the removal of electrons and protons from the substrate. In this model, the role of oxygen is reduced to regenerating the oxidized state of the primary hydrogen acceptor. This hypothesis was further developed in V.I. Palladin's theory of the chemistry of respiration.

According to V.I. Palladin's theory, respiration is mediated by specialized respiratory chromogens capable of reversible redox transformations. Upon oxidation, chromogens are converted into pigments, which upon reduction turn back into chromogens; in other words, respiratory chromogens act as oxygen acceptors in the living cell, whereas respiratory pigments function as hydrogen acceptors.

Water is an obligate participant in respiration according to V.I. Palladin, serving as a hydrogen donor for the reduction of the pigment back into the chromogen. The hydrogen of the oxidized substrate also participates in this pigment-reduction process. Free oxygen is utilized to regenerate the hydrogen acceptors.

In 1912, V.I. Palladin published his theory of the chemistry of respiration, dividing the overall respiration equation into anaerobic (1) and aerobic (2) phases:

where R is a colored respiratory pigment that strips hydrogen from the substrate and is capable of reversible redox transformations; RH₂ is a colorless respiratory chromogen.

V.I. Palladin's theory assigns a major role to the activation of hydrogen from the oxidized substrate and its transfer via intermediate redox systems. This view was shared by the German biochemist H. Wieland, who independently proposed his own theory of hydrogen activation that same year in 1912.

Ahead of his time, V.I. Palladin authored the scientific hypothesis that living cells contain specific carrier Enzymes responsible for transferring hydrogen atoms from substrates and water. This is why the modern understanding of biological oxidation mechanisms is grounded in the Bach–Palladin theory, which posits that respiration requires both hydrogen and oxygen activators in equal measure. Consequently, the core tenets of The Theory of biological oxidation are as follows:

·water is an obligate participant in respiration;

·alongside the substrate undergoing oxidation, water serves as a hydrogen donor;

·THE RESPIRATORY PROCESS involves specific hydrogen activators that detach hydrogen from the substrate;

·the Initial Stages of respiration are anaerobic and do not require molecular oxygen;

·molecular oxygen is required at The final stage of respiration to regenerate hydrogen acceptors, resulting in the formation of water.

It is worth noting that V.I. Palladin pursued research on respiration throughout his entire career, engaging many of his students in these studies. Building directly upon the foundational works of A.N. Bach and V.I. Palladin, Ukrainian biochemists have made a substantial contribution to global science regarding various aspects of respiration—specifically the activation of oxygen and hydrogen, the genetic link between aerobic and Anaerobic respiration, the Chemistry of the respiratory process, and the role of enzyme complexes within it.