Plant Physiology - Musiyenko, M. M. 2001

Respiration
Respiratory strategy. Basic principles

In nature, There are two main processes through лицом which the solar energy stored in organic matter is released: Respiration and Fermentation. Respiration is the aerobic oxidative breakdown of Organic compounds into simple, inorganic ones, accompanied by the release of energy. Fermentation is the anaerobic process of breaking down organic compounds into simpler ones, which is likewise accompanied by the release of energy.

The strategic goal of respiration is to generate energy in the form of ATP, provide reduction equivalents, and form intermediate products for various biosynthetic reactions within the plant Organism.

ATP is produced through The oxidation of Molecules used as Energy Sources, such as glucose, Fatty acids, and Amino Acids. In most biosynthetic reactions, the products are in a more reduced state than their precursors; therefore, In addition to ATP, they require a reduction equivalent. The primary electron donor in reductive biosynthetic reactions is NADPH, along with NADH and FADH₂, which serve as the main electron carriers during the oxidation of respiratory substrates.

During the anaerobic stage of respiration (Glycolysis) and the subsequent aerobic stages (The Tricarboxylic Acid Cycle, The pentose phosphate shunt), Coenzymes are reduced. In the final, Third Stage of respiration, these coenzymes (NADH, FADH₂) are either oxidized by atmospheric oxygen in the mitochondrial Respiratory Chain or utilized in the biosynthetic reactions (NADPH) of The plant Cell:

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The wide variety of metabolic types in plant organisms of different taxonomic positions, ages, physiological states, and environmental conditions is due to the multitude of reactions involved in the respiration process.

Respiration is one of the fundamental indicators of the vitality of any living organism. The assimilates produced by green plants through Photosynthesis are predominantly non-specific reserve substances. Therefore, their utilization by other organisms, as well as the synthesis of species-specific compounds based on them, is possible only after a series of complex biochemical transformations. The chemical energy of photoassimilates, as a transformed form of solar energy, is contained within the chemical bonds of these compounds. When such bonds are broken, typically through oxidation processes, energy is released. The plant organism successfully utilizes this energy because oxidation occurs gradually, through a series of steps, and thus energy is released in small amounts. It can be expended on specific metabolic processes or used to form new energy-rich chemical bonds, such as in ATP molecules, which act as the "energy currency" of The Cell:

The efficiency of converting potential energy into useful work depends on the type of transforming energy system that drives METABOLISM. Since Metabolism as a whole consists of chemical and physical reactions, it obeys the Laws of Thermodynamics.

Anabolic reactions ensure the synthesis of complex molecules from simpler ones; they are endergonic, meaning they require an input of energy:

where ΔG represents The change in Free energy for such a reaction.

Catabolic reactions are reactions in which complex products break down into simpler ones; they are exergonic, meaning they proceed with the release of energy:

The totality of these reactions constitutes metabolism. The newly formed organic substances are used for The Biosynthesis of new cellular components and also serve as a source of chemical energy. Energy is transported to various PARTS OF THE cell, transforming from one form into another. According to the first law of thermodynamics, during energy exchange and interconversions, the energy of the reaction products plus the released energy is always equal to the total energy of the initial reaction substrates.

The Second Law of thermodynamics—the law of Entropy—imposes certain limitations on the spontaneous conversion of thermal energy into other forms, causing all processes to proceed in the direction of increasing entropy. Internal disorder and energy dissipation are characterized by the entropy value S.

The plant organism is a living open system that exists in a state of continuous exchange of energy and entropy with its environment. Therefore, in accordance with the laws of thermodynamics:

where U is internal energy, F is free energy, T is absolute Temperature, and S is entropy.

Respiration is not localized in any specific plant Organs; it occurs in all living Cells, Tissues, and organs, making it a universal process.

Respiration is a biochemical process in which organic compounds are oxidized and broken down in the living organs of a plant organism in the presence of oxygen, with the release of energy. This results in a variety of intermediate products, often analogous to those produced during photosynthesis. These intermediate products are of great importance because they serve as a source for synthesizing the substances necessary for the normal functional activity of the organism. Respiration

thus establishes a relationship within the organism between the breakdown and Synthesis of the main biochemical compounds involved in metabolism. The gas exchange of tissues with the environment during respiration is the opposite of photosynthesis: respiration involves the uptake of oxygen and the release of carbon dioxide. To penetrate the chemical and energetic essence of respiration is to gain a deeper understanding of metabolism.