Plant Physiology - Lecture Notes - O. M. Tarnopilska 2019

5. Plant Respiration
5.1 Main Pathways of Respiratory Substrate Oxidation

Respiration is the oxidative breakdown of Organic compounds involving oxygen, resulting in The formation of Water, carbon dioxide, and high-energy compounds utilized by Cells.

Respiratory substrates. Respiratory quotient is the volumetric or molar ratio of CO₂ released during respiration to O₂ consumed over the same period. Under normal oxygen availability, the value of this quotient depends on the respiratory substrate. When CARBOHYDRATES are utilized, the quotient equals 1. If more oxidized compounds, such as organic acids, undergo breakdown, oxygen consumption decreases, and the quotient exceeds 1. For instance, during the utilization of malic acid, it is 1.33. The oxidation of more reduced compounds (fats, Proteins) requires more oxygen, causing the quotient to drop below 1. For example, when fats are utilized, the quotient is 0.7.

When carbohydrates are in short supply, alternative substrates are utilized. This is particularly evident during seed germination, where reserve nutrients consist of proteins and fats. Proteins are first broken down into Amino Acids. Subsequently, Amino acids are oxidized to acetyl-CoA and keto acids, which participate in the Krebs cycle. Fats are hydrolyzed by lipases into glycerol and Fatty acids.

Glycerol is phosphorylated and then oxidized to 3-phosphoglyceraldehyde, which enters Carbohydrate METABOLISM. Fatty acids are oxidized to form acetyl-CoA.

Oxidoreductases. The oxidation of respiratory substrates during respiration is mediated by Enzymes known as oxidoreductases, because the oxidation of one substance (the electron and proton donor) is coupled with the reduction of another (the acceptor). These Enzymes are classified into several groups.

Anaerobic or pyridine dehydrogenases. These are two-component enzymes whose coenzyme is NAD or NADP. They transfer electrons to various acceptors—excluding oxygen—and remove two protons from the substrate. One proton binds to the coenzyme, while the other is released into the medium. Depending on the protein moiety, over 150 such enzymes are known.

Aerobic or flavin dehydrogenases. These enzymes catalyze the removal of two protons from substrates and transfer electrons from anaerobic dehydrogenases to various acceptors (such as Quinones and Cytochromes), including oxygen. Their prosthetic group consists of vitamin B₂ derivatives: flavin adenine dinucleotide and flavin mononucleotide.

Oxidases. These enzymes transfer electrons from a substrate exclusively to oxygen, yielding water (via The transfer of 4 electrons to O₂), hydrogen peroxide (H₂O₂), or superoxide radical anions (O₂). Both H₂O₂ and O₂ are highly toxic and are therefore rapidly converted into water and oxygen by catalase and superoxide dismutase, respectively.

Oxygenases. These enzymes activate molecular oxygen and catalyze its incorporation into various organic compounds (amino acids, phenols, Unsaturated fatty acids, and xenobiotics—foreign toxic substances).