Plant Physiology - Musienko M. M. 2001

Respiration
Respiration is the central metabolic pathway. Respiration and photosynthesis

Respiration is a central link in METABOLISM (Fig. 111) because the entire complex chain of interrelated processes involving the mobilization of Photosynthesis products through their chemical and energetic activation

Class="center">

Fig. 111. Intermediate products of transformations occurring during carbohydrate oxidation in respiration

is carried out thanks to it. For a long time, the Biological Significance of respiration was reduced merely to the release of energy from respiratory substrates and its utilization by the living Cell. However, modern research data on the biochemical nature and enzymatic mechanisms of respiration not only demonstrate its high complexity but also reveal The Role of intermediate products formed along The pathway of organic molecule conversion into the final products CO₂ and H₂O.

Regardless of the pathway of carbohydrate breakdown, intermediate products of Cleavage are often utilized even before their complete oxidation.

Thus, in the absence of O₂, pyruvic acid can be reduced to lactic acid or, via acetaldehyde, to ethyl alcohol.

Organic acids newly formed in The Tricarboxylic Acid Cycle are capable of binding ammonia via direct amination or Transamination to form Amino Acids, which are subsequently incorporated into Protein Synthesis (Fig. 112).

Fig. 112. Biosynthetic families of amino acids

Amino acids can also give rise to phenols, Flavonoids, anthocyanins, Lignin, and Other Compounds whose formation pathway begins with the deamination of The amino acid phenylalanine. Tryptophan serves as the source for the Synthesis of the phytohormone auxin.

Acetyl-CoA, which is formed during The breakdown of Pyruvate, serves as the starting product for the synthesis of Fatty acids, isoprene polymers (C₅H₈), Terpenes, Steroids, as well as phytohormones such as gibberellin, Abscisic acid, and partially cytokinin

Finally, the Pentose Phosphate Pathway of glucose breakdown yields pentoses, which serve as precursors for the Biosynthesis of Nucleic acid NUCLEOTIDES. This process also generates erythrose 4-phosphate, essential for the synthesis of shikimic acid, a precursor of aromatic rings in various Organic compounds (Fig. 113).

Respiration and Photosynthesis

It is well established that photosynthesis acts against the gradient of decreasing Entropy, whereas respiration proceeds along the gradient of decreasing Free energy and is associated with an increase in entropy. Therefore, for a long time, these processes were considered polar opposites. In reality, however, they are closely interrelated and evolutionarily linked, with Water playing a vital role in both. During photosynthesis, water acts as a hydrogen donor. Hydrogen reduces CO₂, while the oxygen derived from water enriches the atmosphere and helps maintain the ecological balance of the CO₂-to-O₂ ratio in the biosphere. In respiration, water Functions as an oxidant for the carbon in respiratory substrates, whereas its hydrogen, together with the hydrogen of the substrates itself, acts as a reductant. Water is split with the participation of a manganese-containing water-oxidizing complex, whereas its biosynthesis during respiration involves Cytochromes and iron-containing Proteins. The Electron Transport Chain of the chloroplast and its catalytic systems are largely homologous to the respiratory ETC. Furthermore, the CO₂ reduction cycle (The Calvin Cycle) is closely related to the Hexose monophosphate pathway of hexose metabolism.

It is known that, In addition to the consumption of respiratory substrates for biomass accumulation—that is, for growth (the so-called growth component of respiration, or growth respiration)—a certain amount of matter and Energy is required to maintain the functional status of already active cellular structures. These expenditures constitute another component of respiration, namely maintenance respiration.

Fig. 113. The shikimate pathway for The formation of Aromatic Compounds: E-4-P — erythrose 4-phosphate, PEP — phosphoenolpyruvate, DAHP — 3-deoxy-D-arabino-heptulosonate 7-phosphate, DHQ — dehydroquinate, DHS — dehydroshikimate, Chor — chorismate, Pref — prephenate, HPP — 4-hydroxyphenylpyruvate, Phe — phenylalanine, Cinn — cinnamic acid, Coum — p-coumaric acid, Anth — anthranilate, Quin — quinic acid, PCA — protocatechuic acid, Gall — gallic acid. The line in the figure separates Primary and secondary products. The hydroxylation of Phe------Tyr generally plays a minor role, whereas the deamination of Tyr------Coum is found in very few plants (such as grasses). In higher plants, the formation of phenylpropanoid derivatives presumably occurs via an alternative pathway involving quinic acid.

Some authors divide respiration into an even greater number of functional components. Distinguishing these fractions has provided a deeper insight into The Physiological Role of respiration. It turns out that photosynthesis performs functions very similar to those of respiration—specifically, supplying The Cell with the intermediates and energy equivalents required for growth and maintenance. In other words, in the light, photosynthesis is capable of supporting all growth and maintenance processes that are sustained by respiration in the dark. This shifts our understanding of how these processes are integrated at THE CELLULAR LEVEL. Rather than operating sequentially within metabolism, they function simultaneously in a coordinated manner. Thus, to the conventionally accepted sequence: photosynthesis → assimilates → respiration → growth, one must add a link that reflects their concurrent participation in growth processes:

Although this scheme still provides a rather simplified representation of the full complexity of these interactions, there is no doubt that such an intricate system, involving the interplay of diverse metabolic cycles, requires exceptionally delicate regulation of photosynthetic and respiratory intermediates and energy carriers of various origins.

Many researchers believe that respiration is suppressed in green Cells in the light, although some data argue against this view.

It should be noted that at the whole-Organism level as well, photosynthesis can act not merely as a supplier of assimilates, but can also directly contribute to growth and maintenance processes by providing intermediate metabolites or energy. In particular, H. Lambers (1998) suggested that photosynthesis may directly supply energy equivalents to support growth processes.

Consequently, the physiological functions of photosynthesis and respiration, discussed in the preceding chapters, form the foundation of the entire metabolic and energy network in green plants.