Plant Physiology - Musienko, M. M. 2001

Respiration
Evolution of Energy Metabolism Pathways

The Features of the Three types of METABOLISM/26.html">Energy Metabolism in plant organisms that we examined—Photosynthesis, Fermentation, and Respiration—indicate that these fundamental Functions are inextricably linked and represent links in a single evolutionary chain.

It is known that the material and energetic basis for The Emergence of life on our planet was Organic compounds formed from inorganic ones via abiogenic pathways long before the appearance of the first primitive forms of living creatures. The source of energy necessary for such transformations was ultraviolet radiation quanta.

Primitive heterotrophs, in turn, carried out their energy metabolism via an anaerobic, oxygen-free pathway. By its nature, this type of metabolism belongs to fermentation—a set of oxidation-reduction processes in which the mobilized hydrogen of the substrate is used not to reduce O₂, but some other compound.

Modern anaerobes (heterotrophs) possess well-organized membrane and other biologically active structures, as well as catalytically active systems of energy metabolism, including dehydrogenases, isomerases, transferases, ATP synthases, etc.

The next stage in the evolution of energy metabolism pathways was phototrophy. It was driven not only by Changes in the ecological situation on Earth (as discussed in the chapter "Photosynthesis"), but also by The formation of catalytic systems of a fundamentally new type. We are referring to pigments—compounds endowed with the remarkable ability to use the energy of absorbed light quanta to transition to an electronically excited state. The realization of the energy of the free electrons thus generated and their conversion into chemical energy was achieved through the participation of a new system of catalytic mechanisms, namely, the Electron Transport Chain. As a form of energy metabolism, phototrophy passed through a series of successive, interrelated stages in the course of evolution. The first link was phototrophic Bacteria, which carried out Bacterial photosynthesis, i.e., photoreduction. Common to all modern photosynthetic bacteria is their ability to use exclusively highly reduced compounds with a low oxidation potential as electron Donors. This is a weighty argument for defining these organisms as standing closest to anaerobic heterotrophs. Their electron transport system includes Cytochromes, flavin Enzymes, and Quinones, which are also possessed by evolutionarily lower forms of autotrophs, including higher plants. Electron transport in these organisms is coupled with phosphorylation, and among the end products, alongside ATP, is reduced NAD. The reduction of NAD in bacteria is carried out mainly not by direct electron transfer, but via ATP—that is, through a biochemical pathway rather than a photochemical one. While remaining predominantly anaerobic, the group of photosynthetic bacteria nevertheless includes transitional forms. For instance, some anaerobic purple bacteria are capable of surviving in an oxygen-containing environment and growing at the expense of respiratory energy.

The emergence of the phototrophic mode of Nutrition served as a major impetus for biochemical and biological evolution. It was during this evolution that organisms arose which mastered mechanisms allowing them to utilize electrons from the most oxidized form of hydrogen—in which it exists as part of the H₂O molecule. Green plants became such organisms. Their main advantages compared to photosynthetic bacteria are as follows:

·the presence of Chloroplasts, whose ultrastructural Organization features prevent back-reaction between the active oxidizing agent (oxygen) and highly reduced carbon compounds;

·The ability to absorb quanta of the visible spectrum (PAR) at high light intensities;

·the presence of Two Photosystems;

·the ability to directly reduce NADP.

Due to this, green plants became widespread and colonized all ecological niches where Water, light, and CO₂ are available. The formation of free O₂—which possesses the maximum oxidation potential—during photosynthesis (water photolysis) had an extraordinary impact on the subsequent course of energy metabolism evolution and the evolution of living organisms as a whole.

The fact is that the appearance of oxygen necessitated the creation and Selection of mechanisms capable of mildly deactivating oxygen under biological conditions through its reduction. This gave rise to a qualitatively new stage in the evolution of energy metabolism pathways: the transition from the anaerobic to the aerobic type. For the anaerobic organisms existing on our planet at that time, the appearance of free oxygen was a critical, turning point. It became necessary to seek protection against the detrimental effects of this powerful oxidizing agent, to which anaerobes were not adapted. Consequently, the advent of oxygen-producing organisms was separated from the emergence of organisms capable of efficiently utilizing oxygen as an electron acceptor by a considerable span of time. Throughout this period, various transitional forms of organisms arose in which the evolution of O₂ was initially combined with an inability, and later with a gradually increasing ability, to utilize it in energy metabolism. The catalytic systems they possess reflect Various Forms of adaptation for utilizing electron donors and transferring them to O₂. Examples include obligately autotrophic blue-green Algae (or cyanobacteria), which are incapable of utilizing the O₂ generated during their photosynthesis and release it into the atmosphere.

Chemosynthesizing bacteria, which are also heterogeneous in composition, can be classified among the transitional forms of organisms leading to aerobic metabolism. Alongside autotrophs, heterotrophs are found among them; and alongside the predominant aerobes, certain anaerobic species are also known.

Some chemosynthesizers (iron bacteria) possess an almost complete set of The electron transport chain components (NAD and FAD dehydrogenases, cytochromes, ferredoxins, ubiquinones, etc.). However, the energy efficiency of their oxidative metabolism is generally low. One reason for this is the absence of specialized morphological structures like Cell/35.html">Mitochondria, a function performed in chemosynthesizers by a single-layered cytoplasmic membrane lacking cristae.

The Evolution of the ability to utilize O₂ culminated in the emergence of aerobic respiration—a function based on the refinement and partial de novo formation of pre-existing enzyme systems associated with the mobilization of electrons and their transfer to activated oxygen. The high energy efficiency of coupling such a complex catalyst system was due to the formation of mitochondria. Their ultrastructural organization ensured the ability to use oxidation not only to release the energy of respiratory substrates, but also to yield physiologically active intermediate metabolites. It is important to emphasize that during the formation of respiratory catalytic systems, elements of the pre-existing ETC mechanisms of photosynthesizers were widely utilized. As we noted, differences in The Nature of the components functioning in the ETC of chloroplasts and mitochondria concern only minor details and are not of fundamental importance. Furthermore, the energy metabolism mechanisms of anaerobes were fully utilized in constructing the oxygen respiration systems. Finally, the fundamental features of organelle ultrastructure, membrane molecular organization, and so forth, were also put to use.

The deep evolutionary interrelation between respiration and photosynthesis is evidenced by the similarity in the chemistry of carbohydrate Biosynthesis pathways during photosynthesis and their breakdown during respiration (the reductive and oxidative pentose phosphate cycles), the identity in the nature of intermediate products, and other facts. It should also be emphasized that THE PRINCIPLE OF extending the electron transport chain—which underlies the formation of both photosynthetic and respiratory ETCs—is identical. In this way, minimal energy drops between adjacent ETC links are achieved, and consequently, a high efficiency of electron energy utilization.

An important piece of Evidence for the genetic connection between respiration and photosynthesis is also the principle of alternative (multiple) mechanisms and pathways by which both of these vital Functions of the plant Organism can be carried out. The existence of alternative respiration pathways is of great adaptive significance. From the above, it is quite evident that the Improvement of Biological mechanisms and structures in the course of evolution was achieved mainly not by discarding preceding ones, but through their modification and complexification. The purpose of additional mechanisms was to adapt energy metabolism processes to regular changes in the living conditions of plant organisms on Earth.