Plant Physiology - Musiyenko M.M. 2001

Photosynthesis: physiological, biochemical and ecological aspects
Evolution of photosynthesis

Classification of organisms based on their primary carbon and Energy Sources. Carbon is a core element possessing a range of unique chemical properties, making it essential for All living organisms. Its significance is primarily determined by its ability to form stable covalent bonds with both other carbon atoms and atoms of different elements. In particular, it exhibits the unique property of forming carbon-carbon bonds, thereby building long carbon skeletons of molecules in the form of stable chains and/or rings. Carbon also forms multiple (double and occasionally triple) bonds with other carbon atoms, as well as with oxygen and nitrogen. C-C bonds can be viewed as the backbone of carbon molecules. Carbon atoms form covalent bonds with H, N, O, P, and S atoms. Compounds comprising these and other atoms in various combinations are precisely what account for The Diversity of Organic compounds. This diversity manifests itself in molecular sizes and chemical properties—which depend on the attachment of various elements and chemical groups to the backbone—as well as on the degree of saturation of the carbon Skeleton itself.

Energy sources. All Living organisms are capable of utilizing only Two Types of energy, namely light and chemical energy. Organisms that synthesize all the organic substances they require using light energy are called phototrophic (phototrophs), while those that require chemical energy for this purpose are termed chemotrophic (chemotrophs). Phototrophs are characterized by the presence of pigments that absorb light energy and convert it into chemical energy. Energy is necessary to

carry out various vital metabolic processes.

Organochemical evolution. The formation of organic matter preceded THE ORIGIN OF living organisms. Let us trace the most probable path of the evolution of life on our planet. Planet Earth is approximately 4.75 billion years old. The most abundant element in the universe is hydrogen (being 1,000 times more prevalent than oxygen), which is why Earth's primitive atmosphere was strongly reducing. The secondary atmosphere arose As a result of outgassing from Earth's interior; it had a high content of H₂O, NH₃, and CH₄, as well as likely C₂N₂, CO, C₂H₂, other Hydrocarbons, and minor amounts of H₂S. Oxygen was completely absent from it. It is believed that the secondary atmosphere persisted in this state for about 0.5 billion years. Organochemical evolution also spanned 0.5 billion years, with the process being driven primarily by CH₄ formed not through the reduction of CO₂, but rather generated alongside other hydrocarbons via the reaction of carbides from Earth's interior with Water.

The secondary atmosphere was continuously exposed to short-wave ultraviolet rays and ionizing radiation from both the Sun and terrestrial emitters (⁴⁰K, ²³⁵U, ²³⁸U), as well as electrical lightning discharges. This promoted various organic syntheses that eventually led to the appearance of acetaldehyde, formic, acetic, and other carboxylic acids, followed by hexoses, ribose, deoxyribose, and other Monosaccharides. Adenine and guanine were also shown to be capable of forming from a mixture of methane, ammonia, and water. Apparently, in the presence of phosphates, the abiotic synthesis of adenosine, AMP, ADP, and ATP also took place. Today, evidence has been obtained for the possible abiotic formation of Porphyrins—the precursors of later-arising Cytochromes and chlorophylls.

At the next stage of organochemical evolution, polymers could have emerged. It is hypothesized that the earliest Protein Synthesis, unlike modern synthesis, was not controlled by Nucleic Acids; most abiotically produced Proteins possessed catalytic activity.

The Development of these synthetic processes coincided with the gradual cooling of the Earth and the Formation of the primordial ocean in the early Precambrian. Driven by a continuous influx of energy, a diverse and chaotic "METABOLISM" took place in the primordial ocean. However, the gradual weakening of the atmosphere's reducing character (consumption of CH₄, accumulation of CO₂) shifted the reaction equilibrium in the primordial ocean. Organochemical evolution likely ceased around this time. Its definitive termination is associated with the exhaustion of readily available energy sources on a global scale. The stratospheric ozone layer, formed as a result of water dissociation under The Influence of ultraviolet rays, significantly reduced the flux of ionizing and short-wave radiation. All these phenomena, however, pertained to the period when primitive life forms emerged, which were already protected by the ozone shield from the lethal effects of UV radiation.

The earliest organisms were heterotrophs. In the absence of atmospheric oxygen, Fermentation served as the primary type of metabolism. The Pentose Phosphate Pathway, which in its original form functioned as a pathway of anaerobic dissimilation, likely arose on The basis of primary heterotrophy. Since one of the Functions of this pathway is the synthesis of ribose (for nucleic acids), the entire cycle may have originated in response to the depletion of pentose reserves in the primordial ocean.

The gradual depletion of all organic substance reserves in the primordial ocean marked the end of the evolutionary stage of primary heterotrophy. Only organisms that evolved mechanisms of autotrophic assimilation were able to continue along a progressive developmental path. Porphyrins, which likely arose abiotically in the ocean, acquired paramount importance as components of cytochromes and chlorophylls with the advent of autotrophy.

ESTABLISHMENT AND DEVELOPMENT of autotrophic Nutrition. It should be noted that autotrophic nutrition is not synonymous with Photosynthesis, because the group of autotrophs is extremely diverse both in morphological Organization and in the manner in which they synthesize organic substances.

The question of the origin of autotrophic nutrition remains quite debatable, despite the appealing hypotheses put forward by K.S. Mereschkowsky, A.S. Famintzin, and others concerning symbiogenesis and The Emergence of autotrophic nutrition through the Symbiosis of various prokaryotes.

With the appearance of primitive anaerobic heterotrophs, the action of light—mediated by pigments—promoted the emergence of photoorganotrophy (The oxidation of organic substances) in some, while others acquired the ability, under conditions of heterotrophic nutrition, to transfer electrons to sulfate or nitrate with the participation of cytochromes—that is, chemoreduction.

One of the most crucial Prerequisites for the emergence of autotrophic metabolism was the presence of photochemical activity in organisms. Photochemical reactions and the free radicals generated thereby could have initially established an electrochemical gradient of H⁺ ions across Cell membranes, the energy of which was utilized to synthesize ATP and expel excess Na⁺ ions. Later, this mechanism could have been supplemented by proton pumps, which ultimately enhanced The Cell's energetic and transport capabilities (Fig. 81).

The efficiency of such a system could have been further augmented by the involvement of pigments in these processes. According to other researchers, the primary role was played precisely by the initial appearance of pigments within the cell, and consequently, the Utilization of Light for transporting H⁺ ions across the membrane and establishing a transmembrane potential.

Main evolutionary trends of the pigment apparatus. There is no doubt that porphyrin-type compounds—which differ from other substances in their ability to absorb quanta with low energy reserves and to remain in a reactive state for prolonged periods—played a leading role in the evolution of matter and the simplest organisms. Primitive systems that incorporated porphyrins directly from the exogenous environment gained significant energetic advantages as a result. With the appearance of pigments in the cell, the autotrophic era of life begins, undergoing a series of refinements.

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Fig. 81. Schematic representation of thylakoids, Bacteria, and chromatophores, along with the Main Features of electron and proton transport topology: → fast photochemical electrogenic reactions, → slower electrogenic reactions, — — nonelectrogenic electron and hydrogen transfer, — — → pathways of proton transfer

Since all modern photosynthetic bacteria inhabit aquatic environments, it is assumed that phototrophic nutrition originally arose in water at depths reached by light. The phototrophic type of Bioenergetics could have emerged independently and convergently in various groups of living organisms. Autotrophs capable of photoreduction (Bacterial photosynthesis) and utilizing readily oxidizable substances such as H₂S, CH₄, H₂, and others as substrates could have appeared. After all, the METABOLIC ACTIVITY OF anaerobic heterotrophs led to the depletion of abiotically synthesized substances in the environment. Primitive organisms utilized carbides and compounds with partially hydroxylated carbohydrate chains, such as aldols, in their metabolism.

The utilization of organic compounds in the presence of light to generate other substances should be regarded as an evolutionary transitional stage between heterotrophs and photoautotrophs. The subsequent phase of autotrophic nutrition must be considered The ability to reduce CO₂ coupled with the oxidation of organic products, such as propanol, ethanol, sugars, and organic acids (e.g., in nonsulfur purple bacteria and sulfur bacteria).

Next, the ability to synthesize organic substances from inorganic ones likely emerged—in other words, bacterial photosynthesis appeared, marking a significant stabilization of the photoautotrophic type of bioenergetics. During bacterial photosynthesis (photoreduction), water is not yet used as an electron donor because these organisms still lack a second photosystem, operating solely via cyclic transport. However, the absorbed energy in such bacteria is already utilized for ATP synthesis. Some modern photoreducers switch to a heterotrophic type of nutrition in an aerobic environment, meaning they are facultative autotrophs (Fig. 82).

Fig. 82. Proposed evolutionary pathway for the emergence of Photosystem II

Consequently, photoreduction preceded photosynthesis, as evidenced by the ability of A number of cyanelles, green, red, and brown Algae to switch to photoreduction under anaerobic conditions.

It is believed that the transition from photoreduction to algal-type photosynthesis was driven by a deficit of reductants—specifically, an imbalance between ATP storage and reducing power. According to M.V. Gusev, Mutations that led to the functional Separation between such systems could have arisen in populations of motile phototrophs as they migrated toward the water surface, where The impact of Mutagenic Factors was quite pronounced.

This metabolic transition in bioenergetics was closely linked to an increase in the energy capacity and competitiveness of organisms. Already in blue-green algae, the fundamental Components of the photosynthetic apparatus are formed, and the components of the Electron Transport Chain as well as specific Enzymes make their appearance.

The victory of photosynthetic organisms in the competitive Struggle for Existence was facilitated by their use of simple and readily available substances, such as CO₂ and H₂O, to build organic molecules, synthesize more energy-rich products like CARBOHYDRATES, and release oxygen in the process.

Such are the probable stages in the development of autotrophic nutrition. In the course of its evolution, various modes of CO₂ assimilation were eventually superseded by the most specialized method—photosynthesis. Along the way, different electron Donors, energy sources, and structures were tested for efficiency. In this regard, the evolution of photosynthesis progressed through the following respective stages: photorganotrophy (donor—organic matter) → photolithotrophy (donor—inorganic matter) → photohydrotrophy (donor—water) (Fig. 83).

Fig. 83. Evolution of photosynthesis

Thus, the evolution of autotrophic nutrition can be schematically represented as follows: heterotrophs → obligate photoheterotrophs → facultative photoheterotrophs → obligate

photoautotrophs → photoreducers → photosynthetics.