The Basics of Evolution - Korzh O.P. - 2006

Part III. PATHS OF LIFE'S DEVELOPMENT

Chapter 18. Development of Lower Organisms

18.2. Types of Nutrition

Living organisms are classified based on the energy and carbon sources they utilize. Only two forms of energy are available to living beings: light and chemical. Organisms that synthesize the substances they need using light energy are called phototrophs, whereas those requiring chemical energy for this purpose are called chemotrophs. Regarding carbon sources, organisms that use inorganic carbon (CO₂) for life processes are termed autotrophs, while forms requiring carbon solely of organic origin are heterotrophs. Taking into account both of these vital requirements, living organisms can be divided into four groups:

1) photoautotrophs: all green plants, blue-green Algae, green and purple sulfur Bacteria;

2) photoheterotrophs: a small group consisting of certain purple non-sulfur bacteria;

3) chemoautotrophs: certain bacteria involved in The Nitrogen Cycle and similar processes;

4) chemoheterotrophs: all animals and Fungi, the majority of bacteria, and some parasitic flowering plants (such as dodder and mistletoe).

Photoautotrophs (green plants) and chemoheterotrophs (animals and fungi) are considered the most essential and numerous groups. They form the biotic cycle of matter, which is a prerequisite for the existence of the biosphere.

It is currently believed that the primary reactions of Photosynthesis—specifically Water photolysis and the abiogenic synthesis of Organic compounds from carbon dioxide using solar energy—could have taken place even in the prebiological period (the plausibility of such a phenomenon has been experimentally proven).

At this stage, intermediate cycles characteristic exclusively of living Cells—such as phosphorylation and The Calvin Cycle—were absent. With The Emergence of the first photosynthetic organisms, this process acquired an endoenergetic character, involving the absorption of energy from the external environment.

During the Cytology/cytology/16.html">Early stages of evolution, energy assimilation and carboxylation occurred independently of one another through a variety of pathways.

It is traditionally believed that METABOLISM/21.html">Chemosynthesis preceded photosynthesis (an assumption based on the absence of oxygen in Earth's primordial atmosphere). However, it has now been established that in chemosynthesizers, the Respiration process follows the Krebs cycle, just as in animals and plants. In an oxygen-free environment, the latter mechanism makes no sense; therefore, it is likely that photosynthesis arose in parallel.

Alternative views reject the classical hypothesis, placing autotrophs and the initial predominance of metabolism over Replication at the foundation of life's Organization. For instance, according to G. Wächtershäuser's hypothesis, The formation of Primary organic substances occurred under anaerobic, fairly hot aquatic conditions (resembling modern geysers). The energy source for these organisms is hypothesized to be the pyrite-forming reaction:

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An important aspect of this theory is the ability of pyrite crystals to bind organic compounds, allowing relevant reactions to take place not in solution, but On the surface of these crystals, where a surface-bound reaction system (surface metabolism) was established. The segregation of Lipids on the surface could have formed the initial pre-membrane structures. The subsequent direction of evolution led to the exclusion of the pyrite crystal and the transition to a true Cell. A complex issue within this theory is the transition to the first autocatalytic cycle (it is hypothesized that a reductive Citric Acid Cycle may have emerged first).

Thus, in The First stage of life's formation, metabolism appears on the pyrite surface, and only later does the genetic apparatus emerge. It is suggested that the formation of genetic mechanisms initially relied not on RNA and DNA, but on so-called "threose Nucleic Acids" (TNAs). At the same time, the synthetic autocatalytic cycle possessed an inherent tendency toward evolution, involving increased complexity and a corresponding expansion of reactive capabilities.

In the earliest photosynthetic organisms, photosynthesis was carried out exclusively with the help of Photosystem I, where chlorophyll A played the most crucial role. Later, under The Influence of natural Selection, the photosynthetic apparatus steadily grew more complex. Chlorophylls B, C, D, and E, along with phycocyanins and phycoerythrins that appeared subsequently, became the Main Components of Photosystem II, which enhanced photosynthetic efficiency. Substantial Changes in the Evolution of the photosynthetic apparatus began to occur after the formation of Chloroplasts, which provided Spatial Compartmentalization of metabolic reactions within The Cell.

The heterotrophic type of Nutrition has also traversed a long evolutionary path. The uptake of organic substances is not a passive process, but a physiologically active one whose evolution began as early as the prokaryotes. The earliest living systems had no need for metabolic reactions: they could live and grow at the expense of the prebiotic soup. Later, as those reserves became depleted, organisms that produced Enzymes to synthesize organic molecules—rather than merely consuming them in ready-made form—became increasingly widespread. At this stage of evolution, both animals and plants developed the need to synthesize all compounds essential for life from assimilated or synthesized organics, driving the corresponding evolution of their biochemical reactions. Metabolic pathways are fundamentally common to all living beings, featuring highly conserved reactions and the enzymes that catalyze them. Enzymes also evolved alongside organisms, yet their core function remained practically unchanged.

The evolution of all Types of nutrition progressed much more efficiently following the complexification of organisms, that is, after the emergence of multicellularity.