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biological energetics, a science that studies the processes of energy transformation in the life processes of organisms. In other words, bioenergetics examines the phenomena of life processes in their energy aspect. The methods and approaches to the phenomena studied in bioenergetics are physicochemical; its objects and problems are biological. Thus, bioenergetics stands at the juncture of these sciences and is a part of molecular biology, biophysics, and biochemistry.
The work of the German physician J. R. Mayer, who discovered the law of the conservation and transformation of energy (1841) on the basis of research on the energy processes in the human body, may be considered the beginning of bioenergetics. The overall study of the processes which are the energy sources for living organisms and of the energy balance of the organism and its changes under various conditions (rest, work of various intensity, environmental temperature) was for a long time the principal content of bioenergetics. In the mid-20th century, in connection with the general developmental trend in the biological sciences, research on the mechanism of transformation of energy in living organisms occupies a central position in bioenergetics.
All research in the field of bioenergetics is based on the exclusively scientific point of view, according to which the laws of physics and chemistry are fully applied to life phenomena and the basic principles of thermodynamics are applied to the transformation of energy in the organism. However, the complexity and specificity of biologic structures and of the processes realized in them condition a number of profound distinctions between bioenergetics and the energetics of the inorganic world—in particular, technical energetics (power engineering). The first fundamental characteristic of bioenergetics consists in the fact that organisms are open systems, which function only under conditions of constant exchange of materials and energy with the surrounding medium. The thermodynamics of such a system is essentially different from classical thermodynamics. The concept of equilibrium states, which is a basic principle of classical thermodynamics, is replaced by the concept of stationary states. The second principle of thermodynamics (the principle of the increase of entropy) is given a different formulation in the form of Prigozhin’s theorem. A second very important characteristic of bioenergetics is associated with the fact that the processes in the cells occur in the absence of jumps in temperature, pressure, and volume; by virtue of this, the conversion of heat into work is not possible in the organism, and heat emission constitutes an irreversible loss of energy. For this reason, organisms have worked out a number of specific mechanisms in the course of evolution for the direct transformation of one form of free energy into another, bypassing the conversion to heat. In the organism, only a small portion of the energy liberated is converted to heat and lost. The major portion of it is converted into free chemical energy of certain compounds, in which it is extremely mobile—that is, it can be converted to other forms even at constant temperature; specifically, it can perform work or accomplish biosynthesis with extremely high efficiency, attaining, for example, 30 percent in the work of the muscles.
One of the principal results of the development of bioenergetics in recent decades has been the establishment of the uniformity of energy processes in the entire living world, from microorganisms to man. It also appears that those substances in which energy accumulates in mobile, biologically assimilable form and the processes by means of which such accumulation is effected are the same for the entire plant and animal worlds. The same uniformity has been established also in the processes of using the energy accumulated in these substances. For example, the structures of contractile proteins and the mechanism of the chemicomechanical effect (that is, the conversion of chemical energy into work) are basically one and the same during the movement of the flagella of protozoans, the dropping of mimosa leaves, or the most complex movements of birds, mammals, and man. A similar uniformity is characteristic not only for the phenomena studied by bioenergetics but also for other functions inherent in all life forms—the preservation and transmission of genetic information, the basic pathways of biosynthesis, and the mechanisms of enzyme reactions.
The substances through which the energetics of organisms is realized are macroergic compounds, which are characterized by the presence of phosphate groups. The role of these compounds in the processes of energy conversion in the organism was first established by the Soviet biochemist V. A. Engel’gardt while studying muscle contraction. It was shown in the later work of many investigators that these compounds participate in the accumulation and transformation of energy in all life processes. Energy freed during the splitting off of the phosphate groups may be used for synthesis of biologically important substances and for an increased supply of free energy for life processes connected with the conversion of free chemical energy into work (mechanical, active transfer of substances, electrical, and so on). The most important of these compounds, playing the role of almost the only transformer and transmitter of energy in the entire living world, is adenosine triphosphic acid (ATP), which decomposes to adenosine diphosphoric acid (ADP) or adenosine monophosphoric acid (AMP). Hydrolysis of ATP—that is, the splitting off of the end phosphate group from it—proceeds according to the equation
ATP + H2O → ADP + phosphate
and is accompanied by a decrease in free energy by the value ΔF. If this reaction occurs at a concentration of 1.0 moles per liter for all reagents and products, at 25° C and pH = 7.0, the free energy of ADP is less than the free energy of ATP by 29.3 kilojoules (kJ), or 7,000 cal. In the cell this change in free energy is greater: ΔF = 50 kJ/mole (12,000 cal/mole). The value ΔF for the reaction ATP → ADP is higher than in most hydrolysis reactions. The bonds themselves of the third (terminal) and second phosphate groups in the ATP molecule, and analagous bonds in other macroergic compounds, are also called macroergic. These bonds are designated by the symbol ~ (tilde)—for example, the formula for ATP may be written thus: adenine—ribose—phosphate ~ phosphate ~ phosphate. The concept of the energy of macroergic bonds in bioenergetics refers not to the actual energy of the covalent bond between the atoms of phosphorus and those of oxygen (or nitrogen), as would be assumed in physical chemistry, but rather only the difference in the values of free energy (ΔF) for the initial reagents and the products of the hydrolysis of ATP or other analogous reactions. “Bond energy” in this sense is not, strictly speaking, localized in a given bond but characterizes the total reaction.
The energy of macroergic bonds of ATP is the universal form of storing free energy for the entire living world: all transformations of energy in life processes are effected through the accumulation of energy in these bonds and its use when they are severed. The value ΔF for these reactions represents a sort of “biological quantum” of energy, since all transformations of energy in organisms occur in portions approximately equal to ΔF. During the enzyme hydrolysis of ATP in the cell, the phosphate group that is splitting off is always transferred to the substrate, in which, as a result, the store of energy is greater than in the initial compound.
Cell metabolism consists in the uninterrupted decomposition of complex substances to simpler substances (catabolic processes) and the synthesis of more complex substances (anabolic processes). Catabolic processes are exergonic— that is, they operate with a decrease in free energy (ΔF < 0); anabolic processes are endergonic—they operate with an increase in free energy (ΔF>0). According to the general laws of thermodynamics, exergonic processes may occur spontaneously, but endergonic processes require an influx of free energy from without. This is effected in the cell because of the coupling of the two processes: some reactions use energy freed in the process of others. This coupling, which is fundamental to all metabolism and to all the life processes of the cell, is effected by means of the ATP-ADP system, which creates intermediate compounds that are enriched with energy.
For example, the synthesis of saccharose from glucose and fructose occurs using the energy freed during the hydrolysis of ATP, by means of the formation of the activated intermediate compound, glucose- 1-phosphate: (1) ATP + glucose → ADP + glucose-1-phosphate; (2) glucose-1-phosphate + fructose → saccharose + phosphate. Overall reaction: ATP + glucose + fructose → ADP + saccharose + phosphate.
Energy balance of the process: ATP → ADP + phosphate -29.3 kJ/mole (-7,000 cal/mole [decrease in free energy]); glucose + fructose → saccharose + 23 kJ/mole (+5,500 cal/mole [increase in free energy]). Loss of energy to heat, 6.3 kJ/mole (1,500 cal/mole)—that is, the efficiency of the process is 79 percent.
The coupling of reactions according to a similar pattern is also effected during the synthesis of other complex compounds (lipids, polysaccharides, proteins, and nucleic acids). In addition to ATP, several analogous compounds also participate in these processes; nitrogenous bases other than adenine (guanine, cytosine, uridine, and thymidine triphosphates, or creatine phosphates) are present in these compounds. During the synthesis of proteins and nucleic acids not just the end phosphate group, but rather the two end groups (pyrophosphate) are split off from the ATP. Thus, all the energy-accumulation processes of organisms must reduce to the processes of ATP formation—that is, phosphorylation (the inclusion of phosphate groups in ADP or AMP).
The energetics of metabolic processes in which energy is in chemical form is clear in its basic features, but this cannot be said of processes in which energy is transformed from chemical form to mechanical work or to any other form of energy (for example, electrical). Thus, it is known, for instance, that work performed by a contracted muscle is produced using the energy freed by the hydrolysis of ATP, but the mechanism of this transformation of energy is not yet clear. The clarification of the innermost mechanisms of the chemicomechanical effect and of other transformations of chemical energy is an important and urgent problem of bioenergetics, the successful solution of which may open the way toward direct transformation of chemical energy into mechanical or electrical, without its wasteful intermediate conversion to heat.
Both in principle and in practice, the only energy source for life on earth is the radiation energy of the sun, part of which is absorbed by plant pigments and certain bacteria and is accumulated by autotrophic organisms in the form of chemical energy during the process of photosynthesis— partly in the form of ATP (processes of photosynthetic phosphorylation) and partly in the form of the energy of certain specific compounds (reduced nicotinamide adenine dinucleotides), which are the most important intermediate energy accumulators. The entire subsequent process of the synthesis of carbohydrates—and then of lipids, proteins, and other cell components—is effected in a cycle of dark enzyme reactions using the energy of the compounds indicated above.
During the reaction of carbohydrate synthesis (overall: 6CO2 + 6H2O → C6H12O6 + 6O2, the increase in free energy ΔF = 2.87 mJ/mole (686,000 cal/mole), and the heat content of the products (enthalpy) changes by the quantity δH = 2.82 mJ/mole (673,000 cal/mole). Thus, carbohydrates, lipids, proteins, and other foodstuffs represent a form of long-term conservation of the energy of radiation absorbed by the plant.
In heterotrophic organisms, ATP is formed in the process of respiration during the intermediate stages of the oxidation of food substances to CO2 and water. In this process about 40–50 percent of the free energy is converted to the energy of the macroergic bonds of ATP, and the rest is lost in the form of heat. The total quantity of energy stored by plants in a year (on the simplified assumption that all the carbon is fixed in the form of glucose) is approximately equal to 1018-1021 J, which constitutes only 0.001 of the total flow of solar energy falling on the earth (1024 J/yr).
Some quantity of energy is also accumulated in the processes of chemosynthesis owing to the oxidation of reduced inorganic compounds, but the contribution of these processes to the energetics of the biosphere is small.
The above characterizes only the overall balance of energy in the processes of its accumulation and use. The study of the primary mechanisms of energy migration on the cellular and molecular levels has shown that the transfer of electrons along a chain of transmitters plays a decisive role. In certain links of this chain of oxidation-reduction reactions, the liberation of small portions of free energy—which correspond approximately to the ΔF values for the macroergic bonds of ATP—occurs.
Further study of the problems of bioenergetics, in particular of the mechanisms of the transformation of chemical energy into work, requires a change to the examination of these processes on a submolecular level, where the laws of quantum physics and chemistry would apply.
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