From Big Medical Encyclopedia

THERMODYNAMICS — science about the general patterns of energy conversion at its transfer between bodies in the form of heat and work. From positions T. power exchange in live systems, and in particular medico-biol is studied. features of heat exchange of an organism with the environment.

All systems depending on extent of energy exchange and the weight (substance) with the environment are divided into three groups: the isolated, closed and open circuits. The isolated systems do not obkhmeni-vatsya with the environment neither energy, nor weight; closed — exchange only energy, but not weight; open circuits exchange with the environment and energy, and weight. All biological systems belong to open circuits.

The processes proceeding in this or that system and changing it properties can be equilibrium or nonequilibrium. The thermodynamic equilibrium comes in the isolated systems and is characterized by constancy in time of the key thermodynamic parameters (in the elementary case pressure P, volume of V and temperature of T) in all points of system. From the point of view of kinetics (see Kinetics of biological processes), at a thermodynamic equilibrium in system of reversible chemical tests equality of speeds of direct and return processes for each of possible reactions separately is established.

First law T. it is actually equivalent to the law of energy conservation. He claims that the warmth absorbed by system from the environment (ΔQ) goes, on the one hand, for increase in energy of all types of the movement and interaction of components (molecules) in system, i.e. on increase in its internal energy (ΔU), and with another — on commission of work (ΔА) against external forces, and is expressed by a formula:

ΔQ = ΔU + ΔА.

For lack of external work warmth will be allocated or absorbed only due to changes of an internal energy of system. The law of constant heat summation is a consequence of it, on Krom heat effect of chemical reaction with a constant pressure does not depend on a way of transition from initial products to final, and is defined only by a difference of heat content (enthalpy) of end and initial products.

Pilot verification of the first law T. for biol. processes confirms its justice. So, consumption of 1 l of oxygen and allocation of 1 l of carbon dioxide gas at direct burning of nutritious products (e.g., glucose) or in the course of their oxidation in an organism is followed by allocation in both cases of identical amount of heat (~ 21,05 kJ, or 5,047 kcal). Coincidence of the corresponding heat effects and for other reactions says that ways of transformation of products in metabolic processes in an organism and chemical reactions out of living cell, from the point of view of cooperative heat effects, are equivalent. Full heat content, or an enthalpy (N), systems depends on the internal energy, pressure (R) and volume (V) which are key parameters of thermodynamic system.

Course of processes of life activity according to the first law T. means that live systems in itself are not independent sources of any new type of energy.

Nonequilibrium and irreversible processes arise in real systems, to-rye are characterized by various values of gradients (electric, concentration, temperature and others) in different parts of system (see the Gradient). Irreversible, or nonequilibrium, processes proceed spontaneously with final speeds in the direction of alignment of gradients and achievement of a thermodynamic equilibrium.

The second law of thermodynamics establishes the general criterion allowing to estimate an orientation of course of irreversible processes. According to this law, warmth cannot spontaneously pass from system with a smaller temperature to system with more high temperature. Changes of a condition of system are characterized by corresponding change of so-called entropy (ΔS), i.e. the total size of the given warmth absorbed by system (ΔQ/T). At change of a state change of entropy (ΔS) for reversible processes equals:


and for irreversible processes there is more this size:


For the isolated system:

ΔQ = 0 and ΔS >= 0.

Therefore, in the isolated system at course of equilibrium processes entropy remains to a constant, but increases at course of nonequilibrium processes, aiming at achievement of a maximum. Entropy thus is a measure of irreversibility of process, and its growth corresponds to disturbance of organization and increase in a chaotic state in system.

Change of entropy, reflecting degree of disorder of system, serves also as the characteristic of a condition of substance. Gas, liquid, crystal states represent different phases of substance. Transition of substance from one phase of an equilibrium state of the substance which is characterized by certain physical properties and thermodynamic parameters in another (phase change) is connected with a qualitative change. properties of substance. Transitions between different states in biopolymers at change of temperature are also phase changes as are followed by change of structural orderliness. In particular, proteinaceous a globule undergo rather sharp transitions like order — a disorder in rather small interval of temperatures.

On the basis of the second law it is possible to calculate change of free energy ΔF (F = U — TS) or thermodynamic potential ΔZ (Z = U + PV — TS), various biochemical processes used for the characteristic. So, if value of an equilibrium constant To reaction is known, then ΔZ it is possible to calculate by a formula:

ΔZ 0 = — RTlnK,

where ΔZ 0 — standard value ΔZ at t °=298 ° To (on a Kelvin scale); R — the gas constant equal of 8,314 J/mol • K. For reaction glucose-1-phosphate <-> glucose-6-phosphate R equally 17 at pH 7 and, therefore, ΔZ 0 makes approximately — 7020 J/mol (— 8,314 J/mol • To • 298K • 2,83 = — 7020 J/mol where is 2,83 ln17). Negative size ΔZ (t. e. ΔZ 0 < 0) shows that in reference conditions reaction of transphosphorylation proceeds spontaneously and leads to reduction of value of thermodynamic capacity of system.

In balanced state of value of free energy (F) and thermodynamic potential (Z) are minimum, and value of entropy (S) as much as possible.

In open circuits unlike isolated due to constant energy exchange and weight with the environment internal gradients can be supported. Instead of a thermodynamic equilibrium in them a stationary state can be established, at Krom parameters of system keep the constant values. However in open circuits it is reached at the expense of an equilibration of inflow and outflow of substance and energy in system.

Thermodynamic properties of open circuits are studied by the special section T. — the thermodynamics of irreversible processes, in a cut considers change of thermodynamic parameters in time. The original position of thermodynamics of irreversible processes is that infinitesimal change of entropy (dS) of an open circuit can happen or due to processes of exchange of system with the environment (d e S), or due to increase in entropy in the system (d i S) owing to internal irreversible processes (dS = d e S + d i S). In an open circuit in a stationary state

of dS/dT = d e S/dT +d i S/dT = 0

and, therefore, producing positive entropy (d i >S/dT 0) is compensated by its reduction during exchange with the environment (d e <S/dT 0).

So, e.g., formation of complicated molecules of carbohydrates in the course of photosynthesis due to inflow of solar energy can correspond to value d e <S/dT 0, and degradation and disintegration of products of photosynthesis — on the contrary to value (d i >S/dT 0. Growth and development of organisms are followed by complication of their organization and reduction of entropy because in other sites of the environment there are interfaced processes with formation of positive entropy.

In open circuits in the state close to a thermodynamic equilibrium, linear ratios of Onsager are carried out, according to the Crimea speeds of processes are directly proportional to sizes of the corresponding motive powers (gradients):

J 1 = L 11 X 1 + L 12 X 2

J 2 = L 21 X 1 + L 22 X 2

L 12 = L 21

where X1, X2 — motive powers; J1, J2 — speeds of processes; L11, L12, L21, L22 — constant coefficients.

So, for transport of ions through a membrane of a ratio consider communication between speeds of processes of transfer of substance (J1), electric charges (J2) and sizes of concentration gradients (X1) and potential difference (X2) on a membrane.

Rate of production of entropy (d i S/dT) in system can be presented in the form of a ratio:

d i S/dT = J1X1 + >J2X2 0.

For one of processes (interfaced) can be that J2X2 is less than zero, and for another (interfacing) — J1X1 is more than zero, however in the sum of J1X1 + J2X2 is always more than zero. Oxidation of glucose in the course of breath, and interfaced by an example to it — synthesis adenosine triphosphoric to - you going against a gradient of thermodynamic potential (with its increase) due to the released energy can be an example of the interfacing process at oxidation of glucose.

In a stationary state in open circuits, for to-rykh Onsager's ratios, according to the situation stated and proved by I. R. Prigozhin (1960), the speed of formation of positive entropy of d are fair i >S/dT 0 accepts the minimum positive value (I. Prigozhin's theorem). The measurements of speed of heat production taken by A. I. Zotin (1973) on different biol. objects, really showed reduction of size d i S/dT, since the first stages of development of an organism that generally corresponds to I. R. Prigozhin's theorem.

For thermodynamic systems, enough far from an equilibrium state, in particular for the system which is in the periodic oscillatory stationary duty, I. R. Prigozhin's theorem is not carried out. It is possible to find in such systems only stability conditions of stationary states, but not criteria of the direction of the movement of system. Disturbance of the established stability conditions leads to withdrawal of system from a steady stationary point. In open circuits where various chemical components participate in chemical reactions, and also diffuse, moving in space, the autokolebatelny mode can be set (see. Biological system, an autokolebaniye in biological system ). In such structure concentration gradients are supported in time and space due to exchange with Wednesday.

Use of approaches and methods T. in combination with methods of kinetics biol. processes gives the chance to define efficiency and extent of interface of power turning into an organism (see. Bio-energetics ), to understand the general reasons of establishment of the nonequilibrium modes of the chemical reactions which are the cornerstone of processes of growth and development of organisms, etc.

Bibliography: 3 about t and A. I N. Thermodynamic approach to problems of development, growth and aging, M., 1974, bibliogr.; Ivens And. and Skeylakr. Mechanics and thermodynamics of biological membranes, the lane with English, M., 1982; Marshell E. Biophysical chemistry, the lane with English, t. 1, p.1, page 9, M., 1981; N and to about-lisg. and Prigozhin I. Self-organization in nonequilibrium systems, the lane with English, M., 1979; Prigozhin I. Introduction to thermodynamics of irreversible processes, the lane with English, M., 1960, bibliogr.; Rubin A. B. Thermodynamics of biological processes, M., 1976; Chiang R. Physical chemistry with annexes to biological systems, the lane with English page 267, M., 1980.

A. B. Rubin.