Plant Physiology - Musiyenko, M. M. 2001

Chemical and molecular composition, structure, and functions of the plant cell
The cell as an open system. Dynamic equilibrium. Basic concepts of thermodynamics

The plant Cell is a thermodynamically open system in which energy conversion and matter exchange with the environment take place continuously. When the input and loss of energy and matter are balanced, The Cell is considered to be in a state of dynamic equilibrium.

METABOLISM consists of chemical and physical reactions and is therefore subject to the Laws of Thermodynamics. These laws apply equally to enzyme-catalyzed reactions and all other reactions, which is what determines The Significance of thermodynamic laws for any physiological process.

The First Law of thermodynamics, the law of conservation (or conversion) of energy, states quite simply: “Energy can be converted from one form to another, but cannot be created or destroyed.” In relation to the plant cell, this means that during energy exchange and interconversion, the total energy of the reaction products plus The energy released in these reactions is always equal to the initial energy of the reactants.

In an isolated thermodynamic system, the internal energy reserve (U) is a constant value:

U= const;

Internal energy is a state function of the system that depends on thermodynamic parameters:

U = f (m, р, V, Т), where m is mass, р is pressure, V is volume, and Т is Temperature.

In the SI system, energy is measured in joules (J). 1 J= 0,239 cal = 6,25 х 10¹⁸eV, 1 kcal = 4,19 kJ.

At constant normal pressure, to characterize Chemical Reactions within the cell instead of internal energy (U), a new state function describing the thermodynamic system can be introduced. The work done by A change in volume V can be described by the equation: W_p = р Δ\/. The change in internal energy (ΔО) upon transition from one state to another is determined by The amount of heat transferred (Q) or the magnitude of work done (W): ΔU = U₂ - U₁ = Q - W.

In biological processes that occur under constant pressure but with a changing volume, the function of enthalpy (from Greek enthalpo — to warm) is introduced into the process equation instead of the internal energy function. The first law of thermodynamics can now be expressed as:

Q = ΔU + p ΔV = Δ(U+ pV) = ΔН

The new thermodynamic state function, enthalpy (Н), is equal to the sum of the internal energy and the product of volume and pressure. Enthalpy is the heat content of the system; the change in enthalpy corresponds to the amount of absorbed or released energy and can be determined using a calorimeter. Both internal energy U and enthalpy Н are state Functions of a thermodynamic system.

The Second Law of thermodynamics determines the direction of all energy exchange processes. According to the second law, in all energy exchanges or energetic transformations, unless energy losses or external energy inputs into the system occur, the potential energy at The final stage will always be less than the potential energy of the initial state. If a decrease in potential energy occurs during reactions, energy is released externally. Such reactions are called exergonic. According to the second law, only exergonic reactions proceed spontaneously. At the same time, endergonic reactions require an energy input—the greater the energy difference between the final product and the starting reactants, the larger the input required. The Nature of a given reaction is judged by the change in the heat content of the system, ΔН (where Δ denotes change and Н denotes heat content). Thermal energy is “disordered energy.” Internal disorder and the dissipation of energy are characterized by Entropy (from Greek en — in, inside, and trope — turning, change, transformation) — S. This is a measure of disorder, i.e., the disorganization of the system. Changes in the thermal energy content and entropy of the system lead to an overall change in its energetic state. This overall change in the energetic state is called the change in Free energy and is denoted as ΔG in honor of the American physicist J. W. Gibbs, who made a significant contribution to the foundation of thermodynamics. This quantity characterizes the driving force of a reaction.

Reactions with a negative Gibbs free energy change, ΔG, are exergonic and can proceed spontaneously, whereas reactions with a positive ΔG are endergonic. The greater the entropy change, the greater the magnitude of the Gibbs free energy change ΔG. Therefore, another formulation of the second law of thermodynamics, or the law of entropy, is possible: “All natural processes are exergonic.” Living systems continuously expend a significant amount of energy to maintain a state far from equilibrium. Upon reaching equilibrium, chemical reactions in the cell would stop, and no work could be performed. In a state of equilibrium, the cell would soon perish.