## Free Energy Is The Useful Energy In A System

Gibbs change in free energy (AG) is that portion of the total energy change in a system that is available for doing work—ie, the useful energy, also known as the chemical potential.

Biologic Systems Conform to the General Laws of Thermodynamics

The first law of thermodynamics states that the total energy of a system, including its surroundings, remains constant. It implies that within the total system, energy is neither lost nor gained during any change. However, energy may be transferred from one part of the system to another or may be transformed into another form of energy. In living systems, chemical energy may be transformed into heat or into electrical, radiant, or mechanical energy.

The second law of thermodynamics states that the total entropy of a system must increase if a process is to occur spontaneously. Entropy is the extent of disorder or randomness of the system and becomes maximum as equilibrium is approached. Under conditions of constant temperature and pressure, the relationship between the free energy change (AG) of a reacting system and the change in entropy (AS) is expressed by the following equation, which combines the two laws of thermodynamics:

where AH is the change in enthalpy (heat) and T is the absolute temperature.

In biochemical reactions, because AH is approximately equal to AE, the total change in internal energy of the reaction, the above relationship may be expressed in the following way:

If AG is negative, the reaction proceeds spontaneously with loss of free energy; ie, it is exergonic. If, in addition, AG is of great magnitude, the reaction goes virtually to completion and is essentially irreversible. On the other hand, if AG is positive, the reaction proceeds only if free energy can be gained; ie, it is ender-gonic. If, in addition, the magnitude of AG is great, the system is stable, with little or no tendency for a reaction to occur. If AG is zero, the system is at equilibrium and no net change takes place.

When the reactants are present in concentrations of 1.0 mol/L, AG0 is the standard free energy change. For biochemical reactions, a standard state is defined as having a pH of 7.0. The standard free energy change at this standard state is denoted by AG0 .

The standard free energy change can be calculated from the equilibrium constant Keq.

AG0' = -RT ln K', eq where R is the gas constant and T is the absolute temperature (Chapter 8). It is important to note that the actual AG may be larger or smaller than AG0 depending on the concentrations of the various reactants, including the solvent, various ions, and proteins.

In a biochemical system, an enzyme only speeds up the attainment of equilibrium; it never alters the final concentrations of the reactants at equilibrium.

ENDERGONIC PROCESSES PROCEED BY COUPLING TO EXERGONIC PROCESSES

The vital processes—eg, synthetic reactions, muscular contraction, nerve impulse conduction, and active transport—obtain energy by chemical linkage, or coupling, to oxidative reactions. In its simplest form, this type of coupling may be represented as shown in Figure 10—1. The conversion of metabolite A to metabolite B

occurs with release of free energy. It is coupled to another reaction, in which free energy is required to convert metabolite C to metabolite D. The terms exer-gonic and endergonic rather than the normal chemical terms "exothermic" and "endothermic" are used to indicate that a process is accompanied by loss or gain, respectively, of free energy in any form, not necessarily as heat. In practice, an endergonic process cannot exist independently but must be a component of a coupled ex-ergonic-endergonic system where the overall net change is exergonic. The exergonic reactions are termed catab-olism (generally, the breakdown or oxidation of fuel molecules), whereas the synthetic reactions that build up substances are termed anabolism. The combined catabolic and anabolic processes constitute metabolism.

If the reaction shown in Figure 10-1 is to go from left to right, then the overall process must be accompanied by loss of free energy as heat. One possible mechanism of coupling could be envisaged if a common obligatory intermediate (I) took part in both reactions, ie,

Some exergonic and endergonic reactions in biologic systems are coupled in this way. This type of system has a built-in mechanism for biologic control of the rate of oxidative processes since the common obligatory intermediate allows the rate of utilization of the product of the synthetic path (D) to determine by mass action the rate at which A is oxidized. Indeed, these relationships supply a basis for the concept of respiratory control, the process that prevents an organism from burning out of control. An extension of the coupling concept is provided by dehydrogenation reactions, which are coupled to hydrogenations by an intermediate carrier (Figure 10-2).

An alternative method of coupling an exergonic to an endergonic process is to synthesize a compound of high-energy potential in the exergonic reaction and to incorporate this new compound into the endergonic reaction, thus effecting a transference of free energy from the exergonic to the endergonic pathway (Figure 10-3). The biologic advantage of this mechanism is that the compound of high potential energy, —©, unlike I

Figure 10-2. Coupling of dehydrogenation and hydrogenation reactions by an intermediate carrier.

Figure 10-3. Transfer of free energy from an exer-gonic to an endergonic reaction via a high-energy intermediate compound (-©).

in the previous system, need not be structurally related to A, B, C, or D, allowing © to serve as a transducer of energy from a wide range of exergonic reactions to an equally wide range of endergonic reactions or processes, such as biosyntheses, muscular contraction, nervous excitation, and active transport. In the living cell, the principal high-energy intermediate or carrier compound (designated ~© in Figure 10-3) is adenosine triphosphate (ATP).

## Diabetes 2

Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...

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