The Concept of free Energy
Gibbs free energy is a thermodynamic property that was defined in 1876 by Josiah Willard Gibbs to predict whether a process will occur spontaneously at constant temperature and pressure. Gibbs free energy G is defined as,
G = H- TS
Where H, T and S are the enthalpy, temperature and entropy.
Changes in the Gibbs free energy G correspond to changes in free energy for processes at constant temperature and pressure. The change in Gibbs free energy change is the maximum non-expansion work obtainable under these conditions. G is negative for spontaneous processes, Positive for non-spontaneous processes and Zero for processes at equilibrium. Change in a system which is available for doing work it is the useful energy.
Some exergonic and endogonic reactions in biologic systems are coupled in this way. This type of systems has a built-in mechanisms of the rate control at which oxidative processes are allowed to occur since since the existence of a common obligatory intermediate or both exogonic and endogonic reactions 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 hydrogenation reactions, which are coupled to hydrogenation by an intermediate carrier.
Coupling of dehydrogenation and hydrogenation reactions by and intermediate carrier molecule.
An alternative method of coupling of an exergonic to an endergonic process is to synthesize a compound of high - energy potential in the exergonic reaction, thus affecting transference of energy from the exergonic to the endogonic pathway.
In this image E is a compound of high potential energy and E is corresponding compound of low potential energy . The biologic advantage of this mechanism is that E, unlike in the previous system, need not to be structurally related to A, B, C, Or D. This would allow E to serve as a transducer of energy from a wide range of endergonic reactions or processes, as shown in next image.
In the living cell, the principal high-energy intermediate of carrier compound is adenosine troposphere or ATP.
Transference of free energy from and exergonic to an endogonic reaction through the formation of a high-energy intermediate compound.
In this image E is a compound of high potential energy and E is corresponding compound of low potential energy . The biologic advantage of this mechanism is that E, unlike in the previous system, need not to be structurally related to A, B, C, Or D. This would allow E to serve as a transducer of energy from a wide range of endergonic reactions or processes, as shown in next image.
In the living cell, the principal high-energy intermediate of carrier compound is adenosine troposphere or ATP.
Transaction of energy through a common high energy compound to energy-requiring biological process.
The Laws of thermodynamics in Biological systems
The first law of thermodynamic states that "the total energy in a system, plus its surroundings remains constant." This is also the law of conservation of energy it implies that within a total system, energy is neither lost nor gained during any change. However, within the total system, energy may be transferred in to another form in to another form of energy. For example, chemical energy may be transformed into heat, electrical energy or mechanical energy.The second law of thermodynamics states, " the total entropy of a system must be increase if a process is to occur spontaneously". Entropy represents the extent of disorder or randomness of the system and becomes maximum in a system as it approaches true equilibrium. Under the conditions of constant temperature and pressure, the relationship between the free energy (∆G) of a reacting system and the change in the entropy ( ∆S) is given by the following equations which combines the two laws of thermodynamics.
Where delta H is the change in enthalpy and T is the absolute temperature. Under the condition of biochemical reactions, because delta H is approximately equal to delta E, the total change in the internal energy of the reaction, the above relationship may be expressed in the following way.
If delta G is negative in sign, the reaction proceeds spontaneously, it is exogonic. If in addition, delta G id of great magnitude, the reaction goes virtually to completion and is essentially irreversible. On the other hands if (∆G) is positive, the reaction proceeds only if free energy can gain, it is endogonic . If in addition, the magnitude of (∆G) is great, the system is stable with little or no tendency for a reaction to occur. If (∆G) is zero , the system is at equilibrium and no net change takes place.
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