Concept of free energy and its determination

Concept of free energy and its determination

Contents

•       Bioenergetics

•       Concept of free energy

•        Determination of free energy

Objective

At the end of this lecture, student will be able to

•       Explain concept of free energy and its determination

Bioenergetics

•       Bioenergetics or biochemical thermodynamics deals with the study of energy changes (transfer and utilization) in biochemical reactions

•       Reactions are broadly classified as exergonic (energy releasing) and endergonic (energy consuming)

•        It is concerned with the initial and final states of energy component of the reactants and not the mechanism of chemical reaction

•       Free energy (∆G): Energy available to do work

•       Reaction can occur spontaneously if they are accompanied by decrease in free energy

•       Every chemical substance has a certain amount of energy built into it which is the energy of the chemical bonds holding the atoms together

•       Synthetic reactions-energy utilized & breakdown reactions-energy released

•       Exergonic reactions represents catabolic reactions

•       Endergonic reactions represents anabolic reactions

•       Enthalpy(∆H):is a measure of overall amount of energy in the system or sum of internal energy of a system

•       It is positive in endothermic reactions and negative in exothermic reactions

•       Entropy (∆S): represents a change in randomness or disorder of reactants and products

•       It is the measure of disorder or randomness of a system

•       Disordered system has high entropy and ordered system has low energy

•       Entropy attains maximum as the reaction approaches equilibrium

•       First law of thermodynamics: The total energy of a system plus its surroundings remains constant

•       Second law of thermodynamics: The total entropy of a system must increase if a process is to occur simultaneously

•       Under constant temperature and pressure, the relationship between free energy change ∆G and change in entropy ∆S is given by combining the two laws of thermodynamics

•       The relationship between the change of ∆G, ∆H & ∆S is expressed as                                    

∆G= ∆H-T∆S

                            T= Absolute temperature in kelvin (K= 273+ C^o)

                       ∆H = change in enthalpy or heat

•       If ∆G   is negative in sign, indicate loss of free energy (exergonic) and positive sign (endergonic reactions)

Negative and positive ∆G

•       ∆ G is represented by a negative sign, there is a loss of free energy & reaction is said to be exergonic and proceeds spontaneously

•       ∆ G is represented by a positive sign, indicates that energy must be supplied to the reactants & reaction cannot proceed spontaneously and is endergonic in character

•       Exergonic reactions are termed catabolism (breakdown of molecules) and endergonic reactions as anabolism (synthetic reactions)

•       Redox potential: In oxidation and reduction reactions, free energy exchange is proportionate to the tendency of reactants to donate or accept electrons

•       Hydrolysis of ATP is a classical example of exergonic reaction

ATP + H₂O → ADP + Pi (∆ G = -7.3 Cal/mol)

•       The reversal of the reaction (ADP + Pi → ATP) is endergonic and occurs only when there is a supply of energy of at least 7.3 Cal/mol (∆G is positive)

•       The free energy change becomes zero (∆G = 0) when a reaction is at equilibrium

Summary

•       Bioenergetics deals with the study of energy changes in biochemical reactions

•       Reactions are broadly classified as exergonic and endergonic

•       The substances which possess sufficient free energy to liberate  at least 7Cal/mol at pH 7 is called high energy molecules

•       Free energy (∆G): energy available to do work

•       Enthalpy (∆H), measure of the change in heat content of the reactant,  compared to products

•       Entropy (∆S), represents a change in randomness or disorder of reactions and products