# 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-TS

T= Absolute temperature in kelvin (K= 273+ Co)

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