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

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 →
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

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