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+ 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