FREE ENERGY CHANGE(∆G)

Living cells are “Open Systems”. They exchange energy with environment

The reactions inside cells continuously try to attain equilibrium

Energy that cells can use is called Free Energy ‘G’.

It cant be measured, but tells direction of reaction, exact equilibrium position

Free energy can be acquired from environment

eg Heterotropic cells – From Nutrients

Photosynthetic cells – From Sunlight

This is converted to ATP & other energy-rich compounds

Tendency to attain equilibrium is given as Free energy change (∆G)

∆G = ∆H - T∆S

STANDARD FREE ENERGY (∆G°)

• Free Energy change inside cells at constant Temp & Pressure• It’s directly related to equilibrium constant ∆G°= - RT ln Keq• Where Keq = [C] [D]

---------- [A] [B]

i,e the conc of reactants and products at equilibrium for the reaction

[A] + [B] - [C] + [D].

• The units of ∆G° is kcal / mol

• ∆G°= - RT ln Keq

DIRECTION OF REACTIONS & COUPLING

• When ∆G of a reaction is negative, the reaction is exergonic and tends to go toward completion (also called CATABOLIC reactions)

When ∆G is positive, the reaction is endergonic and tends to go in the reverse direction ( also called ANABOLIC reactions)

• So when 2 reactions are coupled, the outcome is the sum of the two ∆G’s

FREE ENERGY OF ACTIVATION ∆G‡ & ENZYMES

• Enzymes help in speeding up (increasing rate) a reaction.• Most exergonic rxns proceed at useful rates only because enzymes help• Enzymes stabilize the transition state & thus reduce the activation energy• This increases reaction rate by many orders of magnitude.

Fig : Free energy-reaction coordinate diagram

ATP

• Helps in transfer of free energy from Exergonic to Endergonic reactions by coupling them

• Made of ADENINE, RIBOSE & 3 PHOSPHATE groups• Inside cells, it exists in complex with Mg2+

ATP & HIGH ENERGY COMPOUNDS

• ATP acts as a “Energy currency” inside cells • The ∆G°` or standard free energy of hydrolysis of ATP is intermediate & divides

the list into 2 gps. • High-Energy phosphates have ∆G°` higher than ATP (hence ADP can use it to form

ATP) • Low-energy phosphates have ∆G°` lower than ATP (hence ATP acts as a donor to

form these cpds)

ATP CYCLE

• It shows the processes that synthesise and utilize ATP inside cells

High energy phosphate can also be stored in muscle, as creatine phosphate. This is called PHOSPHAGENS

OXIDATION – REDUCTION REACTIONS

• These reactions involve “Electron Transfer” (eg., As in ETC)• Electron Donor (which is oxidised) & Electron Acceptor (which is reduced) are

involved. They constitute a “REDOX PAIR”. • The reaction is treated as 2 half reactions.• It can be measured & shown as redox potential (E°’) by the Nernst equation

E = E° + RT ln [electron acceptor] ---- ------------------------ nF [electron donor]

where n= number of electrons transferred per moleculeF= Faraday constantE= reduction potential in VoltsE°= standard reduction potential

Standard reduction potential (E°’) &

Free Energy change (∆G)

• Standard reduction potential (E°’) can be used to calculate ∆G by the equation∆G°’ = - nF ∆E°’

Example: consider reactionAcetaldehyde + NADH + H+ -- ethanol + NAD+

The half reactions are:-Acetaldehyde + 2H+ + 2e- ---- ethanol

E°’ = -0.197VNAD+ + 2H+ + 2e- ---- NADH + H+

E°’ = -0.320V

CALCULATION:By convention, ∆E°’ is expressed as E°’ of electron acceptor - E°’ of electron donor. In

example, acetaldehyde is accepting electrons from NADH ∆E°’ = -0.197 V - (-0.320 V) = 0.123V, and n is 2

Therefore ∆G°’ = -nF∆E°’ = -2 (96.5 KJ/V/mol) (0.123 V) = 23.7 KJ/mol