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 Mg 2+

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 molecule F= Faraday constant E= reduction potential in Volts E°= 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 reaction Acetaldehyde + NADH + H + --  ethanol + NAD + The half reactions are:- Acetaldehyde + 2H + + 2e  ethanol E°’ = V NAD + + 2H + + 2e  NADH + H + E°’ = V CALCULATION: By convention, ∆E°’ is expressed as E°’ of electron acceptor - E°’ of electron donor. In example, acetaldehyde is accepting electrons from NADH ∆E°’ = V - ( V) = 0.123V, and n is 2 Therefore ∆G°’ = -nF∆E°’ = -2 (96.5 KJ/V/mol) (0.123 V) = 23.7 KJ/mol