Chapter 3. Thermodynamic Principles

• Enthalpy and entropy• Free energy• Chemical equilibria

Life obeys the laws of thermodynamics

• Living organisms are open systems• Living system maintain a steady state• Enzymes catalyze biochemical reactions

Enthalpy (H)

• ΔH = heat absorbed or generated at constant pressure

• Spontaneous reactions usually occur without input of heat (ΔH < 0)

Entropy (S)

Second law of thermodynamicsΔSsystem + ΔSsurrounding= ΔSuniverse >0

Spontaneous processes occurs in directions that increase the overall disorder of universe

Randomness

Spontaneous processes

Gibbs free energyG = H – TS

ΔG = ΔH - T ΔS

• ΔG < 0 exergonic (spontaneous)• ΔG > 0 endergonic (non-spontaneous)

• ΔG = 0 equilibrium

Reaction spontaneity

>0-+

<0if T>ΔH/ΔS

++

<0 if T<ΔH/ΔS

--

<0+-

ΔG=ΔH-TΔSΔSΔH

Chemical Equilibria

aA + bB cC + dD

ΔG = ΔG° + RT ln[C]c[D]d

[A]a[B]b

When ΔG=0 (at equilibrium),

ΔG° = -RTlnKeq

Keq = [C]c [D]d

[A]a [B]beq eq

eqeq = e-ΔG°/RT

Variation of Keq with ΔGo at 25oC

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Standard state conventionsin biochemistry

• Temperature = 25°C (298K)• Pressure = 1 atm• activity of pure water is 1 (despite [H2O]=55.5M)• H+ activity at pH 7.0 = 1 (vs. pH 0 the physical

chemical standard state)

cf. For the dilute solution, activity ≈ concentration

Coupled Reactions

(1) A + B C + D ΔG1

(2) D + E F + G ΔG2

(1+2) A + B + E C+ F + G ΔG3

When ΔG3=ΔG1+ΔG2 < 0 : Spontaneous

ATP

ADP +PiATP + H2O ΔG= -30.5 kJ/mol

ATP + H2O AMP + PPi ΔG= -32.2 kJ/mol

Energy release from phosphate hydrolysis

Examples of the coupled spontaneous reactions