4-1

Chapter 4: OutlineThermodynamics

First Law

Second Law

Free Energy

Standard free energy changes

Coupled reactions

Hydrophobic effect (revisited)

Role of ATP

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BioenergeticsEach formation or breakdown of a

biomolelcule involves an associated energy change. Thermodynamics is the field of chemistry that studies these energy changes.

The goal of thermodynamics is to predict whether a reaction will occur spontaneously which, in a chemical sense, means it will continue without energy input once started.

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4.1 ThermodynamicsThe heat and energy transformations

studied by thermodynamics take place in a system (defined by the investigator) connected to the surroundings (the rest of the universe).

Closed system: energy exchanged between system and surroundings.

Open system-matter and energy exchanged between system and surroundings.

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First Law-1Energy is neither created nor destroyed.

or E = q+w

E is the change in the internal energy and is a state function, i. e. independent of path.

q is heat and is not a state function.

w is work and is not a state function.

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First Law-2Biochemical systems function at

constant pressure, volume, and temperature.

H(enthalpy) = E + PV or H = E

and H = q (heat flow)

The change in enthalpy for a reaction is calculated using the equation

Hreactants = Hproducts – Hreactants

-H is exothermic +H is endothermic

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First Law-3Given the equation and the Hf values, calculate

the H for the reaction.

6 CO2 + 6 H2O C6H12O6 + 6 O2

kJ/mol kJ/mol

C6H12O6 -304.7 CO2 -94.0

O2 0 H2O -68.4

[1*-304.7+6*0]-[6*-94.0+6*-68.4)]=+670 kJ Prod - Reactants

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Second LawWith a spontaneous reaction, the

entropy of the universe increases.

Suniv = Ssystem + ssurroundings

In irreversible processes, entropy is a driving force.

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4.2 Gibb’s Free EnergyGibb’s free energy change (G) is the

most useful thermodynamic function for predicting reaction spontaneity.

The two other thermodynamic quantities that contribute to the value for G:

H=enthalpy change (energy change measured at constant pressure)

S=entropy change (related to the state of disorder in a system)

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Gibb’s Free Energy: 2The three thermodynamic quantities are

related by the following equation:

G = H -TSsysFor biochemists, G is usually measured

at 25 oC, one atm for a gas, and at a concentration of 1 M for solutes except hydronium ion which is at pH 7.

These conditions specify a standard G represented as Go’ .

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Gibb’s Free Energy: 3In a spontaneous reaction:

free energy decreases, G is negative

energy is released by the reaction

reaction is said to be exergonic

In a nonspontaneous reaction:

free energy increases, G is positive

energy is absorbed by the reaction

reaction is said to be endergonic

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Examples, G values(From standard tables)

Go’,kJ/mol

(kcal/mol)

Exergonic reaction:

ATP + H2O ADP + Pi -30.5 (-

7.3)

Endergonic reaction:

glucose-6-phosphate to

fructose-6-phosphate + 1.7 (+0.4)

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Go and Keq

For the reaction

aA + bB = cC + dD

G = Go + RT ln [C]c[D]d

[A]a[B]b

At equilibrium, G = 0

Go = - RT ln Keq

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Coupled ReactionsFrequently in biochemistry two reactions

are “coupled” or run as a pair. One reaction is endergonic but the second reaction is exergonic. The sum of the reactions (and the G changes) is overall exergonic and consequently the reaction pair is overall spontaneous.

This is shown on the next slide.

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Coupled Reactions: 2

Go’(kcal/mol)1. glucose-6-P fructose-6-P + 0.42. fructose-6-P + ATP

fructose-1,6-bisP + ADP - 3.43. glucose-6-P + ATP

fructose-1,6-bisP + ADP - 3.0The overall reaction 3 (sum of 1+2) is

exergonic. Sum of Go’ 1 + Go’ 2 is Go’ 3 or –3.0 kcal/mol. Overall reaction is spontaneous

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4.3 Bioenergetics and ATPHydrolysis of adenosine triphosphate

(ATP) provides the free energy to drive most endergonic reactions.

OCH2

HOH

H

OH

HH

N N

N N

NH2

OP

O

O

OP

O

O

OP

O

O

O

ADP + Pi = -7.3 kcal/mol

AMP + PPi = -7.7 kcal/mol

PPi 2 Pi = -8 kcal/mol

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ATPDrives several processes:

Biosynthesis of biomolecules

Active transport across membranes

Mechanical work (e. g. muscle contraction)

Can carry phosphoryl groups from higer-energy compounds to lower-energy compounds.

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kJ/mol kcal/mol

Glucose-6-P -13.8 -3.3

Fructose-6-P -15.9 -3.8

ATPAMP + PPi -32.3 -7.7

ATPADP + Pi -30.5 -7.3

P-creatine -43.1 -10.3

Glycerate-1,3-bP -40.4 -11.8

Phosphoenolpyruvate -61.9 -14.8

Go’ for ROPO32- + H2O