Introduction: Why are we interested in thermodynamics?

Reasons: A. Requires no knowledge of mechanism.

Example: When we discuss metabolism, we will want to relate redox processes to ATP formation: NADH --> NAD+ can drive the reaction: ADP + Pi --> ATP

B. Determines whether a reaction is favorable or not.

Example: We will want to understand why enzymes don't affect the position of a chemical equilibrium and how they speed up reactions.

Coupling of a favorable reaction with unfavorable reaction can drive a chemical system in a biologically desirable direction

How energy and work are related

A review of the laws of thermodynamics:

1. 1st law = chemists don't do physics (no E = mc²)
2. 2nd law = left unattended, things get worse
3. 3rd law = zero is zero 0 K = perfect crystal = zero entropy

A thermodynamic system has a special meaning: the part of the universe under consideration.

Can be closed, as is most usual in chemical thermodynamics. No energy or matter is entering or leaving the system.

In the world of biology the system is always open (i.e. matter and energy are passing into and out of a body)

E = energy (internal) E = Q - W

Q = heat absorbed
W = work done by system

In biochemistry we are interested in having the system do useful work

Internal energy is related to another thermodynamic function called enthalpy by the following relationship

H = E + PV

H = enthalpy
PV = pressure-volume work

In biochemistry, we can equate E and H (both are state {vs. path} functions), if there is no PV work (true to a large extent in most biochemical systems)

Entropy

First Law: For bookkeeping --> energy is neither created or destroyed in chemical reactions.

Second Law: Direction of S = the entropy of an isolated system will tend to increase to a maximum value.

Systems of molecules have a natural tendency to randomization

Degree of randomness is measured as entropy = S = k lnW, where k = Boltzmann constant (or R/n)

Can be converted to S = R ln (Vf/Vi) doubling of volume results in twice as many states.

Third Law: S is zero in a perfect crystal at zero degrees Kelvin, and this law defines this particular temperature scale.

Gibbs free energy and entropy

Now we are ready to combine both energy (enthalpy) and entropy in a complete thermodynamic treatment.

G = H - TS

G = Gibbs free energy; H = enthalpy;
S = entropy.

At equilibrium there is no change in free energy of system (a definition)

The criterion for a favorable process in a non isolated system (at constant temperature and pressure) is that G be negative in value.

A process that has a negative value of free energy is termed an exergonic process (will proceed spontaneously, or is favorable)

A process that has a positive value of free energy is termed an endergonic process (will not proceed spontaneously, or is unfavorable)

Free energy and concentration

G = N RT ln ([A2]/[A1]) can be used to express the reverse of entropy (i. e. pumping or concentrating chemicals)