Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 438	Introductory Biochemistry	Spring 2007
Lecture Notes: 19 March	© R. Paselk 2006
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Thermodynamics in Metabolism

Remember, the cool thing about thermo is that it is pathway independent - we can tell how much energy is available in an M&M by burning it in pure oxygen in a stainless steel container and tell you how far you can run on that M&M!

The tragedy of thermo, the other side of the coin, is that it tells nothing about the details - thermo gives us no idea about how the energy is used, or what steps are involved in its loss.

Remember also that for chemists and biologists the thermodynamic term generally of most interest is the Free Energy for a reaction, that is the energy available to do work.

• The free energy is defined as: G = G_products- G_reactants = H - T S.
• When the free energy is negative we say the reaction is spontaneous, which simply means the reactants are favored in the reaction equation as written.
• Note when a reaction is at equilibrium then the G is zero.

Since free energy depends on conditions, chemists tabulate free energies under Standard Conditions, (G°): 298 K, 1 atm., with all concentrations at 1 M.

For biological systems we define a slightly different standard free energy with [H⁺]= 10^-7 M (pH=7), G° '.

For non-standard conditions we can find the free energy of a reaction using: G = G° ' + RT lnQ. For the special case of equilibrium, the free energy is zero, so G° ' = -RT lnK', G° ' = -5700 log K (in joules). Thus free energy is related to the equilibrium constant, K.

Pathway Diagrams

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Last modified 19 March 2007 (late start)