BCH/PPA 503 -- Plant Biochemistry
Lecture Five

Energetic Considerations






Energy is the capacity to do work and overcome resistance.  Molecules tend to seek the least organized, least energetic state or highest enthalpy, H.

Equilibrium in a biological system is death - i.e. organisms require a regular input of energy.  Energy in biological systems is supplied by certain forms of chemical energy (see review) such as highly reduced molecules or high-energy structures.  Chemical energy has traditionally been measured in calories, abbreviated cal, but the SI unit, the joule, is now recommended.

Gibbs free energy, G, (named for J. Willard Gibbs, one of the founders of thermodynamics) = H - TS where T is the temperature (°K) and S is the entropy.

DG°´ = the change in standard free energy where the pH is maintained at 7.

DG° = -RTlnKeq

with a strongly -DG° at equilibrium there will be a higher molar concentration of C + D than A + B.

For the following reaction series the change in free energy is given for each step

The energetics of would favor glucose + Pi formation over G-6-P.

The reaction can be coupled to an exergonic reaction

Kinase has two substrates glucose and ATP and binds to both.  The Gibbs Free Energy for the overall reaction is DG° = -4 (+3.3 - 7.3).  Therefore this reaction would tend to go toward G-6-P and ADP.

Another example:

DG° = 7.7 - 7.6 - 8.0 = -7.9

All materials © 1998, 1999, 2000, Dr. David Hildebrand or Dr. Bob Houtz, unless otherwise noted.
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This page was last modified January 15, 2000.