Introduction

The foregoing chapters have focused on practical and applied aspects of thermodynamics and statistical mechanics. These subjects provide ways of thinking about energy transformation, methods for determining the direction of spontaneous change, the magnitude of thermodynamic quantities when a system passes from one state to another, and the molecular origin of change. Useful as they are, however, thermodynamics and statistical mechanics do not tell us everything we'd like to know: they give no direct indication of the rate at which a chemical change will occur nor how the rate of change will vary with conditions.

The present chapter seeks to fill a few of the gaps remaining from a strictly thermodynamic treatment of biochemical change. It might seem counter-intuitive for this chapter to appear next to last instead of first, as one of the most basic aspects of our experience of the world is constant change. Plants grow, go to seed, and die, while animals move, eat, reproduce, and die. And the molecules of which bacteria, plants, and animals are made are always moving. But, the title of this book is Biological Thermodynamics, not Biological Kinetics!

As we have seen, analysis of free energy changes provides a way of answering such questions as ‘Why is most of the energy of the glucose molecule obtained in the citric acid cycle and not in glycolysis?’