Biochemistry - The Chemical Reactions of Living Cells, Volume 1 - D. Metzler 1980

Energetics of Biochemical Reactions
Thermodynamics
System and Its Surroundings

Thermodynamics [1–5] deals with the quantitative description of energy and heat changes within a system, particularly those associated with the attainment of chemical equilibrium. By knowing the changes in thermodynamic quantities such as $H$ and $S$, one can predict whether a reaction will proceed under given conditions and describe the state of equilibrium. Furthermore, analysis of thermodynamic values can sometimes shed light on The Nature of the forces holding molecules together.

The most significant limitation in applying the principles of classical thermodynamics stems from the fact that this science invariably deals with equilibrium states and provides no insight into reaction kinetics. This has sometimes led to the erroneous Conclusion that thermodynamics is irrelevant to biochemistry. That is certainly not the case; understanding the energetic relationships of biochemical reactions is crucial. At the same time, it is equally dangerous to fall into the opposite extreme and assume that thermodynamic calculations performed for an equilibrium state can be directly extrapolated to the steady-state conditions characteristic of living Cells (Chapter 1).

When applying thermodynamic equations, it is essential to clearly define the system under consideration. Such a system might be a solution in a flask placed in a thermostated bath. The flask, the bath, and everything else constitute the surroundings. In thermodynamics, the system combined with its surroundings is sometimes referred to as the universe.