Thermodynamics and Biochemical Equilibria

It is important to realize that while thermodynamic information will tell us whether or not a reaction can take place it says nothing about the rate of the reaction. It will not even say whether a reaction will proceed at all within a given period of time. This has led to the occasional assertion that thermodynamics is not relevant to biochemistry. This is certainly not true it is important to understand energy relationships in biochemical reactions. At the same time, one should avoid the trap of assuming that thermodynamic calculations appropriate for equilibrium situations can always be applied directly to the steady state found in a living cell. 

Thermodynamics is an exact science and its laws deal with measurable quantities whose values are determined only by the state of the system under consideration. For example, the system might be the solution in a flask resting in a thermostated bath. To specify its state we would have to say whether it is pure solid, liquid, or gas, or a solution of specified composition and give the temperature and pressure. The flask, the bath, and everything else would be 

We are most often interested in the changes in the thermodynamic functions when a chemical reaction takes place for example, the heat absorbed by the system within a bomb calorimeter where the volume stays constant (Qv) is a direct measure of the change in E  

Processes at constant pressure. Chemical and biochemical reactions are much more likely to be conducted at constant pressure (usually 1 atm) than they are at constant volume. For this reason, chemists tend to use the enthalpy H more often than the internal energy E. 

It follows from Eq. 6-3 that if the pressure is constant, AHp is equal to AEP + P AV. Since in a process at constant pressure, P AV is exactly the pressure-volume work done on the surroundings, the heat absorbed at constant pressure (Qp) is a measure of AHp. 

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