Thermodynamics is the study of energy and its transformations in physical and chemical systems. In biological systems, thermodynamics provides a framework to understand how energy flows in living organisms, and how living organisms utilize energy from their environment to perform work. In this chapter, we will discuss the thermodynamics of biological systems, including the laws of thermodynamics, free energy, enthalpy, and entropy.

Laws of thermodynamics The first law of thermodynamics, also known as the law of conservation of energy, states that energy cannot be created or destroyed, only converted from one form to another. In biological systems, energy is constantly being transformed from one form to another, such as from chemical energy in food to mechanical energy for muscle movement.

The second law of thermodynamics states that the total entropy (disorder) of a closed system will always increase over time. In biological systems, entropy can be decreased locally, but only at the expense of an increase in entropy elsewhere in the system. For example, the decrease in entropy of a cell during a metabolic reaction is accompanied by an increase in entropy of the surroundings.

Free energy Free energy (G) is the amount of energy available to do work in a system. It is the difference between the energy content of the products and the reactants in a reaction, at constant temperature and pressure. If G is negative, the reaction is exergonic and spontaneous (releases energy), while if G is positive, the reaction is endergonic and requires energy input to proceed.

Enthalpy Enthalpy (H) is the heat content of a system, and it represents the total energy of the system, including internal energy and the energy required to maintain pressure and volume. Enthalpy can be calculated from the internal energy of a system (U), the pressure (P), and the volume (V) using the formula H = U + PV.

Entropy Entropy (S) is a measure of the disorder or randomness of a system, and it is related to the number of possible configurations of the system. The entropy of a system tends to increase over time, as energy is dispersed and the number of possible states increases.

Applications in biochemistry The principles of thermodynamics are fundamental to understanding many biological processes, such as cellular respiration, photosynthesis, and enzyme catalysis. Enzymes function by lowering the activation energy of a reaction, making it easier for a reaction to occur. The energy required to do work in biological systems, such as muscle contraction or biosynthesis, comes from the breakdown of energy-rich molecules such as ATP.

In addition, thermodynamics plays an important role in drug development and metabolic engineering. Knowledge of the thermodynamics of a reaction can be used to predict the feasibility and direction of a reaction, and to design drugs that selectively inhibit or activate specific enzymes. Metabolic engineering involves designing and modifying metabolic pathways to optimize the production of desired compounds, and thermodynamic considerations are important in selecting the optimal pathway and conditions for production.