Thermodynamics

REVIEW

Thermodynamics and living organisms: We are all law-abiding citizens

The first two laws of thermodynamics are important to understanding biochemical reactions. The first law states that energy can neither be created nor destroyed, and the second law states that the entropy, or disorder, of the universe must increase with every process. Willard Gibbs derived a formula that aids us in understanding how thermodynamic principles govern chemical reactions:

The Gibbs Free Energy Change of a reaction (DG) tells us whether a reaction is thermodynamically possible. Reactions with a negative DG are spontaneous, meaning they have a reasonable probability of occurring naturally, without energy input.

Keep in mind that the DG function does not impact the rate, or speed, of a reaction. Reactions with large negative DG values may still occur very slowly.

Because the DG of a reaction varies with reactant and product concentrations, temperature, and pressure, biochemists agreed on standard conditions for reporting DG values. The standard conditions are 1 atmosphere of pressure, a temperature of 298 K, and reactant and product concentrations of 1 M, with the exception of H⁺, which may be present at physiological concentrations (neutral pH). The standard free energy change of a reaction is denoted by DG°´.

The DG of a reaction is related to the standard free energy change for the reaction, DG°´, by the following relationship:

DG = DG°´ + RT ln [C][D] [A][B] Where: R is the gas constant (8.3145 J mol–1K–1) T is the temperature (in Kelvin)

Since DG = 0 at equilibrium and the ratio of products to reactants is the equilibrium constant K_eq, it follows that

Thus, DG°´ is the driving force that pushes reactants and products toward equilibrium.

The Thermodynamics Police: No violation of the laws of thermodynamics have ever been observed

There is an argument that has circulated for years that purports to prove that the theory of evolution is incorrect because it violates the laws of thermodynamics. In this argument, evolution (the theory that the complex plants and animals that inhabit the Earth today evolved from simple, self-replicating molecules) is said to violate the second law of thermodynamics, because the entropy of a highly ordered animal is much lower than the entropy of a simple molecule. Thus, the story goes, the molecules that make up living organisms have moved from a state of disorder to a state of order over millions of years, a clear violation of the law that states that entropy is always increasing.

The problem with this line of reasoning is that it treats a living organism as a closed system that does not interact with its environment. Reactions that increase the order of one part of the system are always offset by an increase of disorder in another part of the system, and it takes energy to produce order (anyone who has ever tried to keep a room clean can attest to that!). Thus, our bodies grow at the expense of the order of the food we ingest, ice cubes are formed in the freezer at the expense of heat released by the refrigerator, and cans are stacked on the shelf as our muscles break down glucose to do the work. We are citizens of the universe; the biochemical reactions within our bodies follow the laws of thermodynamics.