Gibbs free energy, enthalpy, and entropy
- ∆H is the enthalpy change. Enthalpy in biology refers to energy stored in bonds, and the change in enthalpy is the difference in bond energies between the products and the reactants. A negative ∆H means heat is released in going from reactants to products, while a positive ∆H means heat is absorbed. (This interpretation of ∆H assumes constant pressure, which is a reasonable assumption inside a living cell).
- ∆S is the entropy change of the system during the reaction. If ∆S is positive, the system becomes more disordered during the reaction (for instance, when one large molecule splits into several smaller ones). If ∆S is negative, it means the system becomes more ordered.
- Temperature (T) determines the relative impacts of the ∆S and ∆H terms on the overall free energy change of the reaction. (The higher the temperature, the greater the impact of the ∆S term relative to the ∆H term.) Note that temperature needs to be in Kelvin (K) here for the equation to work properly.
Endergonic and exergonic reactions
Spontaneity of forward and reverse reactions
Non-standard conditions and chemical equilibrium
How cells stay out of equilibrium
- They may use energy to import reactant molecules (keeping them at a high concentration).
- They may use energy to export product molecules (keeping them at a low concentration).
- They may organize chemical reactions into metabolic pathways, in which one reaction "feeds" the next.