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.Example of how a cell can keep reactions out of equilibrium. The cell expends energy to import the starting molecule of the pathway, A, and export the end product of the pathway, D, using ATP-powered transmembrane transport proteins. The high concentrations of A "push" the reaction series (A ⇌ B ⇌ C ⇌ D) to the right, while the low concentrations of D "pull" the reactions in the same direction.