Activation Energy

Activation energy is the energy needed to initiate a chemical reaction between reactants that are present. The source is often supplied by thermal energy which causes molecules to move faster and collide with one another, causing the bonds of the reactants to break.

Thus, the original reactants required free energy for the reaction to occur. Represented as the uphill part of this curve, this is the amount of activation energy for the reaction. At the peak, known as the transition state, the unbound molecules are now in unstable condition. As atoms reattach with new bonds, they release free energy into the environment, which is shown as the downhill portion of the reaction.

While humans metabolize sugar and fat for energy, if thermal energy were used to break down these molecules, so much free energy would be released as heat that the proteins in the cell would denature. Instead, substances known as catalysts are specifically added to regulate the rate of metabolism, like speeding it up. For example, a biological catalyst, an enzyme, lowers the activation energy required to break bonds, and allows reactions to occur at reasonable rates without cellular damage.