ABSTRACT
The barrier, or transition state, merely changes the path and therefore does not affect the net free energy change, but it clearly does obstruct the process from getting started in the first place. The free energy necessary to overcome such barriers is called the activation energy, it is the free energy necessary to get a process going. The only energy available to an endergonic reaction in the absence of a coupled exergonic one, however, is heat from the surroundings of the reaction materials; at 30°C this amounts to about 0.02 eV/molecule. This thermal energy thus seems inadequate to drive endergonic reactions, which need at least a few tenths of electron volts. Enzymes lower the activation energy barriers that impede the initiation of exergonic reactions, but do not reduce them to zero. The remaining required activation energy can be provided by heat flow from the surroundings.
