∞ generated and posted on 2016.12.25 ∞
Phenomenon that requires no net input of energy to move forward.
Processes in general require an input of energy to be initiated, called activation energy. That input of energy, however, is not the determinant of spontaneity. Instead, it is whether the process, once it proceeds, particularly post any transition state, ends up liberating more energy than was put into the system to get the process started.
That is, if output is greater than input, in terms of a quantity known as free energy, then the process is described as spontaneous.
In biological systems it is typically those chemical reactions that do no require an input of ATP, or equivalent energy source, that are spontaneous reactions. Note that movement down a concentration gradient or, more generally, towards equilibrium or simply exergonic reactions, are spontaneous processes.
Figure legend: Spontaneous processes overall result in a reduction in energy associated with the process. By contrast, if something proceeds not spontaneously then it requires a net input of energy. By net, the implication is that one is considering more than just activation energy, that is, not simply the energy required to get a reaction started. Spontaneous is equivalent to an exergonic reaction whereas non-spontaneous is equivalent to an endergonic reaction.
Exergonic reactions by definition are spontaneous processes whereas endergonic reactions, also by definition, are non-spontaneous processes.