Spontaneity in Biochemical
useful criterion for predicting the spontaneity of a process is the freeenergy, which is indicated by the
symbol G.(Strictly speaking, the use
of thiscriterion requires conditions of constant temperature and pressure,
which are usual in biochemical thermodynamics.) It is not possible to measure
absolute values of energy; only the changes
in energy that occur during a process can be measured. The value of the change
in free energy, ∆G (where
the symbol ∆ indicates change), gives the needed information
about the spontaneity of the process under consideration.
energy of a system decreases in a spontaneous (energy-releasing) process, so ∆G is negative (∆G < 0). Such a process is called exergonic, mean-ing that energy is
released. When the change in free energy is positive (∆G >0), the process is nonspontaneous. For a
nonspontaneous process to occur, energy must be supplied. Nonspontaneous
processes are also called ender-gonic, meaning
that energy is absorbed. For a process at
equilibrium, with nonet change in either direction, the change in free
energy is zero (∆G= 0).
of the change in free energy, ∆G, indicates the direction of the reaction:
example of a spontaneous process is the aerobic metabolism of glucose, in which
glucose reacts with oxygen to produce carbon dioxide, water, and energy for the
+ 6O2→ 6CO2 + 6H2O
example of a nonspontaneous process is the reverse of the reaction that we saw , the phosphorylation of ADP (adenosine
diphos-phate) to give ATP (adenosine triphosphate). This reaction takes place
in liv-ing organisms because metabolic processes supply energy.
The change in free energy (∆G) that accompanies a reaction deter-mines whether that reaction is
spontaneous at a given temperature and pressure.
negative free energy change (∆G< 0) is characteristic of a spontaneous
reaction. A positive free energy change (∆G> 0)
indicates that the reac-tion is not spontaneous, but the reverse process is
spontaneous. When the free energy change is zero (∆G = 0),
the reaction is at equilibrium.