Sunlight is an abundant source of free energy. The chlorophyll molecule in the leaves of plants uses this to unburn water—in other words, to split the H2O molecule into its constituent parts of hydrogen and oxygen. The oxygen is released into the atmosphere, leaving hydrogen, on its own, within the leaves. Isolated hydrogen, of this kind, is itself now a source of free energy because it needs to rebond with oxygen or anything chemically similar to oxygen. This step, when sunlight is used to unburn water, is called the light reaction.
