Such safety devices are integral to all well-designed machines and the natural world is no exception. But only recently have scientists begun to elucidate the cut-outs and safety valves employed in the biochemical world. Plants harvest the power of the sun, converting it into chemical energy initially through the activity of 'photosystem II', a large protein complex. At the heart of photosystem II is chlorophyll, which absorbs light to convert this green pigment into an activated form. The energy contained in this activated chlorophyll is used to make all the complex chemicals on which life depends.
Plants are at their most efficient in average levels of sunlight.
In full sunlight, especially at noon, they absorb much more energy than they can use. If this excess is not disposed of safely, chlorophyll forms a hyper-excited state that can pass its energy on to any nearby oxygen molecules. The resulting activated oxygen, called 'singlet' oxygen, causes indiscriminate damage and ultimately the death of the plant's cells in a process similar to the bleaching of carpets or paintings in direct sunlight. So plants have developed a way of quickly dissipating excess energy as heat, which is technically known as 'nonphotochemical quenching'. For a system so crucial to the continuance of life on Earth, it is very poorly understood.