Photosynthesis is the difference between the Earth and everywhere else we know about. From Mercury to Pluto and, so far as we know, on all the hundreds of other planets we have now discovered, sunlight – or starlight – falls on craters and mountains, clouds or ice, and is either absorbed or reflected. But on the Earth, it is absorbed by purposeful systems that do something with it.That something, photosynthesis, happens on a molecular scale but has planetary effects.

Photosynthesis is the process whereby light is absorbed by plants, algae and other organisms and its energy is stored by a chemical called adenosine triphosphate, ATP. In plants, the solar energy is absorbed by a pigment called chlorophyll. It absorbs most of the light apart from the green. The green light is reflected, which is what makes plants look green. Other organisms that perform photosynthesis, especially some bacteria, use other pigments to absorb the light and can be red or blue.

The light absorption is the “light phase” of photosynthesis. It is followed by the 'dark phase” in which energy stored in ATP is used to turn water which plants draw up from their roots, and carbon dioxide from the atmosphere, into sugars. This reaction creates living matter and also releases oxygen for animals to breathe, including us. So it is the vital transaction in the existence of life on Earth.