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photosynthesis
The process by which green plants, algae, and some bacteria convert the energy of sunlight into chemical energy, which is then stored as carbohydrate. Overall, the process may be written as:
c6H12O6 + 6O2
Although in detail photosynthesis is a complex sequence of reactions, there are two main stages. In the "light reaction," also called photolysis or the photochemical stage, chlorophyll (the key chemical in the whole process) is activated by absorbing a photon of light, initiating a sequence of reactions in which the energy-rich compounds ATP (adenosine triphosphate) and TPNH (the reduced form of triphosphopyridine nucleotide – TPN) are made, water being decomposed to give free oxygen for the process. In the second stage, the "dark reaction," also called the Calvin cycle or the thermochemical stage, the ATP and TPNH provide the energy for the assimilation of carbon dioxide gas, yielding a variety of sugars from which other sugars and carbohydrates, including starch can be built up.
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• BIOCHEMISTRY
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