Definition of Photosynthesis

Photosynthesis is defined as a photochemical oxidation-reduction process: using light energy captured by chlorophyll, water is oxidised (loses electrons) to release oxygen, while carbon dioxide is reduced (gains electrons) to form carbohydrate. It is considered the single most important anabolic (biosynthetic) process on Earth, since it is the only large-scale route by which raw inorganic materials - CO2 and water - are built up into organic carbohydrate, using nothing more than light as the energy source. The commonly cited overall equation is $6CO_2 + 6H_2O \xrightarrow{light,\ chlorophyll} C_6H_{12}O_6 + 6O_2$. Ruben and Kamen (1941), however, showed that six water molecules are not actually enough to account for all the oxygen evolved, since every oxygen atom released comes from water and not from CO2; they revised the equation to $6CO_2 + 12H_2O \xrightarrow{light,\ chlorophyll} C_6H_{12}O_6 + 6H_2O + 6O_2$, in which twelve water molecules are consumed (six are split completely, and six are regenerated as a byproduct), so that the oxygen balance correctly reflects water, not CO2, as its sole source. Because it is fundamentally a redox process, the lig …