PHOTOSYNTHESIS,
the process by which chlorophyll-containing organisms--green plants, algae, and some bacteria--capture energy in the form of light and convert it to chemical energy. Virtually all the energy available for life in the earth's biosphere--the zone in which life can exist--is made available through photosynthesis.
A quite generalized, unbalanced chemical equation for photosynthesis is
CO2 + 2H2A + light energy → (CH2) + H2O + H2A
The formula H2A represents a compound that can be oxidized, that is, from which electrons can be removed; CO2 is carbon dioxide; and CH2 is a generalization for the hydrocarbons incorporated by the growing organism. In the vast majority of photosynthetic organisms--that is, algae and green plants--H2A is water (H2O); in some photosynthetic BACTERIA (q.v.), however, H2A is hydrogen sulfide (H2S). Photosynthesis involving water is the most important and best understood and, therefore, will be discussed here in detail.
Photosynthesis consists of two stages: a series of light-dependent reactions that are temperature independent and a series of temperature-dependent reactions that are light independent. The rate of the first series, called the light reaction, can be increased by increasing light intensity (within certain limits) but not by increasing temperature. In the second series, called the dark reaction, the rate can be increased by increasing temperature (within certain limits) but not by increasing light intensity.