What is Photosynthesis?
Photosynthesis is a biological process by which energy contained within light is converted into chemical energy of bonds between atoms that power processes within cells. It emerged roughly 3.5 billion years ago in geological history and has evolved into complex biochemical and biophysical mechanisms; it occurs today within a variety of single-celled organisms as well as in plants. Earth’s atmosphere and seas contain oxygen because of photosynthesis.
There are three types of photosynthesis: CAM, C4 and C3. C3 is the most well-known type of photosynthesis, is used by most plants, and indeed until recent decades was the only type of photosynthesis known to exist.
Photosynthesis Occurs in Two Stages
The two stages of photosynthesis are known as the light reactions and the dark reactions. During the light reactions of photosynthesis, light from the sun is captured through a series of reactions involving the chemical chlorophyll. This results in the synthesis of two high energy chemical compounds: ATP and NADPH, the latter whose chemical energy is held by electrons which can be easily transferred to other compounds. This set of reactions requires water (H2O) from which oxygen is released during the process. In the dark reactions, which can take place either in the presence of light or in the dark, energy from ATP and NADPH is used to bond–or “fix”–carbon dioxide (CO2) to another compound, which already contains several carbon atoms. The difference between C3 and C4 photosynthesis depends on differences in the chemical compounds to which the incoming CO2 is linked during the dark reactions (CAM photosynthesis differs from both C3 and C4 photosynthesis in that prior to fixation, CO2 is an acid form known as carbonic acid).
In C3 photosynthesis, the compound to which CO2 is integrated first is a 3-carbon compound. The “enzyme” necessary for this process is known as RuBisCO. C3 photosynthesis is more efficient than C4 or CAM photosynthesis when the environment is cool and moist and when light is plentiful. However, since C3 uses more water than the other two types of photosynthesis, it is not as useful to organisms living in hot, arid environments.