Photosynthesis is the process that plants use to convert CO2 into carbohydrates (sugar). These carbohydrates are stored (as starch) and are used by the plant for food. The basic formula for photosynthesis is:
Photosynthesis involves two general steps: light reactions (thylakoid membranes) and the Calvin cycle or dark reactions (which occur in the stroma). The light reactions provide energy (ATP) and a combination of electrons and hydrogen (NADPH) for the Calvin cycle's fixation of CO2 to a 5-carbon sugar (RuBP).
There are 3 photosynthetic pathways in plants known as C3, C4, and CAM. All three use the enzyme rubisco to fix carbon dioxide during the Calvin cycle. Our understanding of photosynthesis has changed dramatically in the last 30 years due to the discovery of different metabolic pathways in some plants.
C3 plants are the most common. These plants attach CO2 to the 5 carbon sugar RuBP (ribulose bisphosphate) with the assistance of the enzyme RuBP carboxylase (rubisco). The initial product is a 6-carbon compound which is unstable and is split into two molecules of the 3-carbon phosphoglycerate (3-PGA). PGA is shunted through a series of reactions (the Calvin cycle) resembling the reverse of glycolysis, eventually producing the 6-carbon sugar glucose and regenerating the resident CO2 acceptor, RuBP. However, there are at least two rather specialized groups of plants (C4 and CAM) in which, prior to CO2 fixation to RuBP, CO2 is attached to the 3 carbon phosphoenolpyruvate (PEP) to form a 4-carbon organic acid (malic acid or oxaloacetic acid).
C4 plants, like corn or sugarcane, first attach CO2 to a 3-carbon sugar to form 4-carbon organic acids with the aid of a very efficient enzyme PEP carboxylase. These acids are transported from surface cells deep into the leaf's bundle sheath cells and made to release CO2 . Only then does Rubisco efficently converted the CO2 to sugar. All this takes place during the day.
CAM plants (e.g. cactus) also attach CO2 to PEP making 4-carbon organic acids. This happens at night. During the day CO2 is released from these 4C-acids, and converted into sugar with the aid of rubisco.
In either case the organic acids are decarboxylated and the CO2 released can be fixed by RuBP carboxylase and the Calvin cycle proceeds just as in C3 plants.
In summary there are three photosynthetic-types commonly recognized; C3, C4, and CAM. C3 plants fix atmospheric CO2 directly onto RuBP and thus into glucose. C4 plants first fix atmospheric CO2 into 4-carbon acids in the mesophyll of the leaf and decarboxylate the 4-carbon acids in the bundle sheath cells where the CO2 is then fixed by RuBP carboxylase (all of this takes place in the day). CAM plants first fix atmospheric CO2 into malic acid and other 4C-acids at night. During the day, malic acid is decarboxylated and the CO2 released is then fixed by Rubisco (all of this takes place in the same cell).
Recent studies with aquatic plants suggest there may be other pathways.
The diurnal switch in metabolismof CAM photosynthesis results in a diurnal fluctuation in malic acid content (and also in acidity). Thus, one can distinguish a CAM plant by measuring the difference in malic acid in photosynthetic tissues between morning and evening.