Carbon dioxide enters plants through the stomata and diffuses into mesophyll cells. It then enters the Calvin-Benson cycle in C3 plants or the C4 cycle in C4 plants. It eventually ends up in sugars and starch.

The conversion of carbon dioxide into sugar and starch is an energy-expensive process. It does not occur spontaneously but rather is driven by the light reactions of photosynthesis. The light reactions of photosynthesis generate ATP and NADPH, which are consumed in the dark, or light-independent, reactions. The fixation of three carbon dioxide molecules through the Calvin-Benson cycles costs the cell six molecules of NADPH and nine molecules of ATP.

In the first stage of the Calvin-Benson cycle, rubisco catalyzes the reaction of carbon dioxide with ribulose-1,5-bisphosphate to ultimately yield sugar and starch. In photorespiration, rubisco catalyzes the reaction of oxygen with ribulose-1,5-bisphosphate. This reaction wastes the ATP and NADPH produced by the light reactions.

Photorespiration is minimized in C4 plants. C4 plants concentrate carbon dioxide in the cells containing rubisco where the Calvin-Benson cycle takes place. The concentration of carbon dioxide favors the reaction of rubisco with carbon dioxide and minimizes its reaction with oxygen.

In the process of photosynthesis, plants capture light energy and use it to transform carbon dioxide into sugar and starch. Plants build almost all of their organic molecules from carbon dioxide and other inorganic compounds.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education