C3 plants use the C₃ carbon fixation pathway as the sole mechanism to convert CO₂ into an organic compound. They make up over 95% of Earth’s plant species and flourish in cool, wet, and cloudy climates, where light levels may be low, because the metabolic pathway is more efficient there. With enough water, the stomata can stay open and let in more carbon dioxide. However, carbon losses through photorespiration are high.

C4 plants are able to raise the intercellular carbon dioxide concentration at the site of fixation, thus reducing and sometimes eliminating carbon losses by photorespiration. C4 plants first fix CO₂ into a compound containing four carbon atoms. They inhabit hot, dry environments, have very high water-use efficiency, and can double the C3 photosynthesis, but C4 metabolism is inefficient in shady/cool environments. Less than 1% of Earth’s plant species can be classified as C4, but they account for around 20% of global gross primary productivity (Wand et al. (1999) Glob. Change Biol. 5, 6).