In C3 plants, the first product of CO2 fixation is 3-phosphoglyceric acid (a three-carbon compound).
These plants are mostly found in temperate regions. Example: rice, barley.
C4 plants:
In C4 plants, the first product of CO2 fixation is oxaloacetic acid (a four-carbon compound).
C4 plants are mostly found in tropical areas. They possess a distinct type of leaf anatomy called ‘Kranz’ anatomy in which the mesophyll cells form a cluster around the bundle-sheath cells. Example: maize, sorghum.
Aquatic plants:
Aquatic plants are mostly C4 plants since the amount of dissolved CO2 is low in aquatic habitats.
These plants have two CO2 acceptors (phosphoenolpyruvate and ribulose bisphosphate) and fix CO2 much faster in comparison to C3 plants.
They also exhibit a higher photosynthesis rate. Example: Udotea flabellum.