photorespiration is the process that creates a major difference between C3 and C4 plants. C4 plants have a mechanism to increase CO2 concentration at the enzyme site, preventing RuBisCO from binding with oxygen, thus they lack photorespiration. In contrast, C3 plants undergo photorespiration, which decreases CO2 fixation. The Calvin cycle occurs in all photosynthetic plants (both C3 and C4).