1. C4 plants have a special enzyme for accepting the atmosphere CO2. It can accept CO2 even if it lies 1-2 ppm (parts per million)
2. RUBISCO enzyme of C3 plants can't accept CO2 below 50 ppm, hence photorespiration takes place.
3. PEP carboxylase is efficient in CO2 accepting and hence photorespiration doesn't take place in C4 plants.