What is the basis for designating C3 and C4 pathways of photosynthesis?

 

 

The basis for designating C3 and C4 pathways of photosynthesis is as follows

CPathway 

C4 Pathway

In Calvin cycle or C3 cycle (dark reaction), 
carbon dioxide is fixed into first stable compound called 3-PGA,(3 phorphoglyceric  acid) which is a 3 carbon compund. 

In C4 plants, the Calvin cycle C3 cycle occurs  in bundle sheath cells. Prior to reaching there, the CO2 is trapped by mesophyll cells and fixed into a 4-carbon stable product called oxalo acetic acid.