Question

What is the basis for designating $C_3$ and $C_4$ pathways of photosynthesis?

Answer 1

$C_3$ plants

The first stable product formed during atmospheric
$C{O}_2$ fixation is 3-carbon (3- phosphoglyceric acid).
Hence, the name $C_3$ cycle.

$C_4$ plants

The first stable product formed during atmospheric $C{O}_2$ fixation is 4- carbon (oxaloacetate). Hence, the name $C_4$ cycle.

Final answer

Based on the number of carbon present in the first stable product of $C{O}_2$ fixation.