This is because these plants (C4 and CAM) needed to avoid the wasteful process of PHOTORESPIRATION.; by first producing C4 -malate , and oxaloacetate compound,s before the usual C3 compunds GLYCERATE PHOSPAHTE is produced (after decaboxylation), to enter CALVIN cylce.
The name C3 and C4 comes from the number of carbon compounds produced -the first compound MALATE in C4 pants is a 4 carbon compound , and first 3C compoud GLYCERATEPHOPAHTE produced in C3 Plants.
Explanation:
C3 plants usually reduce C02 by reaction with RIBULOSE BIPHOSPAHTE;the reaction is catalysed by enzyme RUBISCO (most abundant enzymes in the world) to produce 6 carbon intermediate which splits to 2 molecules of three carbon compounds GLYCERIDE - 3-PHOPHATE (GP)
GP is reduced to 3Carbon sugar Triphosphate in the presence of ATP and NADP from light dependent stage, The Triphospahte is converted to glucose, Amino acids and Lipids.
However, C4 plants e.g maize , shorghum which grow in tropics, of high temperature and high light intensities with low altitudes usually face difficulties with enzyme RUBISCO, beacuse the enzymes can catalyse both the reaction of CO2 with Ribulose biphosphate and that of Oxygen with the same compound.
This competitive inhibition reduce the availability of Ribulose biphospahte for CO2 and therefore Carbohydrate production.
To avoid this the C4 plants have special adaptations that shunt Oxygen from the atmosphere.They have special mesophyll cells with direct contact with atmosphere. Phosphoenol pyruvate (PEP) is present in the mesophyll cells . C02 combined with PEP , a reaction catalysed by enzyme PEPcaboxylase; to form OXALOACATATE,(4C) compounds. The latter is converted to Malate. The malate is pass into the Bundle sheath where enzynme RUBISCO is located,
The malate is decaboxylated (removal of C02). The available is C02 reacts Ribulosebiphosphate in the presence of Rubisco in the normal C3 fashion to form TP.
SUMMARY
30 ATPs ARE NEEDED TO PRODUCE 1 MOLECULE OF GLUCOSE BY C4 PLANTS , WHILE 18ATPS ARE NEEDED TO PRODUCE THE SAME MOLECULE OF GLUCOSE BY C3 PLANTS.
These C4 process required additional STEPS and ENERGY(ATPs), and therefore more expensive than C3 steps.
