Question

How many molecules of ATP are required for synthesis of one molecule of glucose in $C_3$ and $C_4$ pathways?

Answer 1

Since every carbon dioxide molecule has to be fixed twice, first by 4-carbon organic acid and second by RuBisCO, the $C_4$ pathway uses more energy than the $C_3$ pathway. The $C_3$ pathway requires 18 molecules of ATP for the synthesis of one molecule of glucose, whereas the $C_4$ pathway requires 30 molecules of ATP.