Question

For the synthesis of one glucose molecule, the number of ATP required are

• A. 9 ATP for $C_3$ cycle and 20 ATP for $C_4$ cycle.
• B. 18 ATP for $C_3$ cycle and 30 ATP for $C_4$ cycle.
• C. 22 ATP for $C_3$ cycle and 35 ATP for $C_4$ cycle.
• D. 24 ATP for $C_3$ cycle and 36 ATP for $C_4$ cycle.

Answer 1

The correct option is B 18 ATP for $C_3$ cycle and 30 ATP for $C_4$ cycle.

Since every $C{O}_2$ 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.

This energy debt is more than paid for by avoiding losing more than half of photosynthetic carbon in photorespiration as occurs in some tropical plants, making it an adaptive mechanism for minimizing the loss.