Question

The number of ATP required in excess to assimilate atmospheric $C{O}_2$ to four molecules of triose phosphates in $C_4$ plants as compared to $C_3$ plants is

• A. 30
• B. 36
• C. 60
• D. 8

Answer 1

The correct option is D 8

For one glucose molecule, 6 turns of C${}_3$ cycle are required for producing 12 molecules of triose, out of which 2 will be used for glucose. For 6 cycles, 18 ATP are required. ATP produced in one cycle is 18/6=3. If there are 4 molecules required for glucose formation, 4 cycles should take place. So, 3 x 4 = 12 ATP required for 4 molecules of triose.

Similarly, for one glucose molecule, 6 turns of C${}_4$ cycle are required for producing 12 molecules of triose, out of which 2 will be used for glucose. For 6 cycles, 30 ATP are required. ATP produced in one cycle is 30/6=5. If there are 4 molecules required for glucose formation, 4 cycles should take place. So, 5 x 4 = 20 ATP required for 4 molecules of triose.

So, there are 20-12 = 8 molecules of ATP required in excess.

Thus, the correct answer is option D.