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As compared to a $C_3$-plant, how many additional molecules of ATP are needed for net production of one molecule of hexose sugar by $C_4$-plants -
A
Zero
B
Six
C
Two
D
Twelve
Detailed Solution
In $C_3$ plants the Calvin cycle needs 3 ATP and 2 NADPH for the fixation of one $CO_2$, so for one hexose (6 $CO_2$) it needs 18 ATP and 12 NADPH.
$C_4$ plants first fix $CO_2$ in the mesophyll cells and then release it in the bundle sheath cells for the Calvin cycle.
To regenerate phosphoenol pyruvate (PEP) from pyruvate in the mesophyll cells, the enzyme pyruvate phosphate dikinase uses 2 high energy bonds of ATP (ATP is converted to AMP) for every $CO_2$ fixed.
So 2 extra ATP are needed for each $CO_2$, i.e. $2 \times 6 = 12$ extra ATP for one hexose.
Total requirement in $C_4$ plants $= 18 + 12 = 30$ ATP per hexose.
So $C_4$ plants need twelve additional ATP molecules compared with $C_3$ plants.
$C_4$ plants first fix $CO_2$ in the mesophyll cells and then release it in the bundle sheath cells for the Calvin cycle.
To regenerate phosphoenol pyruvate (PEP) from pyruvate in the mesophyll cells, the enzyme pyruvate phosphate dikinase uses 2 high energy bonds of ATP (ATP is converted to AMP) for every $CO_2$ fixed.
So 2 extra ATP are needed for each $CO_2$, i.e. $2 \times 6 = 12$ extra ATP for one hexose.
Total requirement in $C_4$ plants $= 18 + 12 = 30$ ATP per hexose.
So $C_4$ plants need twelve additional ATP molecules compared with $C_3$ plants.