Question

During symbiotic nitrogen fixation, how many ATPs are used in fixing one mole of $N_2$.

• A. $8$
• B. $16$
• C. $5$
• D. $10$

Answer 1

The correct option is B $16$

Some of the plants, especially the roots of legumes live in symbiotic association with $N_2$-fixing bacteria. They can fix nitrogen into biologically usable forms. This phenomenon is called symbiotic nitrogen fixation. The enzyme nitrogenase is required to fix atmospheric nitrogen. The molecule of nitrogen is reduced by the addition of hydrogen atoms provided by a reducing agent like $NADP{H}_2$. It produces dimide ($N_2{H}_2$), hydrazine ($N_2{H}_4$) and finally ammonia ($N{H}_3$) is formed.

The overall reaction involved in $N_2$-fixation is

$N_2+8e^{-}+8H^{+}+16ATP\to 2N{H}_3+H_2+16ADP+16P_i$

Thus, one mole of nitrogen is converted into two molecules of ammonia. For producing 1 molecule of ammonia, 8 ATPs are needed. So, for fixing one mole of $N_2$, 16 ATPs are used.