Question

__________ Molecules of ATP are required to fix one molecule of nitrogen (N2) to 2NH3.

• A. 4
• B. 8
• C. 16
• D. 20

Hint:

Memorize the stoichiometry of nitrogen fixation: 1 \(N_2\) requires 8 electrons and {16 ATP}. This high energy cost is a common topic in exams.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Biological nitrogen fixation involves converting atmospheric nitrogen (\(N_2\)) into ammonia (\(NH_3\)). This process, catalyzed by the nitrogenase enzyme complex, demands significant energy input in the form of ATP.
Step 2: Key Formula or Approach:
The balanced chemical equation representing biological nitrogen fixation is:
\[ N_2 + 8H^+ + 8e^- + 16ATP \rightarrow 2NH_3 + H_2 + 16ADP + 16P_i \] Step 3: Detailed Explanation:
The equation illustrates that reducing one dinitrogen molecule (\(N_2\)) to two ammonia molecules (\(NH_3\)) necessitates the hydrolysis of 16 ATP molecules into ADP and inorganic phosphate (\(P_i\)). The nitrogenase enzyme complex uses this energy to overcome the substantial activation energy needed to cleave the robust triple bond in \(N_2\). The process also requires 8 electrons and 8 protons, and generates hydrogen gas (\(H_2\)) as a byproduct.
Step 4: Final Answer:
The fixation of one \(N_2\) molecule requires the consumption of 16 ATP molecules.