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 is the process where atmospheric nitrogen (\(N_2\)) is converted into ammonia (\(NH_3\)). This reaction is catalyzed by the nitrogenase enzyme complex and is extremely energy-intensive, requiring a significant amount of ATP.
Step 2: Key Formula or Approach:
The overall balanced chemical equation for biological nitrogen fixation is:
\[ N_2 + 8H^+ + 8e^- + 16ATP \rightarrow 2NH_3 + H_2 + 16ADP + 16P_i \] Step 3: Detailed Explanation:
As shown in the equation, the reduction of one molecule of dinitrogen (\(N_2\)) to form two molecules of ammonia (\(NH_3\)) requires the hydrolysis of 16 molecules of ATP to ADP and inorganic phosphate (\(P_i\)). This energy is used by the nitrogenase enzyme complex to overcome the high activation energy required to break the strong triple bond of the \(N_2\) molecule. The process also requires 8 electrons and 8 protons. A byproduct, hydrogen gas (\(H_2\)), is also formed.
Step 4: Final Answer:
To fix one molecule of \(N_2\), 16 molecules of ATP are consumed.