Question

How many ATPs are consumed for production of 5 molecules of NH3 during biological N2-fixation?

• A. 12
• B. 40
• C. 20
• D. 16

Answer 1

The correct option is B

40

The correct option is B

Explanation of the correct option:

1. Biological nitrogen fixation can be represented by the following equation: N₂ + 8H+ + 8e- + 16ATP → 2NH₃ + H₂ + 16ADP + 16Pi
2. We see that one molecule of N₂ requires 16 ATP molecules to produce 2NH₃ molecules.
3. 2NH₃ molecules require 16 ATP.
4. 1NH₃ molecule will require 8 ATP.
5. Therefore 5NH₃ molecules will require 5*8=40 ATP.

Explanation of the incorrect options:

Option A:

The requirement to synthesize one molecule of NH₃ is 8 ATP so 12 cannot be an option.

Option C:

The requirement to synthesize 2 molecules of NH₃ is 16 ATP so 20 cannot be an option.

Option D:

The requirement to synthesize one molecule of NH₃ is 8 ATP so 16 ATP will give 2 molecules of NH₃ not 5 molecules of NH_3.

Final answer: 40 molecules of ATPs are consumed for production of 5 molecules of NH3 during biological N2-fixation.