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Chapter 25: Problem 2

How many ATP equivalents are consumed per \(\mathrm{N}\) atom of ammonium formed by (a) the nitrate assimilation pathway and (b) the nitrogen fixation pathway? (Assume for this problem NADH, NADPH, and reduced ferredoxin are each worth 3 ATPs.

Short Answer

Expert verified
For the nitrate assimilation pathway, 21 ATP equivalent are consumed per N atom, while for the nitrogen fixation pathway, 8 ATP equivalents are consumed per N atom.

Step by step solution

01

Calculate ATP equivalents in nitrate assimilation pathway

In the nitrate assimilation pathway, nitrate (\(\mathrm{NO}_3^-\)) is converted to ammonium (\(\mathrm{NH}_4^+\)). This process requires two steps: The reduction of nitrate to nitrite (\(\mathrm{NO}_2^-\)), and the reduction of nitrite to ammonium. The reduction of nitrate to nitrite uses one molecule NADH (equivalent to 3 ATPs), and the reduction of nitrite to ammonium uses six molecules of reduced ferredoxin (equivalent to 18 ATPs). So, in total \(3 + 18 = 21\)
02

Calculate ATP equivalents in nitrogen fixation pathway

In the nitrogen fixation pathway, molecular nitrogen (\(\mathrm{N}_2\)) is converted to ammonium (\(\mathrm{NH}_4^+\)). This process uses eight protons, eight electrons and 16 ATP to reduce one molecule of molecular nitrogen to two molecules of ammonium, so per \(\mathrm{N}\) atom, it requires half of the total ATP which equals to \(16/2 = 8\)
03

Compare the ATP equivalents consumed in both pathways

Add the total ATP equivalents in both pathways to yield the final results. For the nitrate assimilation pathway, 21 ATPs equivalents are used per \(\mathrm{N}\) atom, and for the nitrogen fixation pathway, 8 ATP equivalents are consumed per \(\mathrm{N}\) atom.

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