white_o
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I've got written in all my notes that Rhizobium converts atmospheric N2 into NH3, but online and in some mark scheme it's saying that it's converted into NH4+ and to reject NH3 as an answer. So what's the correct answer?
Also, what happens to this product, is it used by the legume or does it go into the soil to be used by other plants?
Thanks!
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Spectral
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Great question!

Short disclaimer: 'I'm an A level student and only the basics of the nitrogen cycle is covered in my specification, so take my response with a pinch of salt'

So I'll address your question in the nice parts you gave it in:

1. 'Does N2 get converted to NH3 or NH4+ in nitrogen fixation?'
The answer is NH3 - its a multistep conversion driven by the enzyme complex nitrogenase and the final product is indeed NH3, not NH4+.

2. 'Why then is NH4+ in the mark scheme but not NH3?'
Honestly, I would accept both (especially since I'll be sitting my exams this year also!). The reason I see why NH4+ can be considered 'more correct' is with regards to what happens to the NH3 next. It's quite an interesting question so let me give a bit more insight (hopefully will be useful for your notes)
-Roots acidify soil by 'releasing' CO2 from respiration. This CO2 reacts with H2O to form carbonic acid, this forms an equilibria and well, the important point is that this produces H+ ions as a result. (Roots also directly pump out H+ ions into the soil as well)
-With all the H+ ions available in the soil, it is quite likely that the ammonia produced by your nitrogen fixing bacteria will become protonated (or for chemists it will act as a Bronsted-lowry base accepting a proton) to form NH4+.

So that's why NH4+ is formed, and it is the NH4+, not NH3 that is oxidised by nitrifying bacteria to then subsequently produce NO2- and then NO3- (our good old friend the nitrate).

3. 'So what happens to the product, is it used by the legume or does it go into the soil to be used by other plants?'
I think the part with the nitrates has hopefully answered half this question already. Whether or not the nitrates are used by the plant or other plants, its by the legume itself (I think). The reason is because Rhizobium forms a very close association with the leguminous plant. Essentially:
1. Roots from the legume emits chemical signals to attract Rhizobium.
2. As the Rhizobium migrates over, it stimulates root hairs to elongate and form what is known as an 'infection thread' by an invagination (folding inwards, sort of like endocytosis) of the plasma membrane.
3. It gets a bit complicated here, but skipping through some steps the end product is the formation of a Bacteroid surrounded by schlerenchyma cells. This provides an anaerobic environment that is needed for nitrogen fixation for Rhizobium (some free living nitrogen fixing bacteria are aerobic though, so its not that all nitrogen fixing bacteria require anaerobic conditions, in fact my specification makes it sound like they're all aerobic!).

It forms a symbiotic relationship with the plant, so yes, only for the plant!

I hope I've helped a tiny bit!
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white_o
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(Original post by Spectral)
Great question!

Short disclaimer: 'I'm an A level student and only the basics of the nitrogen cycle is covered in my specification, so take my response with a pinch of salt'

So I'll address your question in the nice parts you gave it in:

1. 'Does N2 get converted to NH3 or NH4+ in nitrogen fixation?'
The answer is NH3 - its a multistep conversion driven by the enzyme complex nitrogenase and the final product is indeed NH3, not NH4+.

2. 'Why then is NH4+ in the mark scheme but not NH3?'
Honestly, I would accept both (especially since I'll be sitting my exams this year also!). The reason I see why NH4+ can be considered 'more correct' is with regards to what happens to the NH3 next. It's quite an interesting question so let me give a bit more insight (hopefully will be useful for your notes)
-Roots acidify soil by 'releasing' CO2 from respiration. This CO2 reacts with H2O to form carbonic acid, this forms an equilibria and well, the important point is that this produces H+ ions as a result. (Roots also directly pump out H+ ions into the soil as well)
-With all the H+ ions available in the soil, it is quite likely that the ammonia produced by your nitrogen fixing bacteria will become protonated (or for chemists it will act as a Bronsted-lowry base accepting a proton) to form NH4+.

So that's why NH4+ is formed, and it is the NH4+, not NH3 that is oxidised by nitrifying bacteria to then subsequently produce NO2- and then NO3- (our good old friend the nitrate).

3. 'So what happens to the product, is it used by the legume or does it go into the soil to be used by other plants?'
I think the part with the nitrates has hopefully answered half this question already. Whether or not the nitrates are used by the plant or other plants, its by the legume itself (I think). The reason is because Rhizobium forms a very close association with the leguminous plant. Essentially:
1. Roots from the legume emits chemical signals to attract Rhizobium.
2. As the Rhizobium migrates over, it stimulates root hairs to elongate and form what is known as an 'infection thread' by an invagination (folding inwards, sort of like endocytosis) of the plasma membrane.
3. It gets a bit complicated here, but skipping through some steps the end product is the formation of a Bacteroid surrounded by schlerenchyma cells. This provides an anaerobic environment that is needed for nitrogen fixation for Rhizobium (some free living nitrogen fixing bacteria are aerobic though, so its not that all nitrogen fixing bacteria require anaerobic conditions, in fact my specification makes it sound like they're all aerobic!).

It forms a symbiotic relationship with the plant, so yes, only for the plant!

I hope I've helped a tiny bit!
Very helpful answer, thanks a lot!! so even though N2 forms NH3, it becomes NH4+ and then undergoes nitrification, and then those nitrates are what the leguminous plant actually uses?
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Spectral
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Yes, that's exactly right!

Just as a side point, once the nitrates have been absorbed they are, funnily enough, reconverted back into ammonia in order to synthesise amino acids.

Also some plants do indeed absorb a very small quantity of ammonium ions (without the whole nitrification process), but honestly for A level purposes its best not to worry about this.

I'm sure you'd be then wondering why not just absorb ammonia in the first place?
The answer is probably something to do with the electrically charged nature of ammonium/nitrates vs ammonia, since both ammonium and nitrates are charged compared to a neutral molecule of ammonia. It means that an electrical gradient can be used to help transport them into the plant (something like symport or antiport perhaps, since although nitrates are actively transported into roots, it could be coupled with the diffusion of some other ion in/out of the root).

Sorry the last paragraph is all my speculation, it might be true but then again who knows, and perhaps more importantly, I don't think our exam boards care!
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white_o
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(Original post by Spectral)
Yes, that's exactly right!

Just as a side point, once the nitrates have been absorbed they are, funnily enough, reconverted back into ammonia in order to synthesise amino acids.

Also some plants do indeed absorb a very small quantity of ammonium ions (without the whole nitrification process), but honestly for A level purposes its best not to worry about this.

I'm sure you'd be then wondering why not just absorb ammonia in the first place?
The answer is probably something to do with the electrically charged nature of ammonium/nitrates vs ammonia, since both ammonium and nitrates are charged compared to a neutral molecule of ammonia. It means that an electrical gradient can be used to help transport them into the plant (something like symport or antiport perhaps, since although nitrates are actively transported into roots, it could be coupled with the diffusion of some other ion in/out of the root).

Sorry the last paragraph is all my speculation, it might be true but then again who knows, and perhaps more importantly, I don't think our exam boards care!
Hahaa thanks very much
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