BethMay17
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Like I get that the small amounts of ATP produced in cyclic photophosphorylation is used in the light-independent reaction

But non-cyclic produces ATP too, and more of it? It also produces the reduced NADP for the light-independent reaction too?

Is cyclic photophosphorylation just a backup then?
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Jpw1097
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(Original post by BethMay17)
Like I get that the small amounts of ATP produced in cyclic photophosphorylation is used in the light-independent reaction

But non-cyclic produces ATP too, and more of it? It also produces the reduced NADP for the light-independent reaction too?

Is cyclic photophosphorylation just a backup then?
Cyclic photophosphorylation only uses photosystem I (P700) whereas non-cyclic photophosphorylation uses both photosystem I and photosystem II (P680).. Cyclic photophosphorylation may occur when the incident light has a wavelength of light which is greater than 680nm as photosystem II will no longer be activated as PS II only absorbs wavelengths of light up to 680nm. As such, only PS I will be activated and so only cyclic photophosphorylation will occur. As you've correctly said, non-cyclic photophosphorylation does not produce any NADPH. As far as I'm aware, they produce the same amount of ATP as they use the same electron transport chain (although I could be wrong!).

Hope that helps.
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Spectral
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Interesting question!

My usual disclaimer of 'I'm an A level student and so this is technically beyond my A level specification knowledge'I'll just add a bit more information:

1. Non-cyclic and cyclic photophosphorylation can occur simultaneously (just in different complexes at the same time)
2. The proportion of each will really depend on the ATP required in the Calvin Cycle
3. When there is high demand for ATP the proportion of cyclic photophosphorylation will increase to provide ATP (yes, it is true that they (probably) provide the same 'amount' of ATP per cycle, but the rate of ATP production with cyclic photophosphorylation is probably greater since it is an overall much shorter process.)
4. I would say also with limited NADP+ it would make sense for the plant to switch over the proportions to favour cyclic-photophosphorylation to 'make the most out of' the light available.

I also wonder that since water (photolysis) isn't required in cyclic photophosphorylation, it could be another nifty bonus when water is scarce. (Although perhaps more likely it was significant a long time ago since there are indeed photosynthetic bacteria that only have one photosystem so only participate in cyclic-photophosphorylation).

Hope my (rather limited) input helped a tiny bit!
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BethMay17
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(Original post by Jpw1097)
Cyclic photophosphorylation may occur when the incident light has a wavelength of light which is greater than 680nm as photosystem II will no longer be activated as PS II only absorbs wavelengths of light up to 680nm.
So this means that light energy with a wavelength of 700nm must hit PSI only for cyclic photophosphorylation to take place. If a wavelength of light of 680nm hit PSII and a wavelength of light at 700nm hit PSI then non-cyclic would take place?
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BethMay17
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(Original post by Spectral)
Interesting question!

I also wonder that since water (photolysis) isn't required in cyclic photophosphorylation, it could be another nifty bonus when water is scarce. (Although perhaps more likely it was significant a long time ago since there are indeed photosynthetic bacteria that only have one photosystem so only participate in cyclic-photophosphorylation).
This is an interesting point that I hadn't considered. It does seem possible that cyclic photophosphorylation could be a trick to aid survival by generating ATP even when water is absent or limited.

I wish the textbook would cover this stuff in more detail. It can get frustrating when you want to know why something exists/happens but it just encourages you to accept it and move on. Especially since it's entirely possible they could ask an application question on this kind of thing in the exam.
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Jpw1097
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(Original post by BethMay17)
So this means that light energy with a wavelength of 700nm must hit PSI only for cyclic photophosphorylation to take place. If a wavelength of light of 680nm hit PSII and a wavelength of light at 700nm hit PSI then non-cyclic would take place?
I think that's the idea. I like the idea about there being a lack of water. Another idea could be that perhaps there isn't enough CO2 present. Without sufficient CO2, the rate of the Calvin cycle would fall and so less NADPH would be oxidisied to NADP+ and so less NADP+ would be available to act as the final electron acceptor in non-cyclic photophosphorylation, meaning less non-cyclic photophosphorylation can take place. It's similar to anaerobic respiration in the sense that the electron transport chain oxidises NADH to NAD+ which replenishes NAD+ so that glycolysis can continue (with the electron transport chain being analogous to the Calvin cycle). Without the electron transport chain, such as in anaerobic respiration when there is not enough oxygen, there is not enough NADH being oxidised back to NAD+ to be used in glycolysis and so lactate fermentation must occur to replenish NAD+. So cyclic photophosphorylation may be somewhat similar to lactate fermentation in the sense that it allows ATP to be produced without the need for further metabolic pathways. So instead of there being a lack of oxygen and so NADH cannot be oxidised to NAD+ electron transport chain, instead there is a lack of carbon dioxide and so NADPH cannot be oxidised to NAPD+ in the Calvin cycle.

So I take it cyclic photophosphorylation would be used when the conditions do not favour non-cyclic photophosphorylation either because there is a lack of light with wavelength less than 680nm, there is a lack of water (and so photolysis cannot occur) or because there is a lack of CO2 (and so the rate of the Calvin cycle falls and so NADPH is not being oxidised quickly enough so there is not enough NAP+ available for non-cyclic photophosphorylation).
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Jpw1097
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(Original post by BethMay17)
Like I get that the small amounts of ATP produced in cyclic photophosphorylation is used in the light-independent reaction

But non-cyclic produces ATP too, and more of it? It also produces the reduced NADP for the light-independent reaction too?

Is cyclic photophosphorylation just a backup then?
I also just read that plants may switch to cyclic photophosphorylation if ATP in the Calvin cycle is running low, to replenish ATP.
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