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martin_r

C3 and C4 photosynthesis

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Posted (edited)

Hello,

i am looking for a plant-biology expert,

what would you say ? Which one is more complex?  

C3 or C4 photosynthesis ?

I had a debate with a guy who has an education in plant biology, and i was told that C4 photosynthesis is a sort of C3-downgrade ...

i was told, that the C4 excluded a very complex photo-respiration step,   therefore,  the C4 is less complex, it is a 'C3 downgrade' and easy to evolve 60+ times over and over (by convergent evolution)

However, from what i could understand, the C4 is more complex, because there are newly evolved steps, newly evolved specialized cell anatomy, which seems to be more complex...

So which one is more complex? Could the C4 be considered as a C3-downgrade ?

Could some plant-biology expert comment on this?


Thank you!

 


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Edited by martin_r

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For the most part such discussions are a bit meaningless or perhaps, are only meaningful in very specific contexts. For example, the view of C4 as a downgrade does not make a lot of sense in most contexts. For starters, C4 is a newer development, records place it ca. 24-35 million years ago. However, it is also incredibly divergent and arose more than 40 times in over a dozen families of angiosperms. Many (predominantly dicots) were rather recent additions (< 5 million years). So from a historic sense it is simply an adaptation to low CO2 conditions (as Rubisco also uses O2 and thereby at low CO2 levels carbon fixation efficiency is low).

A second aspect is that C4 pathway is not a singular pathway, rather it represents a range of adaptations, including structural ones that use a series of enzymes to concentrate CO2 locally to enhance the actions of Rubisco. To this end there are also structural changes to form the Rubisco-and CO2-rich compartment. However, the net fixation of CO2 still depends on the same enzyme as C3 photosynthesis (i.e. Rubisco). So it cannot evolve easily as it really needs established C3 mechanisms as basis. I think it is better to think it is a modification to deal with increasing O2 concentrations (in geological terms), essentially by adding a layer on top of the basic C3 pathways, which are fundamentally integrated into plant metabolism. If you really wanted to create a hierarchy one would generally consider C4 photosynthesis as more complex, not because it evolved later, but rather due to the added functional and structural layers.

Note that this is based on introductory level of knowledge, I am not a plant researcher (though have some familiarity with metabolic pathways and their consequences as a whole).

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Charon,


could you briefly comment on this ?

"i was told, that the C4 excluded a very complex photo-respiration step,   therefore,  the C4 is less complex ..."  




 

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Which one? There are many steps and some variation...

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Charon,

perhaps you don't entirely understand my question,   

in my debate,  the other guy's argument was, that C3 photo-respiration is a set of very complex processes.

C4 photosynthesis excluded these very complex processes of photo-respiration, the photo-respiration is no longer needed in C4, so, the C4 is less complex than C3. 

Is this a good argument ? Can C4 photosynthesis be considered less complex ?






 




 

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7 hours ago, martin_r said:

Charon,

perhaps you don't entirely understand my question,   

in my debate,  the other guy's argument was, that C3 photo-respiration is a set of very complex processes.

C4 photosynthesis excluded these very complex processes of photo-respiration, the photo-respiration is no longer needed in C4, so, the C4 is less complex than C3. 

Is this a good argument ? Can C4 photosynthesis be considered less complex ?

That does not make a lot of sense. Photorespiration is predominantly the action of Rubisco where it uses O2 and produces CO2, thus undoing the fixation of the Calvin cycle. C4 is an adaptation to reduce this loss and therefore increase fixation with the C4 cycle. 

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Posted (edited)

Charon, are you saying, that the guy i debated, he doesn't know what he is talking about ?  And that his photo-respiration argument does not make sense in regards to "more or less complex" ?
I apologize, English is not my first language, so perhaps i misunderstood something...



 

Edited by martin_r

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In general complexity is difficult to assess. But if your description is accurate, it is especially incorrect. I would ask specifically for the pathways in question. And again photo respiration is generally not beneficial.

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