Wikipedia has a kind of contradiction that I would like to understand:

According to this link

The C3 pathway requires 18 ATP for the synthesis of one molecule of glucose while the C4 pathway requires 20 ATP. [...]

According to this other link:

[...]. The C3 pathway requires 18 molecules of ATP for the synthesis of one molecule of glucose, whereas the C4 pathway requires 30 molecules of ATP.[...]

Also, I don't understand much of this, I was just curious. I found this link (not sure if reduction also means generate glucose):

However, the C4 pathway of CO2 reduction expends more energy (5 ATP and 2 NADPH) than C3 pathway (3 ATP and 2 NADPH) (Hopkins 1999).

And this link:

ATP consumed in C4 plants
C4 cycle ( which involves regeneration of PEP from C3 acid) - requires 2 ATP per CO2 fixed.
C3 cycle ( which is common in C2,C3,C4 cycle)- requires 3 ATP per CO2 fixed.
Total - 5 ATP per CO2 fixed.
Thus to form a hexose or to fix 6 CO2 6*5= 30 ATP

All emphasis mine.

So... Does C4 requires 20 or 30 ATP per molecule of glucose? I think I found more evidence over 30 than 20, is that correct?


1 Answer 1


Yes,C4 plants use 30 ATP molecules to make one molecule of Glucose.See since you have stated in your question that you know very less about biology and got some similar vibes from looking at your profile so I'll sum it up in a nutshell but hopefully this might help.In flowering plants,all of them follow a basic pathway know as the C3 cycle to make glucose from CO2.Now,in that process about 18 ATP is used.But apart from CO2, there's O2 also present in the atmosphere which interferes with the compounds taking part in C3 cycle and instead of CO2 participation O2 participates in the reaction and the consequence is that glucose is not produced and the plant wastes some energy in the form of ATP.

Now to prevent that from happening,some plants evolved themselves and made some chemicals which bind only with CO2.But the thing is that they also require energy to function and for each molecule of glucose produced they require 12 ATP this making net amount of ATP required to be 30 in a C4 plant.You can in a way visualise this C4 pathway that the plant placed some guards in the form of chemicals that combines with CO2 and makes sure it reaches its destination and to prevent traitor O2 molecules to disrupt plants from making glucose.But we all know that that guards would require some fee and so that fee is 12ATP.And visualise C3 pathway as some some sort of glucose making factory which makes glucose from CO2 and the production cost from raw materials is 18 ATP.

  • $\begingroup$ Thanks for your answer. Since it seems that you know about this, could I make a not much related question? Does CAM photosynthesis is the same as C4 but instead of absorbing CO2 during the day it does during the night to avoid competitors (other plants) and lose of water due transpiring? $\endgroup$
    – Ender Look
    Commented Feb 7, 2019 at 17:41
  • $\begingroup$ See CAM pathway is found in xerophytic plants.Now,for eg: in deserts,there is a very high chances of water loss.So what they do is that they use some similar compounds as C4 plants and use it to store CO2 in the nighttime as there is chances of high water loss during daytime.Now in the day time,they use the stored CO2 to perform photosynthesis and manufacture glucose via the normal C3 pathway. $\endgroup$ Commented Feb 8, 2019 at 2:32

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