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The following is a minor clarification that I want to make, since it is rarely addressed directly in most of the texts I have gone through.

In $C_4$ pathway, the chloroplasts are dimorphic, that is, the bundle sheath chloroplasts contain RuBisCO, but lack grana and can hence perform only the light-independent reactions. The mesophyll chloroplasts lack RuBisCO but contain the granal apparatus to perform light reactions for synthesis of ATP and NADPH. The bundle sheath cells are impervious to gaseous transport (to prevent the entry of $\ce{O2}$), but recieve the required $\ce{CO2}$ by the intake of an organic acid produced by $\ce{CO2}$ fixation by the mesophyll cells.

But, to effect the Calvin cycle in the bundle sheath cells, it requires a high concentration of the light reaction products, i.e., ATP and NADPH. Since they themselves lack grana, they must obtain the required share from elsewhere. Where do they obtain it from?

My guess is that NADPH enters the bundle sheath cell with the organic acid primarily produced in the mesophyll cells where light reaxction occurs. ATP is met by a similar uptake plus the utilisation of the ATP produced by respiration. I need to confirm this as it is usually never addressed in texts.

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2 Answers 2

I am not sure about the source of ATP but I can tell you something about NADPH.

The conversion of malate to pyruvate and CO2 by malic enzyme is carried out in the bundle sheath cell. This process produces NADPH.

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See the very 1st thing is bundle sheath cell shows very less concentration of Grana, hence we say Grana is absent, the required amount of ATP & NADPH2 are also transported from mesophyll cell to bundle sheath cell via plasmodesmata.

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