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Plant-A has on average N chloroplasts per cell. Plant-B has on average M chloroplasts per cell. If N > M, is plant-A more efficient in turning carbon dioxide into oxygen? Also, is it possible to see chloroplasts with a standard bright-field microscope?

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You can say that but chloroplasts do not have uniform morphology across different species. Moreover some organisms such as red algae have chloroplasts of different origin.

Real indicator would be lets say number of chloroplast ATP synthases and/or light harvesting photosystems; these can be perhaps indirectly approximated by the total surface area of the thylakoids.

But for all practical purposes and comparison between not so distant species density of chloroplasts is a decent enough approximate for photosynthetic power.

About turning carbon dioxide to oxygen:

You should note that the source of oxygen is not CO2. Oxygen comes from photolysis of water; this is the initial step in the electron transport chain (ETC) in chloroplasts. The ETC results in production of ATP and NADPH; the latter is used in the Calvin-Benson cycle to fix CO2 to sugar.

So photosynthesis and carbon fixation, though associated in chloroplasts are basically different biochemical processes. Theoretically as long as NADPH is provided a plant should be able to fix CO2 [this is just my guess].

It is difficult to observe internal structure of chloroplasts with normal bright-field microscope but you can count them easily at 40x magnification. See here.

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As @wysiwig already pointed out the different morphology of chloroplasts is something that is hard to come by. This influences the amount of chlorophyll in these organelles which is the key for photosynthesis. So it is very difficult (to impossible) to compare chloroplasts of different plants as they differ pretty much. There is one paper from 1929 which goes into detail here:

However, there is a paper which studies the photosynthesis rate in connection to the number of chloroplasts when manganese ions are a limiting factor. Manganese is part of the active center of the Photosystem II in the photosynthesis. So without maganese, photosynthesis is not possible.

The article finds the number of chloroplasts in cells approximately reduced to the half compared with leafs without maganese-deficiency. Testing the photosynthesis capacity of these leafs, they find that the photosynthetic activity in the manganese-deficient leaves is reduced by about 40%, which fits nicely to the reduction in chloroplast number. They do not find other significant changes in the biochemistry of the photosynthesis in these plants, so is the ratio between chlorophyll a and b not affected, and also the ability to transfer electrons is not affected. The findings are presented in this paper:

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