This is from a Singapore-Cambridge GCE O-level Biology exam (a national exam for 16-year-olds). The exam board maintains that the question is "correct and accurate"—but isn't there a typo in option C?

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Here's my argument:

  • Merely increasing the light intensity at either Points 1 or 2 speeds up photosynthesis, but varying the light intensity at Points 3 and 4 doesn't.

    Therefore, light intensity is limiting photosynthesis at only Points 1 & 2.

  • At Point 3, merely doubling the $\ce{CO2}$ concentration speeds up photosynthesis; on the other hand, the graph shows no evidence that varying the $\ce{CO2}$ concentrations at Points 1, 2 or 4 speeds up photosynthesis.

    Therefore, $\ce{CO2}$ concentration is limiting photosynthesis conclusively at only Point 3.

As this is a data-inference question, all the four answer choices are invalid deductions.

(Asserting that $\ce{CO2}$ concentration is a limiting factor at Point 4 requires assuming that raising $\ce{CO2}$ concentration beyond 0.08% continues speeding up photosynthesis.)

Excluding Point 4 from Option C will render this question finally answerable.

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    $\begingroup$ With your logic wouldn't that also mean that point 3 requires assuming that decreasing the carbon dioxide concentration would decrease photosynthesis rate? In statistics when you want to test if one parameter affects another it's sufficient to test a high and a low point. $\endgroup$ – Cell Aug 24 '20 at 15:30
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    $\begingroup$ @Cell No--because it stands to reason that $\ce{CO2}$ concentration is limiting photosynthesis in a given setup if varying it (in either direction) speeds up photosynthesis. $\endgroup$ – user61462 Aug 24 '20 at 18:00
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    $\begingroup$ @Cell $\ce{CO2}$ concentration is said to be limiting photosynthesis in a given setup as long as increasing (or decreasing, for that matter) it there speeds up photosynthesis. In other words, the fact that increasing $\ce{CO2}$ at point 3 increases photosynthesis is sufficient to assert that $\ce{CO2}$ is a limiting factor there -- regardless of whether decreasing $\ce{CO2}$ there increases or decreases photosynthesis. $\endgroup$ – user61462 Aug 24 '20 at 22:10
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    $\begingroup$ So, why do you think that logic doesn't apply at point 4? $\endgroup$ – tyersome Aug 24 '20 at 23:24
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    $\begingroup$ @tyersome The graph displays no faster photosynthesis than at point 4. $\endgroup$ – user61462 Aug 25 '20 at 0:53

I think your thinking is fine, but you're being a bit pedantic about it. I don't think there is any typo.

The purpose of the question is for you to understand that there are two limiting factors here: light and CO2.

You can identify points in the graph where light is limiting because there is a slope with increasing light, and you can identify points in the graph where CO2 is limiting because light no longer increases the rate, and that the plateau level changes with different CO2 levels.

Yes, you are right, there could be other yet-unrevealed-to-you limiting factors here, but there is nothing in the graph to show this. You would need to see another set of lines where increasing CO2 further does not change the plateau level, and/or where the shape changes.

Chalk it up to Occam's razor. Without further information besides what is on this graph, if I asked you to draw your prediction about what the line would look like at 0.12% CO2, you should draw another line that follows the other and then plateaus at a third, higher level.


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