#23 - Concept Reference, Must Be True

[Administrator]

Complete Question Explanation

The correct answer choice is (B).

Answer choice (A):

Answer choice (B): This is the correct answer choice.

Answer choice (C):

Answer choice (D):

Answer choice (E):

This explanation is still in progress. Please post any questions below!

[dimi.wassef@yahoo.com]

Could anyone explain question 23? Thank you!!

[Robert Carroll]

dimi.wassef,

We need to identify what advantage C-4 photosynthesis has, according to the last paragraph. The end of the second paragraph identifies a problem that can arise for photosynthesis: if oxygen levels get too high, oxygen binds completely to rubisco, interfering with photosynthesis. The process of C-4 photosynthesis in the last paragraph shows how maize and other plants avoid this problem. Those plants do so by sequestering rubisco in airtight cells, importantly keeping oxygen away from rubisco. If we're going to get a similar advantage in the answer choice, like this question asks, we probably want a similar way to keep oxygen away from rubisco.

Answer choice (A) does no such thing. This aspect of C-4 photosynthesis doesn't seem to contain its advantages - it says in the third sentence of the last paragraph that sequestering rubisco is an additional process to what answer choice (A) is talking about, so this idea alone will not replicate the benefits of C-4 photosynthesis.

Answer choice (B) is great. The replacement enzyme for rubisco doesn't bind with oxygen, so the danger of binding in the last sentence of the second paragraph will be avoided.

Answer choice (C) isn't very good - we actually want carbon. C-4 photosynthesis converts carbon dioxide into carbon, but needs to convert it to carbon dioxide again to use it. This answer doesn't really seem to be a good replacement.

Answer choice (D) involves one part of the C-4 photosynthesis process that doesn't even seem particularly related to the problem it's trying to solve.

Answer choice (E) is not relevant. Reacting with carbon dioxide is not a problem to be avoided.

Robert Carroll

[dimi.wassef@yahoo.com]

Thank you! Do we know why Maize is the most productive crop even though rice and corn are both part of the C4 group?

[Robert Carroll]

dimi.wassef,

That doesn't seem relevant to this question, but I also don't see where the passage says that maize is more productive than other C-4 crops. It does say that ancient agriculturists could use maize to produce more food than with any other crop, but I don't think that proves that maize is still alone as the most productive. In any case, as I said, I don't see how it's relevant to the question.

Robert Carroll

[dimi.wassef@yahoo.com]

Hi,

I am referring to the author's question at the end of the first paragraph, which asks why maize has been more productive than any other crop including rice and other C4 crops? I don't feel that question is ever answered by the author.

[Adam Tyson]

Hey Dimi, read that last line again and you'll see that the author is not placing maize alone at the top of the productivity scale, but rather is saying that maize "and a few similar crops" are more bountiful than others. So maize is part of an especially productive group of crops, rather than being alone.

A different question would be "why did the ancient agriculturalists have more success with maize than with anything else?" The answer there surely has something to do with the biochemical stuff discussed in the rest of the passage, but may also be due to factors like water and temperature that may have made sugar cane, rice, and other crops less successful than maize despite their similarities. You're right that the passage never answered that particular question, but it also never asked that question, so no big deal that it was left unanswered!