Q23

[mag2108]

Regarding: last passage of RC section (maize passage)

I find this whole passage challenging and had trouble with a lot of the questions, so any input on conceptualizing/processing/etc. the passage as a whole would be greatly appreciated.

Question 23: as I understand it, basically asks what process could replace (get same effect of) C4-photosynthesis in a plant which doesn't have the C4 thing. I found most of the answer choices confusing... can someone please explain this one?

[cyruswhittaker]

The passage seems rather innocuous at first, discussing how maize has been a very important crop. Then it proceeds with the question of why it has such high productivity. That leads us into paragraphs two and three, which go into the biochemical mechanism underlying this unique attribute.

A key problem faced by plants with regards to photosynthesis is described in paragraph two, more specifically in lines 30-35. Rubisco is a really important enzyme in photosynthesis, but oxygen causes a problem because it binds "competitively" (line 34) to rubisco, and as a result it interfers with the photosynthesis reaction.

So then in the next paragraph we get this technical description of "C-4 photosynthesis," which is just the name given to the particular process that some plants have evolved in order to remedy the problem noted above. So basically it's that they've evolved a way in which the oxygen doesn't bind competitively to the rubisco. That's the key point.

So (B) indicates this key point. If an enzyme could perform the same role as rubisco, and in which oxygen could not bind, then it would have the similair advantage attributed to plants utilizing C-4 photosynthesis. Remember, the whole problem here is that rubisco--a really important enzyme--is being hindered due to oxygen binding to it.

So I think question 23 simply tests the ability to pinpoint the key point in the topic and sift through all the technical jargon.

[bbirdwell]

I agree! Nice summary!

For better understanding of "technical" passage like this, focus on structural cues like line 36: "Some plants, however..." Those are very important words on the LSAT (Some ____, however), so pay special attention to what follows (ie "a mechanism that prevents oxygen from impairing photosynthesis).

So, for question 23, we need something else that would prevent oxygen from interfering. Why does it interfere? Because it binds to rubisco, etc....

[jones.mchandler]

I initially chose C, and upon reviewing was tempted to choose B, for all of the reasons given above.

However, I stuck with C (unfortunately). I'm having trouble eliminating C. C-4 plants are successful because "oxygen and all other atmospheric gases are excluded from the cells containing rubisco...[which] surround the vascular structures of the leaf." C seems to do a pretty good job of imitating that key element of C-4 plants. Although C doesn't state that all atmospheric gases are excluded, it does say that oxygen is, which is the gas that needs to be excluded in order for the C-4 plants to be more successful than non C-4 plants.

Maybe I'm missing the importance of "all other atmospheric gases" but it seems that the only implication of that statement within the passage is that CO2 gas must be converted to C-4 to bind with rubisco.

So why is B wrong?

[christine.defenbaugh]

I initially chose C, and upon reviewing was tempted to choose B, for all of the reasons given above.

However, I stuck with C (unfortunately). I'm having trouble eliminating C. C-4 plants are successful because "oxygen and all other atmospheric gases are excluded from the cells containing rubisco...[which] surround the vascular structures of the leaf." C seems to do a pretty good job of imitating that key element of C-4 plants. Although C doesn't state that all atmospheric gases are excluded, it does say that oxygen is, which is the gas that needs to be excluded in order for the C-4 plants to be more successful than non C-4 plants.

Maybe I'm missing the importance of "all other atmospheric gases" but it seems that the only implication of that statement within the passage is that CO2 gas must be converted to C-4 to bind with rubisco.

So why is B wrong?

Thanks for posting, jones.mchandler! This is a tough question!

Since the question is asking us what would give the same advantage as C-4 photosynthesis, it's critical to first identify exactly what the advantage is. As cyruswhittaker notes above, the major advantage is outlined in paragraph 3. That advantage is that it avoids the problem raised in paragraph 2 (oxygen getting in the way and screwing up the rubisco).

C-4 plants do this by isolating the rubisco and making sure the oxygen can't get to it. All the other stuff about converting CO2 to C-4 is just to explain how they can still get the photosynthesis job (which requires CO2) done even though the cells are air-tight. The core of the advantage is keeping the oxygen and rubisco away from each other.

And you're right, that if the cells that contained rubisco were impermeable to both CO2 and oxygen, that would do the exact same thing! But is (C) talking about the cells that contain rubisco? It says "vascular structures". Is that where the rubisco is?

To find out, let's see what the passage has to say about the "vascular structures". The only time they are mentioned is in a single sentence, lines 47-50. "...bundle sheath cells, surround the vascular structures of the leaf -- structures that function analogously to human blood vessels."

So, that doesn't tell us that the rubisco is in the vascular structures - in fact, at least in maize, the rubisco is in cells that surround the vascular structures. Making the vascular structures themselves air-tight wouldn't necessarily keep the rubisco and oxygen away from one another, if the rubisco is in some other cell!

(B), however, sidesteps the entire problem of quarantining the rubisco. Instead, this plant is just going to use an entirely different enzyme to do the exact same job, but this time, we won't have to worry about oxygen messing it all up, since oxygen won't bind to the new enzyme!

While we're here, let's take a quick spin through the other incorrect answer choices:

(A)It doesn't matter where we split the water unless we are keeping that separate somehow from the rubisco.

(D)Again, it doesn't matter where the the cells are that split the water unless the rubisco is being kept safely somewhere else.

(E)An enzyme not readily reacting with CO2 kind of seems like bad news for photosynthesis, but even if that were somehow okay, what about the oxygen!? If oxygen will still bind with this enzyme and mess up the process, then we won't have the advantage.

It looks like your thought process started along the right track -- you were looking for a way to keep the rubisco and the oxygen apart. But watch out! Making those vascular structures air-tight would only help if the rubisco were inside them!

Please let me know if this completely answers your question!