Q17

[geverett]

I see why E is the answer. Can someone definitively disprove B for me?

[rdunley417]

Matt I think you have the wrong passage. This is the third passage of PT5 about bacteria and concentration gradients. I also don't see why e is necessarily better than b. both seem like they could be right.

[ManhattanPrepLSAT1]

Thanks rdunley417! I had Q17 from S3 (LR), not the 3rd passage. I've moved my LR explanation to the correct place. Now to explain this one.

Tricky, but answer choice (B) would not actually indicate that the bacteria would move away from the area where the harmful substance is high. These bacteria don't actually seem to be able to control the direction they move in. Instead the move in somewhat erratic directions (lines 5-9). They seem to tumble more when they detect less attractant or more harmful substances and tumble less when the reverse is true (lines 23-32).

Correct Answer
So to move away from the harmful substance the bateria would need to tumble more as the concentration of the harmful substance increased and tumble less as the concentration of the harmful substance decreased - nicely expressed in answer choice (E).

Incorrect Answers
(A) is out of scope. The issue of speed is not mentioned in the passage.
(B) might trigger them to tumble less. The bacteria cannot actually control the direction in which they swim.
(C) is contradicted. The passage states that the bacteria would not respond to changes in the concentration of a chemical attractant (and probably harmful substances as well).
(D) contradicts the passage. This would lead to not changing directions and thus to not moving out of the area.

[WaltGrace1983]

Are we supposed to assume that moving away from the a harmful substances is analogous with moving closer to a more attractive substance? I got my support for #17 from lines 23-32. Was this the correct way to attack the problem?

I initially picked (D) but I realize that not only is it much too strong (we don't know if they ever completely get rid of the tumbling) but also because it was actually the opposite of what I wanted..."Tumbling increases whenever they move away from the attractant" (26).

[christine.defenbaugh]

Are we supposed to assume that moving away from the a harmful substances is analogous with moving closer to a more attractive substance? I got my support for #17 from lines 23-32. Was this the correct way to attack the problem?

Absolutely the correct way to attack the problem! Those lines describe how bacteria end up moving toward something, so we can flip it around to figure out how they would potentially move away from something.

We don't quite have to go so far as to think that attractive and harmful substances are themselves analogous, though. The critical thing is to realize that we've been given a lot of information about how they manage to move toward something, and that means we actually have that same information at our disposal to figure out how they would move away from something else. In other words, we ARE expected to be able to see that moving towards something and moving away from something would accomplished by reciprocal physical mechanisms.

I initially picked (D) but I realize that not only is it much too strong (we don't know if they ever completely get rid of the tumbling) but also because it was actually the opposite of what I wanted..."Tumbling increases whenever they move away from the attractant" (26).

Great work detecting what you did wrong in choosing (D)!