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Complete Question Explanation

The correct answer choice is (E).

Answer choice (A):

Answer choice (B):

Answer choice (C):

Answer choice (D):

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

This explanation is still in progress. Please post any questions below!
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Although I understood the concept, I picked answer choice C instead of E. I was steered away from answer choice E because it did not specify if the substance in question was harmful or not. How come Answer C is not right? Thank you.
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Hi paho1937!

The lines the question stem refers to state: "the cells would behave as if they were swimming up a concentration gradient, with long, smooth runs and relatively few tumbles."

We can look to the second paragraph for more illumination on what this means: "One clue comes from the observation that when a chemical attractant is added to a suspension of such bacteria, the bacteria swim along a gradient of the attractant, from an area where the concentration of the attractant is weaker to an area where it is stronger. As they do so, their swimming is characterized by a decrease in tumbling and an increase in straight runs over relatively longer distances. As the bacteria encounter increasing concentrations of the attractant, their tendency to tumble is suppressed, whereas tumbling increases whenever they move away from the attractant. "

Based on this textual evidence, the specified lines seem to be describing bacteria as swimming as though they were moving from a weaker concentration to a stronger concentration.

I agree, I'd like (E) better if it said that they were swimming from an area where concentration of an attractant was weaker to one where it was stronger. But it's still a much better answer choice than (C) because the passage doesn't give us any description of how bacteria swim when they are swimming away from something that is harmful to them.

Hope this helps!

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I wanted to ask why D would be wrong. I didn't choose C because it talks about the presence rather than the concentration, which is the main focus. I didn't choose E because of the same reason paho mentioned. It didn't say whether it was attractant or a repellent. Like you mentioned, it would have been a much better and 100% accurate answer if it included "attractant".

However, that is exactly what D does except it's the other way around. In the first paragraph, the passage says "They are attracted to materials they need and are repelled by harmful substances. ". So we can infer that just as the bacteria move towards the attractant, they move away from repellent.

The question asks "the author means they were behaving as if they were swimming". If the bacteria is moving away from higher to lower concentrations of repellent (weaker to absence is still higher concentration to lower concentration), it would behave as if they were swimming from lower to higher concentrations of attractant with a few tumbles (which is actually true). I thought this was a reasonable inference from the given information and E seemed to be missing critical information, which is whether the substance was an attractant or a repellent, that gave me enough reason to eliminate it.

Could you help me find my mistake and why E is accurate, let alone the best answer?

The only way I could see why E is an answer is if I were to look at the quoted phrase "swimming up a concentration gradient" by itself without looking at the context in the passage.

Swimming up a concentration gradient = moving from lower concentration of any substance to higher concentration

However, because the question was asking "as if", I don't see why E would be wrong. Also, the statement from E, "from an area where the concentration of a substance is weaker to an area where it is stronger", would be wrong as I could argue that bacteria would not behave like the bacteria in the context if the substance happens to be a repellent, not an attractant.

I'm not sure when to look at the context and when to look solely at the quoted phrase. :cry:
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 Ryan Twomey
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Hey JoshuaDel,

So first we had to understand what a high concentration gradient is, which was defined ten lines prior roughly. A high concentration gradient is where a higher concentration of an attractant is at the front of the cell than at the back of the cell.

So in our rephrase, we were looking for something that said there would be a higher concentration of an attractant where the bacteria were headed than where the bacteria just left.

The work in this question comes after reading the question and then going back to the passage. You should have an exact prephrase here before going to the answer choices. The prephrase should be going from a weaker area of attractant to a stronger area of attractant. I would suggest reading the paragraph again to get this prephrase, but specifically starting with the word "first" and ending with "tumbles."

E matches our prephrase exactly whereas answer choice D does not. Answer choice D discusses repellants which was not involved in the concentration gradient discussion. Answer choice E goes from weak to strong attractant which would be a high concentration gradient, which is what we were looking for.

This is one of those semi-rare instances in reading comp where you need a really good prephrase to get the question right.

I hope all this helps and I wish you all the luck in your studies.


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