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#17 - Concept Reference, Must Be True

lathlee
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the back answer says the final answer is E)
but it is not supported by the text.

Line 23-31: 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. the net effect is that runs in the direction of higher concentrations of the attractant become longer and straighter as a result of the suppression of tumbling, whereas runs away from it are shortened by an increased tendency of the bacteria to tumble and change direction.

E) says exhibited an increased tendency to tumble as it encountered increasing concentrations of the substance, and suppressed tumbling as it detected decreases in the concentration of the substance.
Shannon Parker
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lathlee wrote:the back answer says the final answer is E)
but it is not supported by the text.

Line 23-31: 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. the net effect is that runs in the direction of higher concentrations of the attractant become longer and straighter as a result of the suppression of tumbling, whereas runs away from it are shortened by an increased tendency of the bacteria to tumble and change direction.

E) says exhibited an increased tendency to tumble as it encountered increasing concentrations of the substance, and suppressed tumbling as it detected decreases in the concentration of the substance.


hello,

Line 5 tells us that bacteria are repelled by harmful substances. As you point out, Lines 23-31 tell us that when bacteria suppress tumbling, their runs become longer and straighter, and the runs become shortened and there are increased changes of direction as tumbling is increased. If the bacteria is moving in a direction away from the harmful substance, it is more likely to move away from it if the runs are longer and straighter.

If the bacteria slows down and changes direction when it encounters increased concentrations of the substance, and speeds up and stops changing directions as it encounters decreased concentrations of the substance it is more likely to move away from it.

Therefore, if tumbling is increased as it encounters increased amounts of the substance, and decreased as it encounters decreased amounts of the substance, it will be likely to move away.
bk1111
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Hi, can someone please explain how I can logically deduct the correct answer, E? I read the explanation above but I do not fully understand how that can be inferred. The only fact mentioned about harmful substances is in Line 5 and that they move away from it. I don't understand how to infer the behavior of the bacterium if there are large instances of it.
Daniel Stern
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Hi BK:

So this answer requires us to combine knowledge from a few places in the passage:

Line 5 tells us that bacteria move away from harmful substances; they are repelled by the harmful substances.

Lines 6-10 tell us how bacteria move: they move straight for a while and then "tumble" in a random direction.

If they change direction randomly, how can they get towards substances that are beneficial, such as food, and away from the bad substances?

Lines 23-31 tell us how bacteria are able to move towards substances that they are attracted to, such as food: when they get close to the attracting substance, their tendency to tumble, or move in a different random direction, is suppressed, which increases the chance that they'll move in the same direction.

Question 17 asks us to infer a mechanism for how the bacteria react to harmful substances, and answer E essentially provides us with the opposite of how the passage explains the bacteria get towards the attractive answers: instead of suppressing the tumbles, a big concentration of harmful substances increases the chance of a tumble, and therefore increases the chance the bacteria will move away from the bad stuff. Once the bacteria is far away, in an area of lesser concentration of the harmful substance, the tumbling is diminished, meaning it is more likely that the bacteria will keep moving away.

Good luck in your studies!
Dan
deck1134
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Hi PowerScore Staff,

I hope you are doing well! I'm somewhat confused on question 17.

In attacking this question, I was able to narrow it down to B and E.

It seems to me like B is true, and can be inferred. AS mentioned in the above discussion, the passage talks about how unicellular organisms are repelled to harmful substances. But it also talks about how they are attracted to positive forces. If the organism finds a concentration gradient of something that it is attracted to, the passage clearly indicates that it will move towards that substance. That means, as I see it, that it will move away from its current location (where the substance is high)

I ended up not choosing E because it is not supported, except by conjecture from 5, 26-30, and 50ish. I am at a loss here and would appreciate your help. It seems as if to draw E you have to say that it would tumble more with more harmful substances, and would move towards attractive substances, thereby proving B as well. Thanks!
Adam Tyson
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Looks like you are assuming that the concentration gradient of the attractant would have to take the bacterium away from the harmful substance, and that may not be true. What if that concentration gradient pointed the way through or along the harmful substance instead of away from it? Like seeing the lights of the football stadium you are on your way to and driving straight through a dangerous section of town to get to it, instead of going around? B certainly seems like a contender, but once you realize that you are making that assumption - that towards the good thing must mean away from the bad - you'll realize that answer is not as good as it at first seemed.

I think the best evidence for this is begins in line 20, where we learn that the bacteria move faster towards attractants when they stop tumbling and start going in long, straight lines. Tumbling is slow, straight lines are faster, according to that information, so if we apply that concept to moving away from harmful substances, they would likely move away faster if they went in straight lines when they determined that they were moving towards a decreasing amount of the harmful substance and tumbled when they discovered that they were heading into more of it. Run as you get closer to the safe zone; slow down and change direction if you are heading into trouble!
Adam M. Tyson
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