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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