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Although bacteria are unicellular and among the simplest autonomous

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Although bacteria are unicellular and among the simplest autonomous  [#permalink]

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Updated on: 11 Oct 2019, 23:24
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New Project RC Butler 2019 - Practice 2 RC Passages Everyday
Passage # 118, Date : 31-MAR-2019
This post is a part of New Project RC Butler 2019. Click here for Details

Although bacteria are unicellular and among the simplest autonomous forms of life, they show a remarkable ability to sense their environment. They are attracted to materials they need and are repelled by harmful substances. Most types of bacteria swim very erratically: short smooth runs in relatively straight lines are followed by brief tumbles, after which the bacteria shoot off in random directions. This leaves researchers with the question of how such bacteria find their way to an attractant such as food or, in the case of photosynthetic bacteria, light, if their swimming pattern consists only of smooth runs and tumbles, the latter resulting in random changes in direction.

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. 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.

Biologists have proposed two mechanisms that bacteria might use in detecting changes in the concentration of a chemical attractant. First, a bacterium might compare the concentration of a chemical at the front and back of its cell body simultaneously. If the concentration is higher at the front of the cell, then it knows it is moving up the concentration gradient, from an area where the concentration is lower to an area where it is higher. Alternatively, it might measure the concentration at one instant and again after a brief interval, in which case the bacterium must retain a memory of the initial concentration. Researchers reasoned that if bacteria do compare concentrations at different times, then when suddenly exposed to a uniformly high concentration of an attractant, the cells would behave as if they were swimming up a concentration gradient, with long, smooth runs and relatively few tumbles. If, on the other hand, bacteria detect a chemical gradient by measuring it simultaneously at two distinct points, front and back, on the cell body, they would not respond to the jump in concentration because the concentration of the attractant in front and back of the cells, though high, would be uniform. Experimental evidence suggests that bacteria compare concentrations at different times.

1. It can be inferred from the passage that which one of the following experimental results would suggest that bacteria detect changes in the concentration of an attractant by measuring its concentration in front and back of the cell body simultaneously?

(A) When suddenly transferred from a medium in which the concentration of an attractant was uniformly low to one in which the concentration was uniformly high, the tendency of the bacteria to tumble and undergo random changes in direction increased.
(B) When suddenly transferred from a medium in which the concentration of an attractant was uniformly low to one in which the concentration was uniformly high, the bacteria’s exhibited no change in the pattern of their motion.
(C) When suddenly transferred from a medium in which the concentration of an attractant was uniformly low to one in which the concentration was uniformly high, the bacteria’s movement was characterized by a complete absence of tumbling.
(D) When placed in a medium in which the concentration of an attractant was in some areas low and in others high, the bacteria exhibited an increased tendency to tumble in those areas where the concentration of the attractant was high.
(E) When suddenly transferred from a medium in which the concentration of an attractant was uniformly low to one that was completely free of attractants, the bacteria exhibited a tendency to suppress tumbling and move in longer, straighter lines.

2. It can be inferred from the passage that a bacterium would increase the likelihood of its moving away from an area where the concentration of a harmful substance is high if it did which one of the following?

(A) Increased the speed at which it swam immediately after undergoing the random changes in direction that result from tumbling.
(B) Detected the concentration gradient of an attractant toward which it could begin to swim.
(C) Relied on the simultaneous measurement of the concentration of the substance in front and back of its body, rather than on the comparison of the concentration at different points in time.
(D) Exhibited a complete cessation of tumbling when it detected increases in the concentration of substance.
(E) 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.

3. It can be inferred from the passage that when describing bacteria as “swimming up a concentration gradient” (Highlighted), the author means that they were behaving as if they were swimming

(A) Against a resistant medium that makes their swimming less efficient.
(B) Away from a substance to which they are normally attracted.
(C) Away from a substance that is normally harmful to them.
(D) From an area where the concentration of a repellent is weaker to an area where it is completely absent.
(E) From an area where the concentration of a substance is weaker to an area where it is stronger.

4. The passage indicates that the pattern that characterizes a bacterium’s motion changes in response to

(A) The kinds of chemical attractants present in different concentration gradients.
(B) The mechanism that the bacterium adopts in determining the presence of an attractant.
(C) The bacterium’s detection of changes in the concentration of an attractant.
(D) The extent to which neighboring bacteria are engaged in tumbling.
(E) Changes in the intervals of time that occur between the bacterium’s measurement of the concentration of an attractant.

5. Which one of the following best describes the organization of the third paragraph of the passage?

(A) Two approaches to a problem are discussed, a test that would determine which is more efficient is described, and a conclusion is made, based on experimental evidence.
(B) Two hypotheses are described, a way of determining which of them is more likely to be true is discussed, and one said to be more accurate on the basis of experimental evidence.
(C) Two hypotheses are described, the flaws inherent in one of them are elaborated, and experimental evidence confirming the other is cited.
(D) An assertion that a species has adopted two different mechanisms to solve a particular problem is made, and evidence is then provided in support of that assertion.
(E) An assertion that one mechanism for solving a particular problem is more efficient than another is made, and evidence is then provided in support of that assertion.

6. The passage provides information in support of which one of the following assertions?

(A) The seemingly erratic motion exhibited by a microorganism can in fact reflect a mechanism by which it is able to control its movement.
(B) Biologists often overstate the complexity of simple organisms such as bacteria.
(C) A bacterium cannot normally retain a memory of a measurement of the concentration of an attractant.
(D) Bacteria now appear to have less control over their movement than biologists had previously hypothesized.
(E) Photosynthetic bacteria appear to have more control over their movement than do bacteria that are not photosynthetic.

• Source: LSAT Official PrepTest 5 (June 1992)
• Difficulty Level: 650

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MBA, Class of 2020,

Originally posted by Gnpth on 07 Apr 2015, 07:38.
Last edited by SajjadAhmad on 11 Oct 2019, 23:24, edited 3 times in total.
Updated - Complete topic (949).
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Although bacteria are unicellular and among the simplest autonomous  [#permalink]

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Updated on: 30 Nov 2019, 02:53
6
Hi everyone,
Got 5/6 correct in around 13 minutes.

----------------------------------------------------------------------------------------

P1

Paragraph one starts with a contrast: while unicellular bacteria (UB) are very small they have a sharp ability to sense the environment. They can be attracted by food and repelled by harmful substances.
Their movements consists on smooth runs and tumbles, the latter described as something random.
Their movements puzzle scientists who cannot explain how UB can reach food.

Brief summary: UB puzzle scientists because of their erratic movements

P2

Paragraph two attempts to explain the puzzling fact seen in P1. We can see that a bacteria when approaching an attractant increases its smooth runs (and decreases the tumblings) while when going away fro the attractant the tumbling increases (and the smooth runs decrease).

Brief summary: UB's behavior in presence of an attractant

P3

Paragraph three describes two methods that could be used by Ub to detect concentration of attractants. The first method is to perceive the concentration from the back and the from of the UB simultaneously. If it is perceived on the front side, for example, the UB will move with smooth runs in that direction.
On the other hand the second methods state that the UB could make measurements in different time frames. Then we are given that, if the first method is used and high concentrations of attractants are present both on the front and back side, the UB would not move.
Finally we are given that experiments suggest that the second method be used.

Brief summary: Two methods for detecting attractants concentrations are described and one is considered valid by experimental data

Main point

The main point is to investigate the behavior of UB when they move to attractants

----------------------------------------------------------------------------------------

1. It can be inferred from the passage that which one of the following experimental results would suggest that bacteria detect changes in the concentration of an attractant by measuring its concentration in front and back of the cell body simultaneously?

Pre-thinking

Inference question

Refer to the last paragraph.

"If, on the other hand, bacteria detect a chemical gradient by measuring it simultaneously at two distinct points, front and back, on the cell body, they would not respond to the jump in concentration because the concentration of the attractant in front and back of the cells, though high, would be uniform."

(A) When suddenly transferred from a medium in which the concentration of an attractant was uniformly low to one in which the concentration was uniformly high,the tendency of the bacteria to tumble and undergo random changes in direction increased.
As stated in pre-thinking the bacteria should not move

(B) When suddenly transferred from a medium in which the concentration of an attractant was uniformly low to one in which the concentration was uniformly high, the bacteria’s exhibited no change in the pattern of their motion.
In line with pre-thinking

(C) When suddenly transferred from a medium in which the concentration of an attractant was uniformly low to one in which the concentration was uniformly high, the bacteria’s movement was characterized by a complete absence of tumbling.
does the bacteria increases or decreases its smooth runs or does it stay put?

(D) When placed in a medium in which the concentration of an attractant was in some areas low and in others high, the bacteria exhibited an increasedtendency to tumble in those areas where the concentration of the attractant was high.
it should increases its smooth runs

(E) When suddenly transferred from a medium in which the concentration of an attractant was uniformly low to one that was completely free of attractants, the bacteria exhibited a tendency to suppress tumbling and move in longer, straighter lines.
Cannot be inferred

----------------------------------------------------------------------------------------

2. It can be inferred from the passage that a bacterium would increase the likelihood of its moving away from an area where the concentration of a harmful substance is high if it did which one of the following?

Pre-thinking

Inference question

From the second paragraph we know that as a UB goes away from an high concentration of an attractant its tumbling should increase. We can infer from this that when it moves away from a repellant it's tumbling decreases

(A) Increased the speed at which it swam immediately after undergoing the random changes in direction that result from tumbling.
Not in line with pre-thinking

(B) Detected the concentration gradient of an attractant toward which it could begin to swim.
Cannot be inferred

(C) Relied on the simultaneous measurement of the concentration of the substance in front and back of its body, rather than on the comparison of the concentration at different points in time.
Out of context. Mentioned in P3

(D) Exhibited a complete cessation of tumbling when it detected increases in the concentration of substance.
Opposite

(E) 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.
In line with pre-thinking

----------------------------------------------------------------------------------------

3. It can be inferred from the passage that when describing bacteria as “swimming up a concentration gradient” (Highlighted), the author means that they were behaving as if they were swimming

Pre-thinking

Inference question

Toward an attractant

(A) Against a resistant medium that makes their swimming less efficient.
Not in line with pre-thinking

(B) Away from a substance to which they are normally attracted.
Not in line with pre-thinking

(C) Away from a substance that is normally harmful to them.
Not in line with pre-thinking

(D) From an area where the concentration of a repellent is weaker to an area where it is completely absent.
Not in line with pre-thinking

(E) From an area where the concentration of a substance is weaker to an area where it is stronger.
in line with pre-thinking

----------------------------------------------------------------------------------------

4. The passage indicates that the pattern that characterizes a bacterium’s motion changes in response to

Pre-thinking

Detail question

The concentration of an attractant or repellant

(A) The kinds of chemical attractants present in different concentration gradients.
Not in line with pre-thinking

(B) The mechanism that the bacterium adopts in determining the presence of an attractant.
Not in line with pre-thinking

(C) The bacterium’s detection of changes in the concentration of an attractant.
in line with pre-thinking

(D) The extent to which neighboring bacteria are engaged in tumbling.
Not in line with pre-thinking

(E) Changes in the intervals of time that occur between the bacterium’s measurement of the concentration of an attractant.
Not in line with pre-thinking

----------------------------------------------------------------------------------------

5. Which one of the following best describes the organization of the third paragraph of the passage?

Pre-thinking

Structure question

Two methods are introduced and described----> experimental data suggest that one be the best

(A) Two approaches to a problem are discussed, a test that would determine which is more efficient is described, and a conclusion is made, based on experimental evidence.
Not in line with pre-thinking

(B) Two hypotheses are described, a way of determining which of them is more likely to be true is discussed, and one said to be more accurate on the basis of experimental evidence.
in line with pre-thinking

(C) Two hypotheses are described, the flaws inherent in one of them are elaborated, and experimental evidence confirming the other is cited.
Not in line with pre-thinking

(D) An assertion that a species has adopted two different mechanisms to solve a particular problem is made,and evidence is then provided in support of that assertion.
Not in line with pre-thinking

(E)An assertion that one mechanism for solving a particular problem is more efficient than another is made, and evidence is then provided in support of that assertion.
Not in line with pre-thinking

----------------------------------------------------------------------------------------

6. The passage provides information in support of which one of the following assertions?

Pre-thinking

Inference question

(A) The seemingly erratic motion exhibited by a microorganism can in fact reflect a mechanism by which it is able to control its movement.
supported

(B) Biologists often overstate the complexity of simple organisms such as bacteria.
Not in line with pre-thinking

(C) A bacterium cannot normally retain a memory of a measurement of the concentration of an attractant.
Not in line with pre-thinking

(D) Bacteria now appear to have less control over their movement than biologists had previously hypothesized.
Not in line with pre-thinking

(E) Photosynthetic bacteria appear to have more control over their movement than do bacteria that are not photosynthetic.
Not in line with pre-thinking

----------------------------------------------------------------------------------------

It is a good day to be alive

Originally posted by auradediligodo on 01 Nov 2019, 01:14.
Last edited by auradediligodo on 30 Nov 2019, 02:53, edited 2 times in total.
General Discussion
Intern
Joined: 23 Oct 2016
Posts: 9
Re: Although bacteria are unicellular and among the simplest autonomous  [#permalink]

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19 Nov 2019, 16:28
Hi GMATNinja,

Can you please explain Q1 and Q2?

Look forward,
Intern
Joined: 08 Jul 2019
Posts: 12
Re: Although bacteria are unicellular and among the simplest autonomous  [#permalink]

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14 Dec 2019, 20:50
Hi everyone,
Got 5/6 correct in around 13 minutes.

----------------------------------------------------------------------------------------

P1

Paragraph one starts with a contrast: while unicellular bacteria (UB) are very small they have a sharp ability to sense the environment. They can be attracted by food and repelled by harmful substances.
Their movements consists on smooth runs and tumbles, the latter described as something random.
Their movements puzzle scientists who cannot explain how UB can reach food.

Brief summary: UB puzzle scientists because of their erratic movements

P2

Paragraph two attempts to explain the puzzling fact seen in P1. We can see that a bacteria when approaching an attractant increases its smooth runs (and decreases the tumblings) while when going away fro the attractant the tumbling increases (and the smooth runs decrease).

Brief summary: UB's behavior in presence of an attractant

P3

Paragraph three describes two methods that could be used by Ub to detect concentration of attractants. The first method is to perceive the concentration from the back and the from of the UB simultaneously. If it is perceived on the front side, for example, the UB will move with smooth runs in that direction.
On the other hand the second methods state that the UB could make measurements in different time frames. Then we are given that, if the first method is used and high concentrations of attractants are present both on the front and back side, the UB would not move.
Finally we are given that experiments suggest that the second method be used.

Brief summary: Two methods for detecting attractants concentrations are described and one is considered valid by experimental data

Main point

The main point is to investigate the behavior of UB when they move to attractants

----------------------------------------------------------------------------------------

1. It can be inferred from the passage that which one of the following experimental results would suggest that bacteria detect changes in the concentration of an attractant by measuring its concentration in front and back of the cell body simultaneously?

Pre-thinking

Inference question

Refer to the last paragraph.

"If, on the other hand, bacteria detect a chemical gradient by measuring it simultaneously at two distinct points, front and back, on the cell body, they would not respond to the jump in concentration because the concentration of the attractant in front and back of the cells, though high, would be uniform."

(A) When suddenly transferred from a medium in which the concentration of an attractant was uniformly low to one in which the concentration was uniformly high,the tendency of the bacteria to tumble and undergo random changes in direction increased.
As stated in pre-thinking the bacteria should not move

(B) When suddenly transferred from a medium in which the concentration of an attractant was uniformly low to one in which the concentration was uniformly high, the bacteria’s exhibited no change in the pattern of their motion.
In line with pre-thinking

(C) When suddenly transferred from a medium in which the concentration of an attractant was uniformly low to one in which the concentration was uniformly high, the bacteria’s movement was characterized by a complete absence of tumbling.
does the bacteria increases or decreases its smooth runs or does it stay put?

(D) When placed in a medium in which the concentration of an attractant was in some areas low and in others high, the bacteria exhibited an increasedtendency to tumble in those areas where the concentration of the attractant was high.
it should increases its smooth runs

(E) When suddenly transferred from a medium in which the concentration of an attractant was uniformly low to one that was completely free of attractants, the bacteria exhibited a tendency to suppress tumbling and move in longer, straighter lines.
Cannot be inferred

----------------------------------------------------------------------------------------

2. It can be inferred from the passage that a bacterium would increase the likelihood of its moving away from an area where the concentration of a harmful substance is high if it did which one of the following?

Pre-thinking

Inference question

From the second paragraph we know that as a UB goes away from an high concentration of an attractant its tumbling should increase. We can infer from this that when it moves away from a repellant it's tumbling decreases

(A) Increased the speed at which it swam immediately after undergoing the random changes in direction that result from tumbling.
Not in line with pre-thinking

(B) Detected the concentration gradient of an attractant toward which it could begin to swim.
Cannot be inferred

(C) Relied on the simultaneous measurement of the concentration of the substance in front and back of its body, rather than on the comparison of the concentration at different points in time.
Out of context. Mentioned in P3

(D) Exhibited a complete cessation of tumbling when it detected increases in the concentration of substance.
Opposite

(E) 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.
In line with pre-thinking

----------------------------------------------------------------------------------------

3. It can be inferred from the passage that when describing bacteria as “swimming up a concentration gradient” (Highlighted), the author means that they were behaving as if they were swimming

Pre-thinking

Inference question

Toward an attractant

(A) Against a resistant medium that makes their swimming less efficient.
Not in line with pre-thinking

(B) Away from a substance to which they are normally attracted.
Not in line with pre-thinking

(C) Away from a substance that is normally harmful to them.
Not in line with pre-thinking

(D) From an area where the concentration of a repellent is weaker to an area where it is completely absent.
Not in line with pre-thinking

(E) From an area where the concentration of a substance is weaker to an area where it is stronger.
in line with pre-thinking

----------------------------------------------------------------------------------------

4. The passage indicates that the pattern that characterizes a bacterium’s motion changes in response to

Pre-thinking

Detail question

The concentration of an attractant or repellant

(A) The kinds of chemical attractants present in different concentration gradients.
Not in line with pre-thinking

(B) The mechanism that the bacterium adopts in determining the presence of an attractant.
Not in line with pre-thinking

(C) The bacterium’s detection of changes in the concentration of an attractant.
in line with pre-thinking

(D) The extent to which neighboring bacteria are engaged in tumbling.
Not in line with pre-thinking

(E) Changes in the intervals of time that occur between the bacterium’s measurement of the concentration of an attractant.
Not in line with pre-thinking

----------------------------------------------------------------------------------------

5. Which one of the following best describes the organization of the third paragraph of the passage?

Pre-thinking

Structure question

Two methods are introduced and described----> experimental data suggest that one be the best

(A) Two approaches to a problem are discussed, a test that would determine which is more efficient is described, and a conclusion is made, based on experimental evidence.
Not in line with pre-thinking

(B) Two hypotheses are described, a way of determining which of them is more likely to be true is discussed, and one said to be more accurate on the basis of experimental evidence.
in line with pre-thinking

(C) Two hypotheses are described, the flaws inherent in one of them are elaborated, and experimental evidence confirming the other is cited.
Not in line with pre-thinking

(D) An assertion that a species has adopted two different mechanisms to solve a particular problem is made,and evidence is then provided in support of that assertion.
Not in line with pre-thinking

(E)An assertion that one mechanism for solving a particular problem is more efficient than another is made, and evidence is then provided in support of that assertion.
Not in line with pre-thinking

----------------------------------------------------------------------------------------

6. The passage provides information in support of which one of the following assertions?

Pre-thinking

Inference question

(A) The seemingly erratic motion exhibited by a microorganism can in fact reflect a mechanism by which it is able to control its movement.
supported

(B) Biologists often overstate the complexity of simple organisms such as bacteria.
Not in line with pre-thinking

(C) A bacterium cannot normally retain a memory of a measurement of the concentration of an attractant.
Not in line with pre-thinking

(D) Bacteria now appear to have less control over their movement than biologists had previously hypothesized.
Not in line with pre-thinking

(E) Photosynthetic bacteria appear to have more control over their movement than do bacteria that are not photosynthetic.
Not in line with pre-thinking

----------------------------------------------------------------------------------------

It is a good day to be alive

I have a question about the 2nd question. The second paragraph states "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". In this case how come a bacteria decreases its tumbling when it is moving away from the substance?
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Re: Although bacteria are unicellular and among the simplest autonomous  [#permalink]

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15 Dec 2019, 02:16
ypetrunina wrote:

I have a question about the 2nd question. The second paragraph states "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". In this case how come a bacteria decreases its tumbling when it is moving away from the substance?

If you read carefully You'll notice that there is no decrease in tumbling when the bacteria moves away from the substance but only an increase
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Although bacteria are unicellular and among the simplest autonomous  [#permalink]

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15 Jan 2020, 19:23
swikrityC wrote:
Hi GMATNinja,

Can you please explain Q1 and Q2?

Look forward,

Sure! We'll do Q1 in this post, and I'll post something separately for Q2.

Question 1

Quote:
1. It can be inferred from the passage that which one of the following experimental results would suggest that bacteria detect changes in the concentration of an attractant by measuring its concentration in front and back of the cell body simultaneously?

To answer this question, we need to find where the author discusses how one could tell that bacteria measure the concentration of an attractant at the front and back of their cell body.

That information is located in the third paragraph, which is written to illustrate two ways that bacteria might detect changes in the concentration of an attractant:

• By comparing concentration at the front and back of the cell body simultaneously (this is what we're interested in)
• By measuring concentration at different points in time

Here's what the paragraph says:

Quote:
Researchers reasoned that if bacteria do compare concentrations at different times, then when suddenly exposed to a uniformly high concentration of an attractant, the cells would behave as if they were swimming up a concentration gradient, with long, smooth runs and relatively few tumbles. If, on the other hand, bacteria detect a chemical gradient by measuring it simultaneously at two distinct points, front and back, on the cell body, they would not respond to the jump in concentration because the concentration of the attractant in front and back of the cells, though high, would be uniform. Experimental evidence suggests that bacteria compare concentrations at different times.

So, IF bacteria detect a chemical gradient by measuring it simultaneously at front and back of the cell body, AND those bacteria are suddenly exposed to a uniformly high concentration of an attractant, THEN the bacteria will not respond to the jump in concentration.

In other words, the experiment described in this passage is one where:

• bacteria are suddenly exposed to a uniformly high concentration of an attractant, and
• do not respond to that exposure.

Now, let's see which answer choice matches that description.

Quote:
(A) When suddenly transferred from a medium in which the concentration of an attractant was uniformly low to one in which the concentration was uniformly high, the tendency of the bacteria to tumble and undergo random changes in direction increased.

Choice (A) describes the experimental mechanism we're looking for (bacteria are suddenly exposed to a uniformly high concentration of an attractant). The result, however, is not what we're looking for (no response). Eliminate (A).

Quote:
(B) When suddenly transferred from a medium in which the concentration of an attractant was uniformly low to one in which the concentration was uniformly high, the bacteria’s exhibited no change in the pattern of their motion.

Choice (B) seems like exactly what we want! Both the experimental mechanism and the experimental result match what was described by the author in paragraph 3. Let's keep (B) around and continue eliminating the remaining answer choices.

Quote:
(C) When suddenly transferred from a medium in which the concentration of an attractant was uniformly low to one in which the concentration was uniformly high, the bacteria’s movement was characterized by a complete absence of tumbling.

Like choice (A), choice (C) gets the experimental mechanism right, but describes the wrong result. Eliminate (C).

Quote:
(D) When placed in a medium in which the concentration of an attractant was in some areas low and in others high, the bacteria exhibited an increased tendency to tumble in those areas where the concentration of the attractant was high.

We can eliminate choice (D) because the experimental mechanism described here is NOT in the passage. When the author spelled out how to observe bacteria measuring an attractant at both ends of the cell body, they only presented a case where the bacteria was "suddenly exposed to a uniformly high concentration of an attractant." Because we can't see the premise of the experiment in choice (D) anywhere in the passage, we can't infer the corresponding result, either. Eliminate (D).

Quote:
(E) When suddenly transferred from a medium in which the concentration of an attractant was uniformly low to one that was completely free of attractants, the bacteria exhibited a tendency to suppress tumbling and move in longer, straighter lines.

Like choice (E), choice (D) describes an experimental mechanism that is NOT in the passage. Because we have no information from the passage telling us how bacteria might respond to the complete absence of attractants, we can't infer the corresponding result, and we should eliminate choice (E).

Choice (B) is the only answer choice that is completely reflected in the passage itself.

I hope this helps!
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Re: Although bacteria are unicellular and among the simplest autonomous  [#permalink]

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22 Jan 2020, 18:53
swikrityC wrote:
Hi GMATNinja,

Can you please explain Q1 and Q2?

Look forward,

Question 2

Quote:
2. It can be inferred from the passage that a bacterium would increase the likelihood of its moving away from an area where the concentration of a harmful substance is high if it did which one of the following?

(A) Increased the speed at which it swam immediately after undergoing the random changes in direction that result from tumbling.
(B) Detected the concentration gradient of an attractant toward which it could begin to swim.
(C) Relied on the simultaneous measurement of the concentration of the substance in front and back of its body, rather than on the comparison of the concentration at different points in time.
(D) Exhibited a complete cessation of tumbling when it detected increases in the concentration of substance.
(E) 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.

First, some context:

• In paragraph 1, we are told that bacteria are attracted to what they need ("attractants") and are repelled by what harms them.
• We're also told that the swimming patterns of bacteria consist "only of smooth runs and tumbles, the latter resulting in random changes in direction."

The question is a bit wordy and confusing, but it's asking: If we introduced a HARMFUL substance to a solution, which type of behavior (according to the passage) would increase the chance that a bacterium in that solution would move AWAY from high concentrations of that harmful substance?

To answer that question, let's first review what happens when a chemical attractant is introduced into the bacteria's environment:

• As the bacteria moves towards an area where the concentration of the attractant is stronger, the tumbling decreases, and there's an increase in "straight runs over relatively longer distances."
• Why does this happen? Because "as the bacteria encounter increasing concentrations of the attractant, their tendency to tumble is suppressed."
• So if the bacterium is going the right way (i.e. towards an attractant), it will tumble less and thus change direction less. This increases the chance that it will stay the course and continue moving towards the attractant.
• On the other hand, when the bacterium goes the wrong way (i.e. away from an attractant), its tendency to tumble (and thus change direction) goes up. This increases the chance that it will start going the right way (towards the attractant).

Now, to answer the question, let's switch from an attractant to a harmful substance:

• In this scenario, the wrong way would be towards INCREASED concentrations of the substance (since bacteria would want to avoid something harmful), and the right way would be towards DECREASED concentrations of the substance.
• As we just saw, an increase in tumbling helps a bacterium CHANGE direction, while a decrease in tumbling helps a bacterium stay the course.
• So if a bacterium tumbles MORE as it encounters increasing concentrations of a harmful substance, then it would increase the likelihood of moving AWAY from that substance.
• On the other hand, if bacterium tumbles LESS as it it encounters decreasing concentrations of a harmful substance, then it would increase the likelihood of staying the course and continuing to move away from that substance.
• This is consistent with choice (E).

Remember, we aren't asked to infer from the passage that the behavior described in (E) definitely happens. We're asked to infer that, IF that behavior does in fact happen, it would increase the likelihood that the bacterium moves away from an area where the concentration of a harmful substance is high.

I hope this helps! This is definitely a tough one.
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Re: Although bacteria are unicellular and among the simplest autonomous  [#permalink]

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25 Jan 2020, 01:09

Could you please explain question number 5? Why option C is wrong?
To me, option B seems wrong for the below reason and both A & C seem correct.

Which one of the following best describes the organization of the third paragraph of the passage?

(A) Two approaches to a problem are discussed, a test that would determine which is more efficient is described, and a conclusion is made, based on experimental evidence.
(B) Two hypotheses are described, a way of determining which of them is more likely to be true is discussed, and one said to be more accurate on the basis of experimental evidence.
(C) Two hypotheses are described, the flaws inherent in one of them are elaborated, and experimental evidence confirming the other is cited. -->isn't uniformity concentration example is used to discuss the flaw in one mechanism?
(D) An assertion that a species has adopted two different mechanisms to solve a particular problem is made, and evidence is then provided in support of that assertion.
(E) An assertion that one mechanism for solving a particular problem is more efficient than another is made, and evidence is then provided in support of that assertion. -->Incomplete
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Re: Although bacteria are unicellular and among the simplest autonomous  [#permalink]

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11 Feb 2020, 06:58
1

Could you please explain question number 5? Why option C is wrong?
To me, option B seems wrong for the below reason and both A & C seem correct.

Which one of the following best describes the organization of the third paragraph of the passage?

(A) Two approaches to a problem are discussed, a test that would determine which is more efficient is described, and a conclusion is made, based on experimental evidence.
(B) Two hypotheses are described, a way of determining which of them is more likely to be true is discussed, and one said to be more accurate on the basis of experimental evidence.
(C) Two hypotheses are described, the flaws inherent in one of them are elaborated, and experimental evidence confirming the other is cited. -->isn't uniformity concentration example is used to discuss the flaw in one mechanism?
(D) An assertion that a species has adopted two different mechanisms to solve a particular problem is made, and evidence is then provided in support of that assertion.
(E) An assertion that one mechanism for solving a particular problem is more efficient than another is made, and evidence is then provided in support of that assertion. -->Incomplete

Question 5

Here's the first piece of the third paragraph:

"Biologists have proposed two mechanisms that bacteria might use in detecting changes in the concentration of a chemical attractant. First, a bacterium might compare the concentration of a chemical at the front and back of its cell body simultaneously. If the concentration is higher at the front of the cell, then it knows it is moving up the concentration gradient, from an area where the concentration is lower to an area where it is higher. Alternatively, it might measure the concentration at one instant and again after a brief interval, in which case the bacterium must retain a memory of the initial concentration."

Here, the author describes two mechanisms that bacteria might use in a certain behavior. Let's see how that stacks up against our answer choices:

Quote:
(A) Two approaches to a problem are discussed...
(B) Two hypotheses are described...
(C) Two hypotheses are described...
(D) An assertion that a species has adopted two different mechanisms to solve a particular problem is made...
(E) An assertion that one mechanism for solving a particular problem is more efficient than another is made....

(B) and (C) are looking good! The biologists have two ideas that might explain a certain behavior. This fits well with the word "hypothesis," which means a proposed explanation that requires further testing.

(D) and (E) are plain wrong. Biologists believe that bacteria use one mechanism OR the other, not BOTH mechanisms. For this reason, (D) is out. (E) misses the initial description of the two proposed mechanisms entirely, so it's not a good fit to describe the organization of the passage as a whole.

At this point, I'd say that (A) is on thin ice. The "problem" discussed in the paragraph is how bacteria detect changes in the concentration of a chemical attractant. Stating that two approaches to this problem are discussed seems to imply that the bacteria themselves have a couple of approaches to the problem -- which is a bit of a stretch. The alternative -- that the biologists have two approaches to the problem -- doesn't really work either. They have proposed two different explanations, but are not fundamentally approaching the issue from two different directions.

(A) isn't looking great, but let's look at the second half of the paragraph to see whether it fits.

"Researchers reasoned that if bacteria do compare concentrations at different times, then when suddenly exposed to a uniformly high concentration of an attractant, the cells would behave as if they were swimming up a concentration gradient, with long, smooth runs and relatively few tumbles. If, on the other hand, bacteria detect a chemical gradient by measuring it simultaneously at two distinct points, front and back, on the cell body, they would not respond to the jump in concentration because the concentration of the attractant in front and back of the cells, though high, would be uniform. Experimental evidence suggests that bacteria compare concentrations at different times."

Here, the biologists come up with a test that would support one of their proposed ideas over the other, depending on the result of the experiment. In the last sentence ("Experimental evidence suggests that bacteria compare concentrations at different times"), we learn that the biologists did in fact perform the experiment, and it supported the second of the two proposed mechanisms.

Let's see how that matches up with our remaining answer choices:
Quote:
(A) Two approaches to a problem are discussed, a test that would determine which is more efficient is described, and a conclusion is made, based on experimental evidence.

The main issue with the second piece of (A) is "a test that would determine which is more efficient is described..." The test outlined in the passage doesn't determine which mechanism is more efficient -- it just determines which one the bacteria actually use. This is the death blow for (A), which was already not looking great.

Quote:
(B) Two hypotheses are described, a way of determining which of them is more likely to be true is discussed, and one said to be more accurate on the basis of experimental evidence.

This fits nicely! The biologists came up with a test that would help them determine which of their ideas was accurate. Then, based on evidence from that experiment, the second idea was supported over the first.

(B) is looking good.

Quote:
(C) Two hypotheses are described, the flaws inherent in one of them are elaborated, and experimental evidence confirming the other is cited.

The biggest difference between (B) and (C) is the phrase "the flaws inherent in one of [the hypotheses] are elaborated." So, does the passage elaborate on the flaws of one of the hypotheses?

Not that I can see. Up until the very last sentence, the passage is neutral toward both hypotheses. The author doesn't weigh in on one side or the other -- instead, he/she just describes the two possibilities, then lays out a test that will support one idea over the other. Because the author does not elaborate on the flaws of one hypothesis before revealing that the experiment supported the second hypothesis, we can eliminate (C).

I hope that helps!
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Re: Although bacteria are unicellular and among the simplest autonomous  [#permalink]

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11 Feb 2020, 19:05
GMATNinja wrote:
swikrityC wrote:
Hi GMATNinja,

Can you please explain Q1 and Q2?

Look forward,

Sure! We'll do Q1 in this post, and I'll post something separately for Q2.

Question 1

Quote:
1. It can be inferred from the passage that which one of the following experimental results would suggest that bacteria detect changes in the concentration of an attractant by measuring its concentration in front and back of the cell body simultaneously?

To answer this question, we need to find where the author discusses how one could tell that bacteria measure the concentration of an attractant at the front and back of their cell body.

That information is located in the third paragraph, which is written to illustrate two ways that bacteria might detect changes in the concentration of an attractant:

• By comparing concentration at the front and back of the cell body simultaneously (this is what we're interested in)
• By measuring concentration at different points in time

Here's what the paragraph says:

Quote:
Researchers reasoned that if bacteria do compare concentrations at different times, then when suddenly exposed to a uniformly high concentration of an attractant, the cells would behave as if they were swimming up a concentration gradient, with long, smooth runs and relatively few tumbles. If, on the other hand, bacteria detect a chemical gradient by measuring it simultaneously at two distinct points, front and back, on the cell body, they would not respond to the jump in concentration because the concentration of the attractant in front and back of the cells, though high, would be uniform. Experimental evidence suggests that bacteria compare concentrations at different times.

So, IF bacteria detect a chemical gradient by measuring it simultaneously at front and back of the cell body, AND those bacteria are suddenly exposed to a uniformly high concentration of an attractant, THEN the bacteria will not respond to the jump in concentration.

In other words, the experiment described in this passage is one where:

• bacteria are suddenly exposed to a uniformly high concentration of an attractant, and
• do not respond to that exposure.

Now, let's see which answer choice matches that description.

Quote:
(A) When suddenly transferred from a medium in which the concentration of an attractant was uniformly low to one in which the concentration was uniformly high, the tendency of the bacteria to tumble and undergo random changes in direction increased.

Choice (A) describes the experimental mechanism we're looking for (bacteria are suddenly exposed to a uniformly high concentration of an attractant). The result, however, is not what we're looking for (no response). Eliminate (A).

Quote:
(B) When suddenly transferred from a medium in which the concentration of an attractant was uniformly low to one in which the concentration was uniformly high, the bacteria’s exhibited no change in the pattern of their motion.

Choice (B) seems like exactly what we want! Both the experimental mechanism and the experimental result match what was described by the author in paragraph 3. Let's keep (B) around and continue eliminating the remaining answer choices.

Quote:
(C) When suddenly transferred from a medium in which the concentration of an attractant was uniformly low to one in which the concentration was uniformly high, the bacteria’s movement was characterized by a complete absence of tumbling.

Like choice (A), choice (C) gets the experimental mechanism right, but describes the wrong result. Eliminate (C).

Quote:
(D) When placed in a medium in which the concentration of an attractant was in some areas low and in others high, the bacteria exhibited an increased tendency to tumble in those areas where the concentration of the attractant was high.

We can eliminate choice (D) because the experimental mechanism described here is NOT in the passage. When the author spelled out how to observe bacteria measuring an attractant at both ends of the cell body, they only presented a case where the bacteria was "suddenly exposed to a uniformly high concentration of an attractant." Because we can't see the premise of the experiment in choice (D) anywhere in the passage, we can't infer the corresponding result, either. Eliminate (D).

Quote:
(E) When suddenly transferred from a medium in which the concentration of an attractant was uniformly low to one that was completely free of attractants, the bacteria exhibited a tendency to suppress tumbling and move in longer, straighter lines.

Like choice (E), choice (D) describes an experimental mechanism that is NOT in the passage. Because we have no information from the passage telling us how bacteria might respond to the complete absence of attractants, we can't infer the corresponding result, and we should eliminate choice (E).

Choice (B) is the only answer choice that is completely reflected in the passage itself.

I hope this helps!

Hi GMATNinja

I totally agree with your reasoning!

However, my question is "no change in the pattern of their motion" same as no movement (as passage states no response).

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Re: Although bacteria are unicellular and among the simplest autonomous  [#permalink]

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02 Mar 2020, 08:46
hero_with_1000_faces wrote:
GMATNinja wrote:
swikrityC wrote:
Hi GMATNinja,

Can you please explain Q1 and Q2?

Look forward,

Sure! We'll do Q1 in this post, and I'll post something separately for Q2.

Question 1

Quote:
1. It can be inferred from the passage that which one of the following experimental results would suggest that bacteria detect changes in the concentration of an attractant by measuring its concentration in front and back of the cell body simultaneously?

To answer this question, we need to find where the author discusses how one could tell that bacteria measure the concentration of an attractant at the front and back of their cell body.

That information is located in the third paragraph, which is written to illustrate two ways that bacteria might detect changes in the concentration of an attractant:

• By comparing concentration at the front and back of the cell body simultaneously (this is what we're interested in)
• By measuring concentration at different points in time

Here's what the paragraph says:

Quote:
Researchers reasoned that if bacteria do compare concentrations at different times, then when suddenly exposed to a uniformly high concentration of an attractant, the cells would behave as if they were swimming up a concentration gradient, with long, smooth runs and relatively few tumbles. If, on the other hand, bacteria detect a chemical gradient by measuring it simultaneously at two distinct points, front and back, on the cell body, they would not respond to the jump in concentration because the concentration of the attractant in front and back of the cells, though high, would be uniform. Experimental evidence suggests that bacteria compare concentrations at different times.

So, IF bacteria detect a chemical gradient by measuring it simultaneously at front and back of the cell body, AND those bacteria are suddenly exposed to a uniformly high concentration of an attractant, THEN the bacteria will not respond to the jump in concentration.

In other words, the experiment described in this passage is one where:

• bacteria are suddenly exposed to a uniformly high concentration of an attractant, and
• do not respond to that exposure.

Now, let's see which answer choice matches that description.

Quote:
(A) When suddenly transferred from a medium in which the concentration of an attractant was uniformly low to one in which the concentration was uniformly high, the tendency of the bacteria to tumble and undergo random changes in direction increased.

Choice (A) describes the experimental mechanism we're looking for (bacteria are suddenly exposed to a uniformly high concentration of an attractant). The result, however, is not what we're looking for (no response). Eliminate (A).

Quote:
(B) When suddenly transferred from a medium in which the concentration of an attractant was uniformly low to one in which the concentration was uniformly high, the bacteria’s exhibited no change in the pattern of their motion.

Choice (B) seems like exactly what we want! Both the experimental mechanism and the experimental result match what was described by the author in paragraph 3. Let's keep (B) around and continue eliminating the remaining answer choices.

Quote:
(C) When suddenly transferred from a medium in which the concentration of an attractant was uniformly low to one in which the concentration was uniformly high, the bacteria’s movement was characterized by a complete absence of tumbling.

Like choice (A), choice (C) gets the experimental mechanism right, but describes the wrong result. Eliminate (C).

Quote:
(D) When placed in a medium in which the concentration of an attractant was in some areas low and in others high, the bacteria exhibited an increased tendency to tumble in those areas where the concentration of the attractant was high.

We can eliminate choice (D) because the experimental mechanism described here is NOT in the passage. When the author spelled out how to observe bacteria measuring an attractant at both ends of the cell body, they only presented a case where the bacteria was "suddenly exposed to a uniformly high concentration of an attractant." Because we can't see the premise of the experiment in choice (D) anywhere in the passage, we can't infer the corresponding result, either. Eliminate (D).

Quote:
(E) When suddenly transferred from a medium in which the concentration of an attractant was uniformly low to one that was completely free of attractants, the bacteria exhibited a tendency to suppress tumbling and move in longer, straighter lines.

Like choice (E), choice (D) describes an experimental mechanism that is NOT in the passage. Because we have no information from the passage telling us how bacteria might respond to the complete absence of attractants, we can't infer the corresponding result, and we should eliminate choice (E).

Choice (B) is the only answer choice that is completely reflected in the passage itself.

I hope this helps!

Hi GMATNinja

I totally agree with your reasoning!

However, my question is "no change in the pattern of their motion" same as no movement (as passage states no response).

The answer to your question is an unambiguous no. "No change in the pattern of their motion" is NOT the same as "no movement."

However, "no movement" is NOT the same as "no response." "No movement", without any additional context, implies no movement at all. "No response" implies no change or no reaction to whatever was happening beforehand.

I'm not sure where you're seeing an indication of "no movement" (I don't see this anywhere in the passage or in the question itself), but I hope this resolves your doubt!
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Re: Although bacteria are unicellular and among the simplest autonomous   [#permalink] 02 Mar 2020, 08:46
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