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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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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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Originally posted by Gnpth on 07 Apr 2015, 08:38.
Last edited by SajjadAhmad on 12 Oct 2019, 00:24, edited 3 times in total.
Updated - Complete topic (949).
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Re: Although bacteria are unicellular and among the simplest autonomous  [#permalink]

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New post 12 Oct 2019, 00:18
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Re: Although bacteria are unicellular and among the simplest autonomous   [#permalink] 12 Oct 2019, 00:18
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