Passage
The Venus flytrap [Dionaea muscipula] needs to
know when an ideal meal is crawling across its leaves.
Closing its trap requires a huge expense of energy,
and reopening the trap can take several hours, so
5 Dionaea only wants to spring closed when it’s
sure
that the dawdling insect visiting its surface is large
enough to be worth its time. The large black hairs on
their lobes allow the Venus flytraps to literally feel
their prey, and they act as triggers that spring the
10 trap closed when the proper prey makes its way
across the trap. If the insect touches just one hair, the
trap will not spring shut; but a large enough bug will
likely touch two hairs within about twenty seconds,
and that signal springs the Venus flytrap into action.
15 We can look at this system as analogous to
short-term memory. First, the flytrap encodes the
information (forms the memory) that something (it
doesn’t know what) has touched one of its hairs.
Then it stores this information for a number of
20 seconds (retains the memory) and finally
retrieves
this information (recalls the memory) once a second
hair is touched. If a small ant takes a while to get
from one hair to the next, the trap will have forgotten
the first touch by the time the ant brushes up against
25 the next hair. In other words, it loses the storage
of
the information, doesn’t close, and the ant
happily meanders on. How does the plant encode
and store the information from the unassuming
bug’s encounter with the first hair? How does it
30 remember the first touch in order to react upon
the
second?
Scientists have been puzzled by these questions
ever since John Burdon-Sanderson’s early report on
the physiology of the Venus flytrap in 1882. A
35 century later, Dieter Hodick and Andreas Sievers
at
the University of Bonn in Germany proposed that
the flytrap stored information regarding how many
hairs have been touched in the electric charge of its
leaf. Their model is quite elegant in its simplicity.
40 In their studies, they discovered that touching
a
trigger hair on the Venus flytrap causes an electric
action potential [a temporary reversal in the
electrical polarity of a cell membrane] that
induces calcium channels to open in the trap (this
45 coupling of action potentials and the opening
of
calcium channels is similar to the processes that
occur during communication between human
neurons), thus causing a rapid increase in the
concentration of calcium ions.
50 They proposed that the trap requires a
relatively
high concentration of calcium in order to close
and that a single action potential from just one
trigger hair being touched does not reach this level.
Therefore, a second hair needs to be stimulated to
55 push the calcium concentration over this
threshold
and spring the trap. The encoding of the information
requires maintaining a high enough level of calcium
so that a second increase (triggered by touching the
second hair) pushes the total concentration of
60 calcium over the threshold. As the calcium ion
concentrations dissipate over time, if the second
touch and potential don’t happen quickly, the final
concentration after the second trigger won’t be high
enough to close the trap, and the memory is lost.
65 Subsequent research supports this model.
Alexander Volkov and his colleagues at Oakwood
University in Alabama first demonstrated that it is
indeed electricity that causes the Venus flytrap to
close. To test the model they rigged up very fine
70 electrodes and applied an electrical current to
the
open lobes of the trap. This made the trap close
without any direct touch to its trigger hairs (while
they didn’t measure calcium levels, the current
likely led to increases). When they modified this
75 experiment by altering the amount of
electrical
current, Volkov could determine the exact electrical
charge needed for the trap to close. As long as
fourteen microcoulombs—a tiny bit more than the
static electricity generated by rubbing two balloons
80 together—flowed between the two electrodes,
the
trap closed. This could come as one large burst or as
a series of smaller charges within twenty seconds. If it
took longer than twenty seconds to accumulate the
total charge, the trap would remain open.
This passage is adapted from Daniel Chamovitz, What a Plant Knows: A
Field Guide to the Senses.
©2012 by Daniel Chamovitz.
Question 1
The primary purpose of the passage is to
A) discuss findings that offer a scientific explanation for the Venus flytrap’s closing action.
B) present research that suggests that the Venus flytrap’s predatory behavior is both complex and unique among plants.
C) identify the process by which the Venus flytrap’s closing action has evolved.
D) provide a brief overview of the Venus flytrap and its predatory behavior.
Solution
Choice A is the best answer. The first two paragraphs of the passage describe the physical process by which the Venus flytrap closes its trap but also note certain long-standing questions about that process: “How does the plant encode and store the information from the unassuming bug’s encounter with the first hair? How does it remember the first touch in order to react upon the second?” The passage then answers those questions by discussing, in the third and fourth paragraphs, a study conducted by Dieter Hodick and Andreas Sievers that identified the physiological means behind the closing of the Venus flytrap’s trap and, in the last paragraph, a study conducted by Alexander Volkov that confirmed and built on Hodick and Sievers’s findings. The primary purpose of the passage can therefore be seen as discussing scientific findings that explain how the Venus flytrap closes its trap.
Choice B is incorrect because the passage doesn’t discuss the Venus flytrap’s ability to close its trap in the context of the abilities of other plants. Choice C is incorrect because the passage discusses how the closing action operates but not how it has evolved. Choice D is incorrect because the passage doesn’t provide an overview of the Venus flytrap and its predatory behavior; it merely notes in passing that the closing action has a predatory function.
Question 2
Based on the passage, a significant advantage of the Venus flytrap’s requirement for multiple triggers is that it
A) enables the plant to identify the species of its prey.
B) conserves the plant’s calcium reserves.
C) safeguards the plant’s energy supply.
D) prevents the plant from closing before capturing its prey.
Solution
Choice C is the best answer. The first paragraph discusses the challenge posed to the Venus flytrap by the opening and closing of its trap: “Closing its trap requires a huge expense of energy, and reopening the trap can take several hours, so Dionaea only wants to spring closed when it’s sure that the dawdling insect visiting its surface is large enough to be worth its time.” Since closing and reopening the trap requires the expense of precious energy, it can be inferred that by guarding against unnecessary closing, multiple triggers safeguard the plant’s energy supply.
Choice A is incorrect because the passage never indicates that multiple triggers allow the Venus flytrap to identify which species its prey belongs to, only that they allow it to gauge the prey’s size. Choice B is incorrect because although the passage implies that the plant needs to conserve energy and indicates that calcium is involved in the trapclosing mechanism, there is no indication that the plant’s calcium reserves themselves require conservation. Choice D is incorrect because it can be inferred from the passage that the advantage of multiple triggers is that they prevent the Venus flytrap from closing on the improper prey rather than from prematurely closing on the proper prey; the passage never implies that when touched by its proper prey, the Venus flytrap is at risk of closing too soon to capture it.
Question 3
Which choice provides the best evidence for the answer to the previous question?
A) Lines 3-7 (“Closing… time”)
B) Lines 7-11 (“The large… across the trap”)
C) Lines 11-14 (“If the… action”)
D) Lines 16-18 (“First… hairs”)
Solution
Choice A is the best answer. The previous question asks what the Venus flytrap gains from requiring multiple triggers before closing. The answer, that multiple triggers allow the plant to conserve energy, is best supported near the beginning of the first paragraph: “Closing its trap requires a huge expense of energy, and reopening the trap can take several hours, so Dionaea only wants to spring closed when it’s sure that the dawdling insect visiting its surface is large enough to be worth its time.”
Choices B, C, and D are incorrect because the cited lines don’t support the answer to the previous question. Instead, they describe how the hairs on the Venus flytrap function and how the system of multiple triggers works (choices B and C) and explain how the plant preserves a memory, as it were, that something has touched the trigger hairs (choice D).
Question 4
The use of the phrases “dawdling insect” (line 6), “happily meanders” (line 27), and “unassuming bug’s encounter” (lines 28-29) in the first two paragraphs establishes a tone that is
Solution
Choice C is the best answer. The phrases “dawdling insect,” “happily meanders,” and “unassuming bug’s encounter” are less typical of word choices made in formal, scientific writing than of those made in less formal writing modes. Therefore, the tone that these phrases establish is best described as informal.
Choices A, B, and D are incorrect because the phrases establish a tone that is informal, not academic (choice A), melodramatic (choice B), or mocking (choice D).
Question 5
In the second paragraph (lines 15-31), the discussion of short-term memory primarily functions to
A) clarify an explanation of what prompts the Venus flytrap to close.
B) advance a controversial hypothesis about the function of electric charges found in the leaf of the Venus flytrap.
C) stress the distinction between the strategies of the Venus flytrap and the strategies of human beings.
D) emphasize the Venus flytrap’s capacity for retaining detailed information about its prey.
Solution
Choice A is the best answer. The first paragraph describes the mechanism that prompts the Venus flytrap to close its trap. The second paragraph makes an analogy of each step of that mechanism to an aspect of short-term memory formation in humans and then poses questions about the precise physiological terms in which those steps are carried out. It can therefore be said that the discussion of shortterm memory serves to clarify the first paragraph’s explanation of what prompts the trap of the Venus flytrap to close.
Choice B is incorrect because it is the third paragraph, not the second, that discusses the function of electric charges in the Venus flytrap; moreover, the passage presents this function as a fact, not as a controversial hypothesis. Choice C is incorrect because rather than stressing the differences between Venus flytraps and humans, the analogy in the second paragraph stresses their superficial similarities. Choice D is incorrect because the second paragraph implies that the Venus flytrap’s capacity for retaining information is far from detailed: “something (it doesn’t know what) has touched one of its hairs.”
Question 6
According to the passage, which statement best explains why the Venus flytrap requires a second trigger hair to be touched within a short amount of time in order for its trap to close?
A) The second trigger produces an electrical charge that reverses the charge produced by the first trigger.
B) The second trigger stabilizes the surge of calcium ions created by the first trigger.
C) The second trigger prompts the calcium channels to open.
D) The second trigger provides a necessary supplement to the calcium concentration released by the first trigger.
Solution
Choice D is the best answer. The third paragraph explains that touching a single trigger hair results in “a rapid increase in the concentration of calcium ions” in the plant. The fourth paragraph further explains that the calcium concentration produced by this initial touch isn’t enough to cause the trap to close, but that a second hair touch will bring the total concentration to the level necessary to close the trap: “a second hair needs to be stimulated to push the calcium concentration over this threshold and spring the trap.”
Choices A and B are incorrect because the fourth paragraph explains that the second trigger supplements the action of the first trigger, not that it reverses it (choice A) or stabilizes its effect (choice B). Choice C is incorrect because the third paragraph clearly states that the calcium channels open after the first trigger hair is touched, not the second.
Question 7
Which choice describes a scenario in which Hodick and Sievers’s model predicts that a Venus flytrap will NOT close around an insect?
A) A large insect’s second contact with the plant’s trigger hairs results in a total calcium ion concentration above the trap’s threshold.
B) A large insect makes contact with a second trigger hair after a period of inactivity during which calcium ion concentrations have diminished appreciably.
C) A large insect’s contact with the plant’s trigger hairs causes calcium channels to open in the trap.
D) A large insect’s contact with a second trigger hair occurs within ten seconds of its contact with the first trigger hair.
Solution
Choice B is the best answer. The fourth paragraph explains that the Venus flytrap will close only if a second hair is stimulated to “push the calcium concentration over this threshold and spring the trap.” But the last sentence of the paragraph notes that the calcium concentrations “dissipate over time,” and if enough time elapses after the first trigger, “the final concentration after the second trigger won’t be high enough to close the trap.” It can be inferred, then, that if a large insect didn’t touch a second trigger hair until after the calcium ion concentrations had diminished appreciably, the Venus flytrap would fail to close.
Choice A is incorrect because the fourth paragraph makes clear that if the calcium concentration goes above the trap’s threshold, the plant will close, not remain open. Choice C is incorrect because as the third paragraph explains, the touching of the trigger hair and opening of the calcium ion channels don’t act to keep the trap open but are instead a precondition for the closing of the trap (though closing will occur only if a second trigger hair is touched). Choice D is incorrect because the last sentence of the fifth paragraph explains that the threshold for the time that can elapse between the touching of the first and second trigger hairs is twenty seconds, meaning that a large insect touching two hairs within ten seconds would almost certainly make the plant close.
Question 8
As used in line 67, “demonstrated” most nearly means
A) protested.
B) established.
C) performed.
D) argued.
Solution
Choice B is the best answer. The second sentence of the last paragraph says that Alexander Volkov and his colleagues “first demonstrated that it is indeed electricity that causes the Venus flytrap to close.” In this context, the word “demonstrated” most nearly means established. Choices A, C, and D are incorrect because in the context of scientists showing what causes the Venus flytrap to close, the word “demonstrated” most nearly means established, not protested (choice A), performed (choice C), or argued (choice D).
Question 9
Based on the passage, what potential criticism might be made of Volkov’s testing of Hodick and Sievers’s model?
A) Volkov’s understanding of Hodick and Sievers’s model was incorrect.
B) Volkov’s measurements did not corroborate a central element of Hodick and Sievers’s model.
C) Volkov’s direct application of an electrical current would have been objectionable to Hodick and Sievers
D) Volkov’s technology was not available to Hodick and Sievers.
Solution
Choice B is the best answer. As described in the third paragraph, Hodick and Sievers’s model emphasizes that the Venus flytrap closes by means of an electrical charge triggered when the plant’s hairs are touched. But as explained in the last paragraph, when Alexander Volkov tested this model, the design of his experiment involved the direct application of an electrical charge, which “made the trap close without any direct touch to its trigger hairs.” Therefore, Volkov’s work could be criticized because his design omitted, rather than corroborated, a central element of Hodick and Sievers’s model— namely, the physical stimulation of the hairs.
Choice A is incorrect because although the last paragraph explains that Volkov omitted an element of Hodick and Sievers’s model when designing his own experiment, there is no suggestion that he did so out of a faulty understanding of their model. Choice C is incorrect because it is impossible to know from the passage if Hodick and Sievers would have objected to Volkov’s methods. Choice D is incorrect because the passage doesn’t indicate whether the technology Volkov used had been available to Hodick and Sievers when they formulated their model.
Question 10
Which choice provides the best evidence for the answer to the previous question?
A) Lines 66-69 (“Alexander… close”)
B) Lines 69-71 (“To test… trap”)
C) Lines 71-74 (“This… increases”)
D) Lines 74-77 (“When… close”)
Solution
Choice C is the best answer. The previous question asks what potential criticism might be made of Volkov’s testing of Hodick and Sievers’s model. The answer, that a central element of that model wasn’t corroborated by Volkov’s measurements, is best supported in the last paragraph: “This made the trap close without any direct touch to its trigger hairs (while they didn’t measure calcium levels, the current likely led to increases).” Because the physical touch to the hairs figured in Hodick and Sievers’s model, it can be said that Volkov’s decision to apply an electrical current directly to the plant means that he failed to corroborate a central element of their model.
Choices A, B, and D are incorrect because the cited lines don’t support the answer to the previous question. Instead, they summarize the basic agreement of Volkov’s work with Hodick and Sievers’s model (choice A) and describe steps in Volkov’s experimental design that are related to the application of an electrical current but don’t directly address the omission of the central element of the physical touch to the hairs (choices B and D).
Question 11
Based on the passage, in studying the Venus flytrap, Volkov and his colleagues made the most extensive use of which type of evidence?
A) Mathematical models to predict the electrical charge required to close the Venus flytrap
B) Analysis of data collected from previous researchers’ work involving the Venus flytrap’s response to electricity
C) Information obtained from monitoring the Venus flytrap’s response to varying amounts of electrical current
D) Published theories of scientists who developed earlier models of the Venus flytrap
Solution
Choice C is the best answer. The second sentence of the last paragraph says that the focus of Volkov’s work was the role of electricity in the Venus flytrap’s closing mechanism. The paragraph goes on to explain that by applying electricity directly to the plant and “altering the amount of electrical current, Volkov could determine the exact electrical charge needed for the trap to close.” It is therefore accurate to say that Volkov and his colleagues made the most extensive use of information obtained from measuring the plant’s response to varying amounts of electrical current.
Choice A is incorrect because although the last paragraph explains that Volkov’s work was based on Hodick and Sievers’s mathematical model in which an electrical charge is required to close the Venus flytrap, that model isn’t described as predicting the precise amount of charge required; moreover, although Volkov made use of this earlier model, it served as a starting point, and his work made greater use of the findings generated by his experiment. Choice B is incorrect because the passage doesn’t describe Volkov’s work as having involved analysis of data from earlier studies on the plant’s response to electricity. Choice D is incorrect because although the last paragraph explains that Volkov based his work on Hodick and Sievers’s earlier model, this was the sole model that Volkov relied on, and there is no suggestion that he made use of multiple “published theories” or “earlier models”; moreover, he made more extensive use of data generated by his own experiment than of Hodick and Sievers’s model.