SAT OG 2018 Reading - Test 8 reading 5

Questions 42-52 are based on the following

This passage is adapted from Daniel Chamovitz, What a Plant Knows: A Field Guide to the Senses. ©2012 by Daniel Chamovitz.

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 insectvisiting 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

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.

Question 42 The primary purpose of the passage is to