由SAT考试官方出品(SAT daily practice)，每天一道权威SAT阅读练习题，在苦刷SAT真题之余，是保持备考状态并适当放松身心的绝佳调剂资料。众SATer可善用，本文为SAT阅读每日一练，考察行文理解能力。
Like an opera house, which has its public entrance separate from that for the performers, a cell has different doors for different molecules. Each gets scrutinized at its door before it can enter the cell. Now researchers from the University of California at San Francisco have revealed in the journal Science the three-dimensional structure of one such door, or membrane channel, that specializes in granting entry to a membrane component known as glycerol. Specifically the channel is called the glycerol facilitator (GlpF), from the bacterium Escherichia coli.
Bearing three alcohol groups, glycerol is a basic building block for the cell membrane. (Other components include fatty acids and small charged molecules.) And not just in E. coli. Indeed, although the channel the researchers studied is from a bacterium, it belongs to a large protein family dubbed the aquaporins, which are found in species ranging from bacteria to humans.
GlpF is highly specific for glycerol and similar polyalcohols. Somehow, even though water molecules are much smaller, they cannot enter. The new study reveals why. In order for glycerol to clear the four-channel configuration in the cell membrane, it must pass through a narrow selectivity filter in the center of a channel. Here it is surrounded by amino acids that closely match its own structure, which is hydrophilic ("water-loving") on one side and hydrophobic ("water-fearing") on the other. Water molecules, in contrast, can only pass through this area in single file, which is not energetically favorable, because they like to bond to one another. And ions, which are charged, are unable to pass the "water-fearing" side of the channel. This cell entrance, it seems, is truly exclusive.
In the first paragraph, the example of an opera house primarily serves to _________
A. clarify the reader's understanding of the role of a cell's membrane channels.
B. highlight differences between the structural components of buildings and the structural components of cells.
C. help the reader visualize the size of a glycerol facilitator relative to that of a cell.
D. suggest the complexity of the process by which cell membranes develop.
Choice A is the best answer. The author uses the familiar concept of a building with different entrances -- in this case, an opera house with one door for the audience and another door for the performers -- to help the reader understand that cell membrane channels are entrances that allow different molecules into a cell.
block[blɔk]n. 街区，木块，石块 . 阻塞(物), 障
bond[bɔnd]n. 债券，结合，粘结剂，粘合剂 vt. 使结
configuration[kən.figju'reiʃən]n. 结构，布局，形态，格式塔心理完形 [计算机] 配置
filter['filtə]n. 筛选，滤波器，过滤器，滤色镜 v. 过滤
exclusive[iks'klu:siv]adj. 独占的，唯一的，排外的 n. 独家
understand[.ʌndə'stænd]vt. 理解，懂，听说，获悉，将 ... 理解为，认为