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SAT官方每日一题附答案和解析[阅读](2019年10月23日)

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Critical Reading>Select an Answer

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.

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cell [sel]

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n. 细胞,电池,小组,小房间,单人牢房,(蜂房的)巢室

 
favorable ['feivərəbl]

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adj. 有利的,赞许的,良好的,顺利的,偏袒的

 
block [blɔk]

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n. 街区,木块,石块
n. 阻塞(物), 障

 
filter ['filtə]

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n. 筛选,滤波器,过滤器,滤色镜
v. 过滤

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complexity [kəm'pleksiti]

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n. 复杂,复杂性,复杂的事物

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critical ['kritikəl]

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adj. 批评的,决定性的,危险的,挑剔的
a

 
selectivity [silek'tiviti]

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n. 选择性;分离性;选择度

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species ['spi:ʃiz]

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n. (单复同)物种,种类

 
protein ['prəuti:n]

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n. 蛋白质

 
configuration [kən.figju'reiʃən]

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n. 结构,布局,形态,格式塔心理完形 [计算机] 配置

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