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SAT官方每日一题附答案和解析[阅读](2017年7月3日)

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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.

The main purpose of the passage is to ____

A.explain the ways proteins function in various species.
B.analyze and argue against a common claim about bacteria.
C.assert that a new research technique will enhance future studies in the field of cellular biology.
D.discuss a study that offers a new understanding of a fundamental cellular process.

重点单词   查看全部解释    
bond [bɔnd]

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n. 债券,结合,粘结剂,粘合剂
vt. 使结

 
critical ['kritikəl]

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adj. 批评的,决定性的,危险的,挑剔的
a

 
block [blɔk]

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n. 街区,木块,石块
n. 阻塞(物), 障

 
bacterium [bæk'tiəriəm]

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n. 细菌

 
species ['spi:ʃiz]

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n. (单复同)物种,种类

 
function ['fʌŋkʃən]

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n. 功能,函数,职务,重大聚会
vi. 运行

 
favorable ['feivərəbl]

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adj. 有利的,赞许的,良好的,顺利的,偏袒的

 
assert [ə'sə:t]

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vt. 主张,声明,断言

联想记忆
separate ['sepəreit]

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n. 分开,抽印本
adj. 分开的,各自的,

 
certain ['sə:tn]

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adj. 确定的,必然的,特定的
pron.

 

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