SAT考试9月每日精选题19

>>SAT考试练习：SAT考试9月每日精选题19

>>SAT考试精品高端课程辅导

Critical Reading>Select an Answer

Like an opera house, which has its public entranceseparate from that for the performers, a cell hasdifferent doors for different molecules. Each getsscrutinized at its door before it can enter the cell. Now researchers from the University of California atSan Francisco have revealed in the journal Science the three-dimensional structure of onesuch door, or membrane channel, that specializes in granting entry to a membranecomponent 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 alarge protein family dubbed the aquaporins, which are found in species ranging from bacteriato humans.

GlpF is highly specific for glycerol and similar polyalcohols. Somehow, even though watermolecules are much smaller, they cannot enter. The new study reveals why. In order forglycerol to clear the four-channel configuration in the cell membrane, it must pass througha narrow selectivity filter in the center of a channel. Here it is surrounded by amino acidsthat closely match its own structure, which is hydrophilic ("water-loving") on one side andhydrophobic ("water-fearing") on the other. Water molecules, in contrast, can only passthrough this area in single file, which is not energetically favorable, because they like to bondto one another. And ions, which are charged, are unable to pass the "water-fearing" side of thechannel. 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 cellularbiology.

D.discuss a study that offers a new understanding of a fundamental cellular process.

温馨提示：各sat考生在看过上述sat阅读试题之后，自己分析解答，再看相应的sat阅读答案及解析，会更有效哦!

答案：D

解析：

Choice D is the best answer. The author's focus is on discussing work done by researchers studying the structure of membrane channels in cells. According to the author, the researchers' study revealed how certain molecules move through cells' membranes.

以上就是小编带来的精彩分享，在此，坦途网SAT考试频道祝各位考生，考试顺利，早日出国留学。