为了让我们更好的适应SAT考试，SAT考试官网每天都会有SAT官方每日一题，下面是澳际小编为您整理的2016年9月21日SAT题目，是阅读题，让我们一起来看一看这道题目吧。

　　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 bore 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.

　　澳际培训提醒各位参加准备sat考试的同学，仔细看完以上的sat考试官网题目，然后再看下面的SAT题目答案更有效哦。

　　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.

　　以上就是澳际小编为根据SAT考试官网最新信息，为您整理的2016年9月21日SAT题目，目的是为了让我们更好的适应SAT考试，如果在备考过程中遇到任何问题，欢迎随时咨询澳际培训，为您提供最专业、最全面的解答。

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