手機APP下載

您現在的位置: 首頁 > SAT > SAT每日一題 > 正文

SAT官方每日一題附答案和解析[閱讀](2019年10月23日)

來源:可可英語 編輯:max ?  VIP免費外教試聽課 |  可可官方微信:ikekenet

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.

重點單詞   查看全部解釋    
cell [sel]

想一想再看

n. 細胞,電池,小組,小房間,單人牢房,(蜂房的)巢室

 
favorable ['feivərəbl]

想一想再看

adj. 有利的,贊許的,良好的,順利的,偏袒的

 
block [blɔk]

想一想再看

n. 街區,木塊,石塊
n. 阻塞(物), 障

 
filter ['filtə]

想一想再看

n. 篩選,濾波器,過濾器,濾色鏡
v. 過濾

聯想記憶
complexity [kəm'pleksiti]

想一想再看

n. 復雜,復雜性,復雜的事物

聯想記憶
critical ['kritikəl]

想一想再看

adj. 批評的,決定性的,危險的,挑剔的
a

 
selectivity [silek'tiviti]

想一想再看

n. 選擇性;分離性;選擇度

聯想記憶
species ['spi:ʃiz]

想一想再看

n. (單復同)物種,種類

 
protein ['prəuti:n]

想一想再看

n. 蛋白質

 
configuration [kən.figju'reiʃən]

想一想再看

n. 結構,布局,形態,格式塔心理完形 [計算機] 配置

聯想記憶
?

新東方SAT網絡課程:試聽更多SAT網絡課程>>

發布評論我來說2句

    可可英語官方微信(微信號:ikekenet)

    每天向大家推送短小精悍的英語學習資料.

    添加方式1.掃描上方可可官方微信二維碼。
    添加方式2.搜索微信號ikekenet添加即可。
    河南今天11选5开奖结果