手机APP下载

您现在的位置: 首页 > SAT > SAT每日一题 > 正文

江苏快三号码—官方网址22270.COM_SAT官方每日一题附答案和解析[阅读](2019年10月23日)

来源:广东快三app免费下载—官方网址22270.COM广东快三app免费下载—官方网址22270.COM英语 编辑:max   VIP免费外教试听课 |  广东快三app免费下载—官方网址22270.COM广东快三app免费下载—官方网址22270.COM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.

重点单词   查看全部解释    
cell [sel]

想一想再看

n. 细胞,电池,小组,小房间,单人牢房,(蜂房的)巢室

 
favorable ['feivərəbl]

想一想再看

adj. 有利的,赞许的,良好的,顺利的,偏袒的

 
block [blɔk]

想一想再看

n. 街区,木块,石块
n. 阻塞(物), 障

 
filter ['filtə]

想一想再看

n. 筛选,滤波器,过滤器,滤色镜
v. 过滤

联想记忆
complexity [kəm'pleksiti]

想一想再看

n. 复杂,复杂性,复杂的事物

联想记忆
critical ['kritikəl]

想一想再看

adj. 批评的,决定性的,危险的,挑剔的
a

 
selectivity [silek'tiviti]

想一想再看

n. 选择性;分离性;选择度

联想记忆
species ['spi:ʃiz]

想一想再看

n. (单复同)物种,种类

 
protein ['prəuti:n]

想一想再看

n. 蛋白质

 
configuration [kən.figju'reiʃən]

想一想再看

n. 结构,布局,形态,格式塔心理完形 [计算机] 配置

联想记忆


新东方SAT网络课程:试听更多SAT网络课程>>

发布评论我来说2句

    广东快三app免费下载—官方网址22270.COM广东快三app免费下载—官方网址22270.COM英语官方微信(微信号:ikekenet)

    每天向大家推送短小精悍的英语学习资料.

    添加方式1.扫描上方广东快三app免费下载—官方网址22270.COM广东快三app免费下载—官方网址22270.COM官方微信二维码。
    添加方式2.搜索微信号ikekenet江苏快三号码—官方网址22270.COM添加即广东快三app免费下载—官方网址22270.COM。