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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.

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.

閲嶇偣鍗曡瘝   鏌ョ湅鍏ㄩ儴瑙i噴    
cell [sel]

鎯充竴鎯冲啀鐪

n. 缁嗚優锛岀數姹狅紝灏忕粍锛屽皬鎴块棿锛屽崟浜虹墷鎴匡紝(铚傛埧鐨)宸㈠

 
favorable ['feivərəbl]

鎯充竴鎯冲啀鐪

adj. 鏈夊埄鐨勶紝璧炶鐨勶紝鑹ソ鐨勶紝椤哄埄鐨勶紝鍋忚鐨

 
block [blɔk]

鎯充竴鎯冲啀鐪

n. 琛楀尯锛屾湪鍧楋紝鐭冲潡
n. 闃诲(鐗), 闅

 
filter ['filtə]

鎯充竴鎯冲啀鐪

n. 绛涢夛紝婊ゆ尝鍣紝杩囨护鍣紝婊よ壊闀
v. 杩囨护

鑱旀兂璁板繂
complexity [kəm'pleksiti]

鎯充竴鎯冲啀鐪

n. 澶嶆潅锛屽鏉傛э紝澶嶆潅鐨勪簨鐗

鑱旀兂璁板繂
critical ['kritikəl]

鎯充竴鎯冲啀鐪

adj. 鎵硅瘎鐨勶紝鍐冲畾鎬х殑锛屽嵄闄╃殑锛屾寫鍓旂殑
a

 
selectivity [silek'tiviti]

鎯充竴鎯冲啀鐪

n. 閫夋嫨鎬э紱鍒嗙鎬э紱閫夋嫨搴

鑱旀兂璁板繂
species ['spi:ʃiz]

鎯充竴鎯冲啀鐪

n. 锛堝崟澶嶅悓锛夌墿绉嶏紝绉嶇被

 
protein ['prəuti:n]

鎯充竴鎯冲啀鐪

n. 铔嬬櫧璐

 
configuration [kən.figju'reiʃən]

鎯充竴鎯冲啀鐪

n. 缁撴瀯锛屽竷灞锛屽舰鎬侊紝鏍煎紡濉斿績鐞嗗畬褰 [璁$畻鏈篯 閰嶇疆

鑱旀兂璁板繂

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