GRE阅读题目解析:人体细胞修复效率随年龄增长减弱

　　P2

　　1

　　As people age, their cells become less efficient and less able to replace damaged components.

　　随着年纪增大，人体细胞的修复效率越来越低，越来越不能替换受损部分。

　　2

　　At the same time their tissues stiffen.

　　同时人体组织变得僵硬。

　　3

　　For example, the lungs and the heart muscle expand less successfully, the blood vessels become increasingly rigid, and the ligaments and tendons tighten.

　　比如肺和心脏的肌肉伸展不足，血管越发硬化，韧带和肌腱变紧。

　　4

　　Few investigators would attribute such diverse effects to a single cause.

　　很少有研究者把这些现象归于单个原因。

　　5

　　Nevertheless, researchers have discovered that a process long known to discolor and toughen foods may also contribute to age-related impairment of both cells and tissues.

　　但研究者已经发现，一个早就已知的让食物褪色变硬的过程，也可能导致细胞和组织老化损伤。

　　6

　　That process is nonenzymatic glycosylation, whereby glucose becomes attached to proteins without the aid of enzymes.

　　这个过程就是 NG(非酶糖基化)，通过这个过程，葡萄糖在没有酶参与的情况下被连接到蛋白质。

　　7

　　When enzymes attach glucose to proteins (enzymatic glycosylation), they do so at a specific site on a specific protein molecule for a specific purpose.

　　当酶连接葡萄糖到蛋白质(EG 酶糖基化)时，会在特定位置连接特定蛋白质，出于特定目的。

　　8

　　In contrast, the nonenzymatic process adds glucose haphazardly to any of several sites along any available peptide chain within a protein molecule.

　　NG 过程则相反，葡萄糖被随意链接到任何就近的蛋白质分子的肽链的任何位置。

　　9

　　This nonenzymatic glycosylation of certain proteins has been understood by food chemists for decades, although few biologists recognized until recently that the same steps could take place in the body.

　　数十年前，食品化学家就掌握了特定蛋白质的 NG，尽管直到最近才有生物学家认识到人体内也会发生相同过程。

　　10

　　Nonenzymatic glycosylation begins when an aldehyde group (CHO) of glucose and an amino group (NH2) of a protein are attracted to each other.

　　当葡萄糖的一个醛基连接到蛋白质的一个氨基，就会发生 NG 。

　　11

　　The molecules combine, forming what is called a Schiff base within the protein.

　　分子连接在一起，在蛋白质内形成了被称为 SB 希夫氏碱的结构。

　　12

　　This combination is unstable and quickly rearranges itself into a stabler, but still reversible, substance known as an Amadori product.

　　这种连接并不稳定，会迅速重组成一种更为稳定，但仍可逆的产物，被称为 A (葡萄糖重排)产物。

　　13

　　If a given protein persists in the body for months or years, some of its Amadori products slowly dehydrate and rearrange themselves yet again, into new glucose-derived structures.

　　假设一种蛋白质在人体内存在数月或数年，它的一些 A(重排)产物就会缓慢脱水并再次重排，形成新的葡萄糖衍生结构。

　　14

　　These can combine with various kinds of molecules to form irreversible structures named advanced glycosylation end products (AGE's).

　　这些衍生物可能与多种分子结合，形成一些不可逆的结构，被称为晚期糖基化终末产物(AGE's)。

　　15

　　Most AGE's are yellowish brown and fluorescent and have specific spectrographic properties.

　　多数 AGE‘s 呈棕黄色，伴有荧光且具有独特的光谱特征。

　　16

　　More important for the body, many are also able to cross-link adjacent proteins, particularly ones that give structure to tissues and organs.

　　对人体而言，更重要的是许多 AGE's 还能交联邻近的蛋白质，特别是塑造组织和器官结构的蛋白质。

　　17

　　Although no one has yet satisfactorily described the origin of all such bridges between proteins, many investigators agree that extensive cross-linking of proteins probably contributes to the stiffening and loss of elasticity characteristic of aging tissues.

　　尽管尚无令人满意的研究描述这些蛋白质间连接的起源，但许多学者同意，蛋白质的大量交联可能导致了老化组织的硬化，以及失去弹性。

　　18

　　In an attempt to link this process with the development of cataracts (the browning and clouding of the lens of the eye as people age), researchers studied the effect of glucose on solutions of purified crystallin, the major protein in the lens of the eye.

　　一项研究致力于探究白内障(上年纪的人晶状体变成褐色，朦胧不清)的发展与蛋白质交联的关系，研究者考察了葡萄糖对晶体蛋白溶液的作用，晶体蛋白是眼球晶状体内的主要蛋白质。

　　19

　　Glucose-free solutions remained clear, but solutions with glucose caused the proteins to form clusters, suggesting that the molecules had become cross-linked.

　　不滴加葡萄糖的溶液保持清澈，而滴入葡萄糖的溶液出现了蛋白质结簇，表明分子已经发生交联。

　　20

　　The clusters diffracted light, making the solution opaque.

　　用光照射形成的蛋白质簇会发生衍射，使溶液呈半透明状。

　　21

　　The researchers also discovered that the pigmented cross-links in human cataracts have the brownish color and fluorescence characteristic of AGE's.

　　研究者还发现，白内障患者晶状体的交联蛋白质呈棕色，且有着 AGE's 的荧光特性。

　　22

　　These data suggest that nonenzymatic glycosylation of lens crystallins may contribute to cataract formation. (454 words)

　　这些数据表明晶状体内的晶体蛋白的 NG 过程是白内障的成因之一。

　　For the following question, consider each of the choices separately and select all that apply

　　3. According to the passage, which of the following is NOT characteristic of enzymatic glycosylation of proteins?

　　(A) Proteins affected by the process are made unstable.

　　(B) Glucose attachment impairs and stiffens tissues.

　　(C) Glucose is attached to proteins for specific purposes.

　　选 AB

　　定位在句 7、8：

　　When enzymes attach glucose to proteins (enzymatic glycosylation), they do so at a specific site on a specific protein molecule for a specific purpose.

　　In contrast, the nonenzymatic process adds glucose haphazardly to any of several sites along any available peptide chain within a protein molecule.

　　虽然不能做到在考场上完全读懂，但起码应该反应出：EG 是好伙的，NG 是坏蛋。EG 是规矩干事的，NG 是不受控制乱来的。

　　现在题目问我们，哪项不是 EG 的特征。

　　A 不是。涉及到 unstable 的讨论，都是 NG 部分的内容。

　　B 不是。Glucose attachment 分 EG 和 NG 两种，前者是好的，后者是坏的。而 B 描述的显然是坏作用，impairs and stffens tissues，所以不是在说 EG。

　　C 是 EG 正确描述，不选。

　　4. According to the passage, which of the following statements is true of Amadori products in proteins?

　　(A) They are more plentiful in a dehydrated environment.

　　(B) They are created through enzymatic glycosylation.

　　(C) They are composed entirely of glucose molecules.

　　(D) They are derived from Schiff bases.

　　(E) They are derived from AGE's.

　　选 D

　　细节题，整个过程在句 10-14 叙述的很清楚。

　　A 无法判断。关于脱水是 A 产物本身脱水重排，环境是否脱水的信息，文中没有交代。

　　B 错。显然是通过 NG，前面我们已经判断过了，EG 是正常动作。

　　C 错。句 8、10 都告诉我们，参与过程的是葡萄糖和蛋白质。

　　5. Which of the following best describes the function of the third paragraph of the passage?

　　(A) It offers evidence that contradicts the findings described in the first two paragraphs.

　　(B) It presents a specific example of the process discussed in the first two paragraphs.

　　(C) It explains a problem that the researchers mentioned in the second paragraph have yet to solve.

　　(D) It evaluates the research discoveries described in the previous paragraph.

　　(E) It begins a detailed description of the process introduced in the previous two paragraphs.

　　选 E

　　没什么说的。

　　6. The passage suggests that which of the following would be LEAST important in determining whether nonenzymatic glycosylation is likely to have taken place in the proteins of a particular tissue?

　　(A) The likelihood that the tissue has been exposed to free glucose

　　(B) The color and spectrographic properties of structures within the tissue.

　　(C) The amount of time that the proteins in the tissue have persisted in the body

　　(D) The number of amino groups within the proteins in the tissue

　　(E) The degree of elasticity that the tissue exhibits

　　选 D

　　问：在可能发生 NG 现象的特定器官蛋白质中，哪个因素是最不重要的，亦即最无关的，not relevant / not contributing。

　　A 有关。游离的葡萄糖越多，可能发生 NG 的概率越高。因为 NG 就是葡萄糖和蛋白质肽链乱亲导致的。糖尿病人感到这和自己有关。

　　B 有关。从句 15 可以看到，AGE‘s 的颜色和光谱特征可以帮我们判断其存在。

　　C 有关。根据句 13：If a given protein persists in the body for months or years, some of its Amadori products slowly dehydrate and rearrange themselves yet again, into new glucose-derived structures.

　　D 无关。

　　E 有关。发生交联的器官会变硬，失去弹性，见句 17：Although no one has yet satisfactorily described the origin of all such bridges between proteins, many investigators agree that extensive cross-linking of proteins probably contributes to the stiffening and loss of elasticity characteristic of aging tissues.

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