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GRE阅读题目解析:人体细胞修复效率随年龄增长减弱

2017-07-24 08:25:00来源：网络

　　【GRE阅读题目解析:人体细胞修复效率随年龄增长减弱】GRE考试考什么？GRE阅读是重点。新东方在线GRE频道为大家带来GRE阅读题目解析:人体细胞修复效率随年龄增长减弱，答案见下一页，希望对大家GRE备考有所帮助。更多精彩尽请关注新东方在线GRE频道!

　　查看全部：GRE阅读答案解析(汇总)

　　GRE阅读题目解析:人体细胞修复效率随年龄增长减弱

　　P2

　　As people age, their cells become less efficient and less able to replace damaged components. At the same time their tissues stiffen. For example, the lungs and the heart muscle expand less successfully, the blood vessels become increasingly rigid, and the ligaments and tendons tighten.

　　Few investigators would attribute such diverse effects to a single cause. 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. That process is nonenzymatic glycosylation, whereby glucose becomes attached to proteins without the aid of enzymes. 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.

　　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. Nonenzymatic glycosylation begins when an aldehyde group (CHO) of glucose and an amino group (NH2) of a protein are attracted to each other. The molecules combine, forming what is called a Schiff base within the protein. This combination is unstable and quickly rearranges itself into a stabler, but still reversible, substance known as an Amadori product.

　　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. These can combine with various kinds of molecules to form irreversible structures named advanced glycosylation end products (AGE's). Most AGE's are yellowish brown and fluorescent and have specific spectrographic properties. More important for the body, many are also able to cross-link adjacent proteins, particularly ones that give structure to tissues and organs. 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.

　　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. Glucose-free solutions remained clear, but solutions with glucose caused the proteins to form clusters, suggesting that the molecules had become cross-linked. The clusters diffracted light, making the solution opaque. The researchers also discovered that the pigmented cross-links in human cataracts have the brownish color and fluorescence characteristic of AGE's. These data suggest that nonenzymatic glycosylation of lens crystallins may contribute to cataract formation. (454 words)

　　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.

　　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.

　　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.

　　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

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