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  3. As people age, their cells become less efficient and less able replace damaged components. At the same time their tissues stiffe

As people age, their cells become less efficient and less able replace damaged components. At the same time their tissues stiffe

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问题    As people age, their cells become less efficient and less able 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 (HN2) 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.
Which of the following best describes the function of the third paragraph of the passage?
选项 A、It presents a specific example of the process discussed in the first two paragraphs.
B、It explains a problem that the researchers mentioned in the second paragraph has yet to solve.
C、It evaluates the research discoveries described in the previous paragraph.
D、It begins a detailed description of the process introduced in the previous two paragraphs.
答案D
解析 该题问：下面哪一个最好地描述了文章第三段所起的作用？D项意为“第三段开始详细描述前两段介绍的过程”。第三段提到：“某种蛋白的无酶催化糖基化过程，几十年来食品化学家早已了解，虽然生物学家直到最近才认识到同样的步骤也能发生在人体中。当葡萄糖醛基(CHO)和一个蛋白质的氨基(NH2)相互吸引时，无酶催化糖基化过程就开始。这些分子的结构在蛋白内部形成所谓的“席夫碱”(Schiff base)。这种结合并不稳定，很快重新组织成更为稳定，但仍然可逆转的物质，称之为阿马多利产品”。A项意为“这段提供了第一、二段内论述的过程的特定例子”；B项意为“这段解释了第二段内研究者提到问题得解决”；C项意为“这段评估了前一段描述的研究发现”。这三项都有欠缺。
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