首页
登录
职称英语
As people age, their cells become less efficient and less able replace damage
As people age, their cells become less efficient and less able replace damage
游客
2023-12-20
39
管理
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. [br] 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。
转载请注明原文地址:https://tihaiku.com/zcyy/3289692.html
相关试题推荐
Humanshavealwaysbeenfascinatedbydreams.Thevividdreamspeopleremembe
Humanshavealwaysbeenfascinatedbydreams.Thevividdreamspeopleremembe
Humanshavealwaysbeenfascinatedbydreams.Thevividdreamspeopleremembe
Codeswitchingiswhenpeopleswitch,orchange,fromonelanguagetoanother
[originaltext]Therearetimeswhenpeoplearesotiredthattheyfallasleepal
Wehaveallheardofcounterfeitingbefore.Usuallyitreferstopeoplemakin
Wehaveallheardofcounterfeitingbefore.Usuallyitreferstopeoplemakin
Wehaveallheardofcounterfeitingbefore.Usuallyitreferstopeoplemakin
Somepeoplebelievethatinternationalsportcreatesgoodwillbetweenthenat
Somepeoplebelievethatinternationalsportcreatesgoodwillbetweenthenat
随机试题
[originaltext]Beethovenprobablybegantogodeafin1797,buthetriedto
Faces,likefingerprints,areunique.Didyoueverwonderhowitispossible
加工一批零件,甲单独做20天可以完工,乙单独做30天可以完工,现两队合作来完成这
A.5周 B.12周 C.6~8周 D.6周 E.18~20周胎动出现的
(2017年真题)下列说法错误的是()。A.公司型股权投资基金、合伙型股权
患者,女性,34岁。因持续高热4日食欲下降,未能进食,自述口干、尿少色黄。查体:
下列关于经济资本和监管资本的说法中,错误的是( )。A.账面资本是银行资本金的
影响较大的教学组织形式有个别教学制、班级授课制、分组教学制、道尔顿制和(
某人对所熟悉的环境突然感到气氛不对,周围环境已发生了某种对他不利的变化,使得此人
下列工程内容,属于临时性土建工程和安装工程的是( )。A.稳车和稳车棚 B.
最新回复
(
0
)