首页
外语
计算机
考研
公务员
职业资格
财经
工程
司法
医学
专升本
自考
实用职业技能
登录
外语
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
admin
2011-01-02
114
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.
According to the passage, which of the following statements is true of the process that discolors and toughens foods?
选项
A、It takes place more slowly than glycosylation in the human body.
B、It requires a higher ratio of glucose to protein than glycosylation requires in the human body.
C、It does not require the aid of enzymes to attach glucose to protein.
D、It proceeds more quickly when the food proteins have a molecular structure similar to that of crystallin proteins.
答案
C
解析
该题问:根据文章,下面哪一个关于使食物失色干燥的过程的说法是否正确?C项意为“它无需酶的帮助也能把葡萄糖依附于蛋白质”。A项意为“它比人体内糖基化过程进行(发生)得更慢”;B项意为“它需要比人体内糖基化所需的葡萄糖、蛋白比例更高”;D项意为“当食物蛋白具有和晶体状蛋白相似的结构时,它进行得更快”。
转载请注明原文地址:https://www.kaotiyun.com/show/iJeO777K
0
专业英语八级
相关试题推荐
Theworldisinaself-destructionmode,BythisstatementImeanthatthepeopleoftheworldarebentonmakingthisplanetin
"I’mSORRY."Fordaysthat’sbeenaboutallJapanhasheardfromitsOlympicathletes.Thosewerethefirstwordsutteredbya
Proponentsofdifferentjazzstyleshavealwaysarguedthattheirpredecessors,musicalstyledidnotincludeessentialcharacte
OncefoundalmostentirelyinthewesternUnitedStatesandinAsia,dinosaurfossilsatenowbeingdiscoveredonallsevencont
______referstoasociolinguisticsituationtwovarietiesofalanguageexistsidebysidethroughoutthecommunity,witheach
ThereisagreatconcerninEuropeandNorthAmericaaboutdecliningstandardsofliteracyinschools.InBritain,thefactthat
ACareerinAccountingAsafieldofstudyandwork,accountingisexpandingthroughouttheworld.Ajobinaccountingpromi
WhoisfamousforhishistoricalnovelofRobRoy?
Humanmigration:thetermisvague.Whatpeopleusuallythinkofisthepermanentmovementofpeoplefromonehometoanother.M
1ThiscalmingdownisperhapsthemainreasonwhyIkeepadiary.Itisincrediblehowthewrittensentencecancalmandta
随机试题
交流弧焊机外观检查的内容有哪些?
A.第一心音增强B.第一心音减弱C.第一心音分裂D.肺动脉瓣区第二心音增强伴分裂E.主动脉瓣区第二心音增强完全性房室传导阻滞【】
慢性肾衰竭病人血肌酐大于多少时提示进入尿毒症期
男,体重6kg,呼吸80/min,体温38℃,脉搏180/min,面色苍白,烦躁气促,鼻扇及三凹征明显,两肺可闻及中小湿性啰音,心音低钝,肝肋下3.5cm,诊断为肺炎合并心力衰竭。强心应用的毛花苷丙首次剂量为
关于头皮裂伤,下列哪项是错误的
A、可待因B、喷他佐辛C、二氢埃托啡D、美沙酮E、吗啡常用的吗啡和海洛因成瘾者脱毒治疗时的替代药物是( )。
编制物资需求计划的依据包括()计划。
框筒结构及筒中筒结构内筒的组成一般有()。
研究发现,周氏啮小蜂在杀灭美国白蛾方面,与其他鸟类吃其蛹有所不同。它是在瞬间将受精卵注入美国白蛾的蛹内,利用蛹内的营养物质繁殖自身后代,从而杀死白蛾。这一事实说明,动物在自然界中能够()。
在大型游乐公园里,现场表演是刻意用来引导人群流动的。午餐时间的表演是为了减轻公园餐馆的压力;傍晚时间的表演则有一个完全不同的目的——鼓励参观者留下来吃晚餐。表面上不同时间的表演有不同的目的,但这背后,却有一个统一的潜在目标,即________。填入划横
最新回复
(
0
)