北美gre考试阅读真题解析(A)
<p> <span word="As">As</span> <span word="people">people</span> <span word="age">age</span>, <span word="their">their</span> <span word="cells">cells</span> <span word="become">become</span> <span word="less">less</span> <span word="efficient">efficient</span> <span word="and">and</span> <span word="less">less</span> <span word="able">able</span> <span word="to">to</span> <span word="replace">replace</span> <span word="damaged">damaged</span> <span word="components">components</span>. <span word="At">At</span> <span word="the">the</span> <span word="same">same</span> <span word="time">time</span> <span word="their">their</span> <span word="tissues">tissues</span> <span word="stiffen">stiffen</span>. <span word="For">For</span> <span word="example">example</span>, <span word="the">the</span> <span word="lungs">lungs</span> <span word="and">and</span> <span word="the">the</span> <span word="heart">heart</span> <span word="muscle">muscle</span> <span word="expand">expand</span> <span word="less">less</span> <span word="successfully">successfully</span>, <span word="the">the</span> <span word="blood">blood</span> <span word="vessels">vessels</span> <span word="become">become</span> <span word="increasingly">increasingly</span> <span word="rigid">rigid</span>, <span word="and">and</span> <span word="the">the</span> <span word="ligaments">ligaments</span> <span word="and">and</span> <span word="tendons">tendons</span> <span word="tighten">tighten</span>.</p><p> <span word="Few">Few</span> <span word="investigators">investigators</span> <span word="would">would</span> <span word="attribute">attribute</span> <span word="such">such</span> <span word="diverse">diverse</span> <span word="effects">effects</span> <span word="to">to</span> <span word="a">a</span> <span word="single">single</span> <span word="cause">cause</span>. <span word="Nevertheless">Nevertheless</span>, <span word="researchers">researchers</span> <span word="have">have</span> <span word="discovered">discovered</span> <span word="that">that</span> <span word="a">a</span> <span word="process">process</span> <span word="long">long</span> <span word="known">known</span> <span word="to">to</span> <span word="discolor">discolor</span> <span word="and">and</span> <span word="toughen">toughen</span> <span word="foods">foods</span> <span word="may">may</span> <span word="also">also</span> <span word="contribute">contribute</span> <span word="to">to</span> <span word="age">age</span>-<span word="related">related</span> <span word="impairment">impairment</span> <span word="of">of</span> <span word="both">both</span> <span word="cells">cells</span> <span word="and">and</span> <span word="tissues">tissues</span>. <span word="That">That</span> <span word="process">process</span> <span word="is">is</span> <span word="nonenzymatic">nonenzymatic</span> <span word="glycosylation">glycosylation</span>, <span word="whereby">whereby</span> <span word="glucose">glucose</span> <span word="becomes">becomes</span> <span word="attached">attached</span> <span word="to">to</span> <span word="proteins">proteins</span> <span word="without">without</span> <span word="the">the</span> <span word="aid">aid</span> <span word="of">of</span> <span word="enzymes">enzymes</span>. <span word="When">When</span> <span word="enzymes">enzymes</span> <span word="attach">attach</span> <span word="glucose">glucose</span> <span word="to">to</span> <span word="proteins">proteins</span> , <span word="they">they</span> <span word="do">do</span> <span word="so">so</span> <span word="at">at</span> <span word="a">a</span> <span word="specific">specific</span> <span word="site">site</span> <span word="on">on</span> <span word="a">a</span> <span word="specific">specific</span> <span word="protein">protein</span> <span word="molecule">molecule</span> <span word="for">for</span> <span word="a">a</span> <span word="specific">specific</span> <span word="purpose">purpose</span>. <span word="In">In</span> <span word="contrast">contrast</span>, <span word="the">the</span> <span word="nonenzymatic">nonenzymatic</span> <span word="process">process</span> <span word="adds">adds</span> <span word="glucose">glucose</span> <span word="haphazardly">haphazardly</span> <span word="to">to</span> <span word="any">any</span> <span word="of">of</span> <span word="several">several</span> <span word="sites">sites</span> <span word="along">along</span> <span word="any">any</span> <span word="available">available</span> <span word="peptide">peptide</span> <span word="chain">chain</span> <span word="within">within</span> <span word="a">a</span> <span word="protein">protein</span> <span word="molecule">molecule</span>.</p><p> <span word="This">This</span> <span word="nonenzymatic">nonenzymatic</span> <span word="glycosylation">glycosylation</span> <span word="of">of</span> <span word="certain">certain</span> <span word="proteins">proteins</span> <span word="has">has</span> <span word="been">been</span> <span word="understood">understood</span> <span word="by">by</span> <span word="food">food</span> <span word="chemists">chemists</span> <span word="for">for</span> <span word="decades">decades</span>, <span word="although">although</span> <span word="few">few</span> <span word="biologists">biologists</span> <span word="recognized">recognized</span> <span word="until">until</span> <span word="recently">recently</span> <span word="that">that</span> <span word="the">the</span> <span word="same">same</span> <span word="steps">steps</span> <span word="could">could</span> <span word="take">take</span> <span word="place">place</span> <span word="in">in</span> <span word="the">the</span> <span word="body">body</span>. <span word="Nonenzymatic">Nonenzymatic</span> <span word="glycosylation">glycosylation</span> <span word="begins">begins</span> <span word="when">when</span> <span word="an">an</span> <span word="aldehyde">aldehyde</span> <span word="group">group</span> <span word="of">of</span> <span word="glucose">glucose</span> <span word="and">and</span> <span word="an">an</span> <span word="amino">amino</span> <span word="group">group</span> <span word="of">of</span> <span word="a">a</span> <span word="protein">protein</span> <span word="are">are</span> <span word="attracted">attracted</span> <span word="to">to</span> <span word="each">each</span> <span word="other">other</span>. <span word="The">The</span> <span word="molecules">molecules</span> <span word="combine">combine</span>, <span word="forming">forming</span> <span word="what">what</span> <span word="is">is</span> <span word="called">called</span> <span word="a">a</span> <span word="Schiff">Schiff</span> <span word="base">base</span> <span word="within">within</span> <span word="the">the</span> <span word="protein">protein</span>. <span word="This">This</span> <span word="combination">combination</span> <span word="is">is</span> <span word="unstable">unstable</span> <span word="and">and</span> <span word="quickly">quickly</span> <span word="rearranges">rearranges</span> <span word="itself">itself</span> <span word="into">into</span> <span word="a">a</span> <span word="stabler">stabler</span>, <span word="but">but</span> <span word="still">still</span> <span word="reversible">reversible</span>, <span word="substance">substance</span> <span word="known">known</span> <span word="as">as</span> <span word="an">an</span> <span word="Amadori">Amadori</span> <span word="product">product</span>.</p>
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