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
It was once believed that the brain was independent of metabolic processes occurring elsewhere in the body. In recent studies, h
It was once believed that the brain was independent of metabolic processes occurring elsewhere in the body. In recent studies, h
admin
2019-02-01
19
问题
It was once believed that the brain was independent of metabolic processes occurring elsewhere in the body. In recent studies, however, we have discov- ered that the production and release in brain neurons of the neurotransmitter serotonin(neurotransmitters are compounds that neurons use to transmit signals to other cells)depend directly on the food that the body processes.
Our first studies sought to determine whether the increase in serotonin observed in rats given a large injection of the amino acid tryptophan might also occur after rats ate meals that change tryptophan levels in the blood. We found that, immediately after the rats began to eat, parallel elevations occurred in blood tryptophan, brain tryptophan, and brain serotonin levels. These findings suggested that the production and release of serotonin in brain neurons were normally coupled with blood-tryp-tophan increases. In later studies we found that injecting insulin into a rat’s bloodstream also caused parallel elevations in blood and brain tryptophan levels and in serotonin levels. We then decided to see whether the secretion of the animal’s own insulin similarly affected serotonin production. We gave the rats a carbohydrate-containing meal that we knew would elicit insulin secretion. As we had hypothesized, the blood trypto- phan level and the concentrations of tryptophan serotonin in the brain increased after the meal.
Surprisingly, however, when we added a large amount of protein to the meal, brain tryptophan and serotonin levels fell. Since protein contains tryptophan, why should it depress brain tryptophan levels? The answer lies in the mechanism that provides blood tryp- tophan to the brain cells. This same mechanism also provides the brain cells with other amino acids found in protein, such as tyrosine and leucine. The consumption of protein increases blood con- centration of the other amino acids much more, proportionately, than it does that of tryptophan. The more protein in the meal, the lower is the ratio of the resulting blood-tryptophan concentration to the concentration of competing amino acids, and the more slowly is tryptophan provided to the brain. Thus the more protein in a meal, the less serotonin subsequently produced and re- leased.
The authors’ discussion of the "mechanism that provides blood tryptophan to the brain cells"(lines 44-45)is meant to
选项
A、stimulate further research studies.
B、summarize an area of scientific investigation.
C、help explain why a particular research finding was obtained.
D、provide supporting evidence for a controversial scientific theory.
E、refute the conclusions of a previously mentioned research study.
答案
C
解析
作者为什么要讨论“色氨酸进入脑细胞的机制”?A.激励继续研究。无。B.总结科学研究。无。C.正确。解释为何会有一特殊观测结果。这一特殊结果即第三段一开始surprisingly的内容。D.支持一有争议论点。无。E.驳斥一个前面的研究结论。第三段举一特例并不是为了将前面观点驳倒,只是给一个补充说明而已。
转载请注明原文地址:https://jikaoti.com/ti/NgdYFFFM
本试题收录于:
GMAT VERBAL题库GMAT分类
0
GMAT VERBAL
GMAT
相关试题推荐
WashingtonIrvingwasAmerica’sfirstmanofletterstobeknowninternationally.Hisworkswerereceivedenthusiasticallyboth
Thechildrenwillhaveto_________theirplay-timehourswhenschoolopens.
Thathebroughtthecompanybigprofitswouldn’t______puttingthecompany’smoneyintohisownpockets.
Mr.andMrs.Wrighthavegotninechildren—it’sreallyabig______.
"Helicopterparenting"describesastyleofraisingchildrenwhereparentsareover-protectiveanddotoomuch.Thetermwasuse
Nooneknowshowmanlearnedtomakewords.Perhapshebeganbymakingsoundslikethosemadebyanimals.Perhapshegruntedlik
Whiletheotherlevelsinvolvephysicalsatisfaction,thatis,thefeeding,comfort,safety,andtransportation,thislevelstre
"I’mnot100%convincedthatmemorizingthedictionaryisthebestwayofimprovingyourvocabulary."saysthecharacterplayed
KidsoftenaskScoutBassett,ofPalmDesert,California,ifshewishesshehadtwonormallegs.Scout,18,answers,"No.Ihave
Americansusetheterm"collegestudents"tomeanstudentseitherincollegesoruniversities.Notonlythat,Americansalmost
随机试题
称取0.7875g硝酸银,定容至500mL,此溶液中银的质量浓度是0.1mg/mL。(银的相对原子质量是108)()
王某在使用信用卡过程中,超过规定的限额进行投资,发卡银行再三催讨欠款.王某故意不予理会,经银行核实,王某完全没有偿还能力。王某的行为()。
以下微量元素中具有潜在毒性,但低剂量时可能具有人体必需功能的是()。
()能够引起测评结果的误差。(2008年5月二级真题)(2007年5月二级真题)
人面鱼纹彩陶盆最可能出现在下列哪个地方?()
桥下的清波昼夜不停地淌过,仿佛快丢失了时间。自从和友人在桥上聚过一次之后,我就依恋上了这桥。每日傍晚,比那粗茶淡饭更悠恬的便是这桥与流水。因此每日饭后,我便踱步来到这里。等着夕阳渐渐藏入大山之后,晚霞洒遍天际,造物者再将这凄冷的意象映入冰冷的水中,成全了追
某人编写了向随机文件中写一条记录的程序,代码如下:TypeRTypeNameAsString*10TelAsString*20EndTypePrivateSubCommand1_Click(
Thegreatestadvantageofbooksdoesnotalwayscomefromwhatwerememberofthem,butfromtheir【C1】______.Agoodbookoften【
(1)FredericChopinwasborninZelazowaWola,Poland,orFebruary22,1810,toaFrenchfatherandPolishmother.Hisfather,N
TheTop10CarsthatAmericansHateTimesaresodifficultfortheautoindustrythatevenToyotaandHondahavenowexperie
最新回复
(
0
)