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1、GMAT考试阅读资料(五)(2)Passage 24All of the cells in a particular pla nt start out with thesame compleme nt of gen es. How the n can these cellsdifferentiate and form structures as different as roots, stems ,leaves , and fruits ? The answer is that only a (5) small subset of the genes in a particular kind
2、of cell areexpressed , orturned on , at a give n time. This is accom- plished by a complex system of chemical messe ngersthat in pla ntsin clude horm ones and other regulatorymolecules. Five major horm ones have bee n ide ntified: (10) aux in , abscisic acid ,cytokinin , ethylene , and gibberel-lin.
3、 Studies of plants have now ide ntified a new class ofregulatory molecules called oligosacchari ns.Un like the oligosacchari ns, the five well-k nownplanthormones are pleiotropic rather than specific, that is ,(15) each has more tha n one effect on the growth anddevel-opme nt of pla nts. The five ha
4、s so manysimulta neouseffects that they are not very useful inartificiallyc on trolli ng the growth of crops. Aux in, for in sta nee ,stimulates the rate of cell elon gatio n, causes shoots to (20)grow up and roots to grow dow n, and in hibits thegrowth oflateral shoots. Auxi n also causes the pla n
5、t todevelop a vascular system , to form lateral roots , and toproduce ethylene.The pleiotropy of the five well-studied pla nt(25)horm ones is somewhat an alogous to that of certa inhorm ones in ani mal. For example , horm ones from thehypothalamus in the brain stimulate the an terior lobeof the pitu
6、itary gla nd to syn thesize and release many differe nthormones , one of which stimulates the release (30) ofhorm ones from the adre nal cortex. These horm oneshavespecific effects on target orga ns all over the body. Onehormone stimulates the thyroid gland, forexample , anotherthe ovaria n follicle
7、 cells , and so forth.In other words , there is a hierarchy of horm on es. (35) Such a hierarchy may also exist in pla nts. Oligosac-char ins are fragme nts of the cell wall released bye nzymes : differe nt en zymes release differe nt oligosac-chari ns. There are in dicati ons that pleiotropic pla n
8、thorm ones may actually fun cti on by activat ing the(40)en zymes that release these other , more specific chemical messe ngers from the cell wall.the five well-k nown1. Accord ing to the passagepla nthorm ones are n ot useful in con troll ing the growth ofcrops because(A) it is not known exactly wh
9、at fun cti ons thehorm ones perform(B) each horm one has various effects on pla nts(C) none of the hormones can function without the others(D) each hormone has different effects on different kinds of pla nts(E) each horm one works on only a small subset of a cell's genes at any particular time2.
10、 The passage suggests that the place ofhypothalamichorm ones in the horm onal hierarchies ofani mals is similar to the place of which of the followi ng in plants ?(A) Pla nt cell walls(B) The compleme nt of genes in each pla nt cell(C) A subset of a pla nt cell's gene compleme nt(D) The five maj
11、or horm ones(E) The oligosacchari ns3. The passage suggests that which of the followi ng isa fun cti on likely to be performed by an oligosacchari n(A) To stimulate a particular pla nt cell to become part of a pla nt's root system(B) To stimulate the walls of a particular cell to produce other o
12、ligosacchari ns(C) To activate en zymes that release specific chemical messe ngers from pla nt cell walls(D) To duplicate the gene compleme nt in a particular pla nt cell(E) To produce multiple effects on a particular subsystem of pla nt cells4. The author men ti ons specific effects that aux in has
13、on pla nt developme nt in order to illustrate the(A) point that some of the effects of pla nthorm ones can be harmful(B) way in which hormones are produced by plants(C) hierarchical n ature of the fun cti oning of pla nt horm ones(D) differe nces among the best-k nown pla nthorm ones(E) con cept of
14、pleiotropy as it is exhibited by pla nthorm ones5. Accord ing to the passage , which of the followi ngbest describes a fun cti on performed by oligosacchari ns?(A) Regulat ing the daily fun cti oning of a pla nt'scells(B) In teract ing with one ano ther to producediffere nt chemicals(C) Releas i
15、ng specific chemical messe ngers from a pla nt's cell walls(D) Produc ing the horm ones that cause pla nt cellsto differe ntiate to perform differe nt fun cti ons(E) In flue ncing the developme nt of a pla nt's cells bycon troll ing the expressi on of the cells'ge nesto6. The passage sug
16、gests that ,un like the pleiotropichormones , oligosaccharins could be used effectively(A) trace the passage of chemicals through the walls of cells(B) pinpoint fun cti ons of other pla nt horm ones(C) artificially con trol specific aspects of the developme nt of crops(D) alter the compleme nt of ge
17、nes in the cells of pla nts(E) alter the effects of the five major horm ones onpla nt developme nt7. The author discusses ani mal horm ones primarily in order to(A) in troduce the idea of a hierarchy of horm ones(B) explai n the effects that aux in has on pla nt cells(C) contrast the functioning of
18、plant hormones and ani mals horm ones(D) illustrate the way in which particular horm ones affect an imals(E) explai n the disti ncti on betwee n horm ones and regulatory moleculesWord是学生和职场人士最常用的一款办公软件之一，99.99% 的人知道它，但其实，这个软件背后，还有一大批隐藏技能你不知道。掌握他们，你将开启新世界的大门。Tab+Enter,在编过号以后，会自动编号段落Ctrl + D 调出字体栏，配合
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