GMAT补充阅读练习题!GMAT阅读部分的提分，还是要得益于大家在平时的刻苦练习。大家可以多寻找一些练习题，来检测自己在备考中的不足，这样能够辅助我们拿到高分。下面小编为大家整理了详细的内容，供大家参考!
All the cells in a particular plant start out with the same complement of genes. How then can these cells differentiate and form structures as different as roots, stems, leaves, and fruits? The answer is that only a small subset of the genes in a particular kind of cell are expressed, or turned on, at a given time. This is accomplished by a complex system of chemical messengers that in plants include hormones and other regulatory molecules. Five major hormones have been identified: auxin, abscisic acid, cytokinin, ethylene, and gibberellin. Studies of plants have now identified a new class of regulatory molecules called oligosaccharins.
Unlike the oligosaccharins, the five well-known plant hormones are pleiotropic rather than specific; that is, each has more than one effect on the growth and development of plants. The five have so many simultaneous effects that they are not very useful in artificially controlling the growth of crops. Auxin, for instance, stimulates the rate of cell elongation, causes shoots to grow up and roots to grow down, and inhibits the growth of lateral shoots. Auxin also causes the plant to develop a vascular system, to form lateral roots, and to produce ethylene.
The pleiotropy of the five well-studied plant hormones is somewhat analogous to that of certain hormones in animals. For example, hormones from the hypothalamus in the brain stimulate the anterior lobe of the pituitary gland to synthesize and release many different hormones, one of which stimulates the release of hormones from the adrenal cortex. These hormones have specific effects on target organs all over the body. One hormone stimulates the thyroid gland, for example, another the ovarian follicle cells, and so forth. In other words, there is a hierarchy of hormones.
Such a hierarchy may also exist in plants. Oligosaccharins are fragments of the cell wall released by enzymes: different enzymes release different oligosaccharins. There are indications that pleiotropic plant hormones may actually function by activating the enzymes that release these other, more specific chemical messengers from the cell wall.
4、The author mentions specific effects that auxin has on plant development in order to illustrate the
A.point that some of the effects of plant hormones can be harmful
B.way in which hormones are produced by plants
C.hierarchical nature of the functioning of plant hormones
D.differences among the best-known plant hormones
E.concept of pleiotropy as it is exhibited by plant hormones
正确答案：E
GMAT阅读题目解析
逻辑简图：
文章属于自然科学类题材，写作结构为现象解释型。
第一段：
第一层：提出现象，一组相同的基因产生出的细胞形成不同的组织结构，如根茎叶。
第二层：解释现象，因为只有一部分的基因被表达了，并且是通过五大荷尔蒙和OL物质表达的。
第二段：
第一层：五大荷尔蒙与OL物质不同，具有多效性。
第二层：五大荷尔蒙因为具有多效性，而在控制植物生长方面不是很有效。
第三段：
第一层：五大荷尔蒙在植物中的多效性和动物中相似。
第二层：五大荷尔蒙的专一性体现在动物的目标器官中。
第四段：
第一层：分级系统也出现在植物中。
第二层：植物中的分级系统表现为荷尔蒙释放酶，酶释放OL物质。
4. The author mentions specific effects that auxin has on plant development
in order to illustrate the
(细节题：定位到第二段的第二层;举例中的信息，注意例子前的总结句。)
(A) point that some of the effects of plant hormones can be harmful. 新说法
(B) way in which hormones are produced by plants. 新说法
(C) hierarchical nature of the functioning of plant hormones. 段落定位混淆
(D) differences among the best-known plant hormones. 新说法
(E) concept of pleiotropy as it is exhibited by plant hormones. 例子前的总结句
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