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[Reading Comprehension]
试题详情
文章：
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.
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.
题目：
The author mentions specific effects that auxin has on plant development in order to illustrate the
选项:
A、point that some of the effects of plant hormones can be harmfulway in which hormones are produced by plants
B、hierarchical nature of the functioning of plant hormones
C、differences among the best-known plant hormones
D、concept of pleiotropy as it is exhibited by plant hormones
答案:
E
提问：
1.请张慧雯老师回答，做本题的时候，我定位到原文第二段auxin，for instance..的上一句，说的是这五种荷尔蒙会同时产生很多影响，这些影响对控制玉米的生长没有什么用处，判断为负态度。所以我选了A,因为也看到harmful也为负态度。
2.看了OG答案,这道题正确定位句子应为本段第一句（主旨句），所以是否以后碰到类似题目（即问举一个例子的目的）的时候，找到对应段落的主旨句即可。
解答：
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阅读797
提问：
1.请张慧雯老师回答，做本题的时候，我定位到原文第二段auxin，for instance..的上一句，说的是这五种荷尔蒙会同时产生很多影响，这些影响对控制玉米的生长没有什么用处，判断为负态度。所以我选了A,因为也看到harmful也为负态度。
2.看了OG答案,这道题正确定位句子应为本段第一句（主旨句），所以是否以后碰到类似题目（即问举一个例子的目的）的时候，找到对应段落的主旨句即可。
解答：
点赞0
阅读798
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