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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.
According to the passage, which of the following best describes a function performed by oligosaccharins?
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暂无雷哥网文字解析
ol对植物生长的影响是特定的
A错误，与daily functioning无关
B错误，文章未曾提及此种互动
C错误，有该项功能的是多效性荷尔蒙
D错误，文章未提及ol可以产生激素
E正确，该选项说明了ol对植物的影响是特定的
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.
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2018-05-23 12:03:31
没有看到第一段有OL 是因为没有好好读第一段的最后一句 所以段首和段尾都必须以提出SVO的方式好好读 其次 OL有两处定位 没有一处被推翻 所以必须两处定位都看 加以结合而选答案 在两处定位以外的都是无关 此题中 A regulating daily function 没有提及 排除 Binteracting with another 没有提及 排除 Creleasing specific chemical 是H的作用 所以文意不符 排除 Dproducing H 这个是En的作用 文意不符 排除
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2017-11-04 10:51:27
D选项：Producing the hormones that cause plant cells to differentiate to perform different functions没有说它产生H，文中对应部分可能是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.但是也没有说O可以释放H。 E选项：Influencing the development of a plant’s cells by controlling the expression of the cells’ genes对的。根据第一段 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. 这里有提到expressed，就是所有的植物都有这个过程，文章只是在讨论什么产生这种过程。那么E选项就回答了它是怎么影响这种过程的。 错因：未找到expression的原文定位
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2016-12-30 22:05:21
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