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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.

The passage suggests that which of the following is a function likely to be performed by an oligosaccharin?

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当前版本由 nmbdy 更新于2017-11-04 10:53:44 感谢由 nmbdy 对此题目的解答所做出的贡献。

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 stimulate cell to become root system 刚好和第一段的三句话below结合可以推出
How then can these cells differentiate and form structures as different as roots, stems, leaves, and fruits?
This is accomplished by a complex system of chemical messengers that in plants include hormones and other regulatory molecules.
Studies of plants have now identified a new class of regulatory molecules called oligosaccharins.
Bproduce other OL 未提及 排除
Cactivate enzymes that release specific chemical  是H的作用 文意不符 排除
Dduplicate 未提及 排除
E multiple effects 层次未提及 排除


题目讨论 (4条评论)

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    杀鸡大业

    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.
    硬生生看成pleiotropic 的功能~太粗心鸟。
    0 0 回复 2017-10-06 19:49:02
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    KK立志71党

    选A,还可以从第一段问题入手,How then can these cells differentiate and form structures as different as roots, stems, leaves, and fruits? This is accomplished by hormone and regulatory molecules(就是oligosaaccharines) 所以ol这个东西可以促进形成一个具体的植物器官。
    B错,没有说ol产生ol
    C错,那是hormone的功能
    D错,没有提到复制
    E错,ol的功能是specific的
    0 0 回复 2017-07-29 14:56:16
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      天真麒麟猫回复KK立志71党

      同意应该定位在第一段,定位到最后一段反倒会被别的选项迷惑吧。。

      0 0 回复 2018-09-13 22:26:56

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    xiaoyi

    pleiotropic plant hormones
    0 0 回复 2017-02-20 21:59:24
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    yanhairiluo113

    从文中第四段可知,ol是more specific chemical messengers from the cell wall,因此要选择一个传递出specific意思的选项
    A正确,表达出了ol对植物所产生的specific的效果
    B错误,没有表达specific的意思
    C错误,该选项描述的是pleiotropic plant hormones的功能
    D错误,没有表达specific的意思
    E错误,没有表达specific的意思
    1 0 回复 2015-11-12 15:39:46
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