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DeliciaAn

今天又做开这篇文章了。弄懂了当时没有懂的问题。

All of the cells in a particular plant start out (start out: 开始, 着手) with the same complement of genes. How then can these cells differentiate and form structures as different as roots, stems, leaves, andfruits? 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 (abscisicacid: [生化]脱落酸),cytokinin, ethylene, and gibberellin. Studies of plants have now identified a new class of regulatory molecules called oligosaccharins.（这道题目－>在cell里的genes are expressed.这个过程is accomplished by a complex system of chemical messengers that include hormones and other regularoty molecules(oligosaccharins))

Unlike theoligosaccharins, the five well-known plant hormones are pleiotropic rather thanspecific; that is, each has more than one effect on the growth and developmentof plants. The five has so many simultaneous effects that they are not veryuseful in artificially controlling the growth of crops. Auxin, for instance,stimulates the rate of cell elongation, causes shoots to grow up and roots togrow down, and inhibits the growth of lateral shoots. Auxin also causes theplant to develop a vascular system, to form lateral roots, and to produceethylene.

The pleiotropy ofthe five well-studied plant hormones is somewhat analogous to that of certainhormones in animal. For example, hormones from the hypothalamus in the brainstimulate the anterior lobe of the pituitary gland (pituitary gland: n.[解]脑下垂体) to synthesize and releasemany different hormones, one of which stimulates the release of hormones fromthe adrenal cortex (adrenal cortex: 肾上腺皮质). These hormones havespecific effects on target organs all over the body. One hormone stimulates thethyroid gland, for example, another the ovarianfollicle (ovarian follicle: 卵泡) cells,and so forth. In other words, there is a hierarchy of hormones. Such ahierarchy may also exist in plants. Oligosaccharins are fragments of the cellwall released by enzymes: different enzymes release different oligosaccharins.There are indications that pleiotropic plant hormones may actually function byactivating the enzymes that release these other, more specific chemicalmessengers from the cell wall.

5.     Accordingto the passage, which of the following best describes a function performed byoligosaccharins?

(A) Regulating the daily functioning of aplant’s cells

(B) Interacting with one another toproduce different chemicals

(C) Releasing specific chemical messengersfrom a plant’s cell walls

(D) Producing the hormones that causeplant cells to differentiate to perform different functions（E）

(E) Influencing the development of aplant’s cells by controlling the expression of the cells’ genes         

2643301345

E 的答案在第一段。
第一段你自己看讲了什么。

DeliciaAn

2643301345 发表于 2015-3-22 12:17
E 的答案在第一段。
第一段你自己看讲了什么。

嗯嗯。好的。谢谢啦