OG-12-71

onlysan2002

From the explanations of ETS, E might be the right answer. But I think
that D is right too or is even better than E.  In the first
paragraphy, the author mentioned that "All of the cells in a particular
plant start out with the same complement of genes, how can
they............". Fom my point of view, it is the oligoshchair that
differentiates the structures of roots, leaves........... . So D cannot
be evaluated as a wrong answer.



Informative explanations will be highly appreciated.

wangyu73cn

请看看此帖：http://forum.chasedream.com/dispbbs.asp?boardID=25&ID=75457&page=2

这里ETS解释的第6句明确说了D的错误。即O这个东东不能产生任何的荷尔蒙。所以选项错了。

onlysan2002

Thanks for your kindness.

Actually, I made a mistake, since the number of the question I really want to figure out is OG12-72 rather than OG12-71.

wangyu73cn

从ETS对72题的解释中可见，C选项还是蛮清楚的。将原文列出：

Passage 12

All of 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

(5) small subset of the genes in a particular kind of cell are

expressed, or turned on, at a given time. This is accom-

plished by a complex system of chemical messengers

that in plants include hormones and other regulatory

molecules. Five major hormones have been identified:

(10) auxin, abscisic acid, cytokinin, ethylene, and gibberel-

lin. 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,

(15) each has more than one effect on the growth and devel-

opment of plants. The five has 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

(20) 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

(25) hormones is somewhat analogous to that of certain

hormones in animal. 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

(30) 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.

(35) Such a hierarchy may also exist in plants. Oligosac-

charins are fragments of the cell wall released by

enzymes: different enzymes release different oligosac-

charins. There are indications that pleiotropic plant

hormones may actually function by activating the

(40) enzymes that release these other, more specific chemical

messengers from the cell wall.

72. The passage suggests that, unlike the pleiotropic hormones, oligosaccharins could be used effectively to

(A) trace the passage of chemicals through the walls of cells

(B) pinpoint functions of other plant hormones

(C) artificially control specific aspects of the development of crops

(D) alter the complement of genes in the cells of plants

(E) alter the effects of the five major hormones on plant development

The best answer is C.

Lines 16-18 state that because each pleiotropic hormone has so many different effects on a plant, pleiotropic hormones “are not very useful in artificially controlling the growth of crops” (lines 17-18).

In contrast, the passage indicates that oligosaccharins have specific effects on the growth and development of plants.

Thus, in comparison to the pleiotropic hormones, oligosaccharins could potentially be effective in artificially controlling specific aspects of crop development.

Choices A, B, D, and E can be eliminated because they describe functions that are not attributed in the passage either to the pleiotropic hormones or to oligosaccharins.

[此贴子已经被作者于2004-11-12 14:38:39编辑过]

onlysan2002

OK. Thanks

haijung

71. According to the passage, which of the following best describes a function performed by oligosaccharins?

(A)  Regulating the daily functioning of a plant’s cells

(B)  Interacting with one another to produce different chemicals

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

(D)  Producing the hormones that cause plant cells to differentiate to perform different functions

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

答案(E)--- 看了OG的解釋仍不是很明白是如何推出來的.. 煩請牛牛指點，謝謝!