首页
登录
职称英语
All of the cells in a particular plant start out with the same complement of
All of the cells in a particular plant start out with the same complement of
游客
2024-01-13
43
管理
问题
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 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 sys- tern of chemical messengers that in plants include hormones and other regulatory molecules. Five major hormones have been identified: auxin, abscisic acid, cytokinin, ethylene, and gibberel- lin. Studies of plants have now identified a new class of regulatory molecules called oligosaccharide.
Unlike the oligosaccharide, 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 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 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 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 of hormones from the adrenal cortex. These hormones have specific effects on target organs all over the body. One hormone stimulates the thy- roid 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. Oligosaccharides are fragments of the cell wall released by enzymes: different enzymes release different oligosaccharides. There are indications that pleiotropic plant hormones may ac- tually function by activating the enzymes that release these other, more specific chemical messengers from the cell wall. [br] The passage suggests that which of the following is a function likely to be performed by an oligosaccharide?
选项
A、To stimulate a particular plant cell to become part of a plant’ s root system.
B、To stimulate the walls of a particular cell to produce other oligosaccharides.
C、To activate enzymes that release specific chemical messengers from plant cell walls.
D、To duplicate the gene complement in a particular plant cell.
E、To produce multiple effects on a particular subsystem of plant cells.
答案
A
解析
以下哪一种是低聚糖的功能?低聚糖在文中是化学传送物质的一种(L15—17),而这些传送物质的一个作用就是促使特定细胞中一小部分基因被表达,从而解决L3—5提出的疑问。∴A正确,刺激特定的细胞使其成为植物根系的一部分。正是L3—5设问的内容。B.“produce other oligosaccharides”文中未提。C.“activate enzymes” 的是多向性荷尔蒙(如five well—known hormones),见L53—56。D.“duplicate”文中无。E.“multiple effects on a particular subsystem”文中无。
转载请注明原文地址:https://tihaiku.com/zcyy/3359232.html
相关试题推荐
Coralsareparticularlysensitivetochangesintemperature.珊瑚礁对温度的变化特别敏感。
Visitorstothezoousuallypitytheanimalsowingtotheirparticularemoti
Visitorstothezoousuallypitytheanimalsowingtotheirparticularemoti
Visitorstothezoousuallypitytheanimalsowingtotheirparticularemoti
Visitorstothezoousuallypitytheanimalsowingtotheirparticularemoti
Visitorstothezoousuallypitytheanimalsowingtotheirparticularemoti
Visitorstothezoousuallypitytheanimalsowingtotheirparticularemoti
Visitorstothezoousuallypitytheanimalsowingtotheirparticularemoti
Visitorstothezoousuallypitytheanimalsowingtotheirparticularemoti
Visitorstothezoousuallypitytheanimalsowingtotheirparticularemoti
随机试题
Healwaysdidwellatschool______havingtodopart-timejobseverynowandth
Accordingtothearticle,thevideogameplayercan______.[br]Theauthor’sge
Trendstowardreformhighlighttheinternationalappealofdecentra
在直接标价法下,如果人民币升值,则美元兑人民币()。A、增加或减少无法确定B、不变C、减少D、增加C直接标价法是用1个单位或100个单位的外国货
下列概念中与行为主义学习理论有关的是()。A.强化 B.社会交往
如果用一个圆来表示词语所指称的对象的集合,那么以下哪项中三个词语之间的关系符合下
(2017年真题)某医师治下元虚冷、虚阳上浮之上热下寒证,常选肉桂,此因肉桂除补
肾脏的功能主要是分泌和排泄尿液、废物、毒物和药物,调节和维持体内环境的平衡。变态
旅客运输需求来源于()领域。A:生产和分配 B:分配和交换 C:生产和消费
常用的进度比较方法中,可以形象、直观地反映实际进度与计划进度的比较情况的是()
最新回复
(
0
)