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
7
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] According to the passage, the five well-known plant hormones are not useful in controlling the growth of crops because
选项
A、it is not known exactly what functions the hormones perform.
B、each hormone has various effects on plants.
C、none of the hormones can function without the others.
D、each hormone has different effects on different kinds of plants.
E、each hormone works on only a small subset of a cell’ s genes at any particular time.
答案
B
解析
为什么已知五种荷尔蒙不能用来控制作物生长?A.对其功能不能精确了解。无。B.正确。每种都对植物有多种功用。见原文L22—25。C.不能单独起作用。无。D.对不同植物作用不同。无。E.每种在特定时期只作用于一小部分基因。无。
转载请注明原文地址:https://tihaiku.com/zcyy/3359230.html
相关试题推荐
Visitorstothezoousuallypitytheanimalsowingtotheirparticularemoti
Visitorstothezoousuallypitytheanimalsowingtotheirparticularemoti
Visitorstothezoousuallypitytheanimalsowingtotheirparticularemoti
Visitorstothezoousuallypitytheanimalsowingtotheirparticularemoti
Visitorstothezoousuallypitytheanimalsowingtotheirparticularemoti
Visitorstothezoousuallypitytheanimalsowingtotheirparticularemoti
Visitorstothezoousuallypitytheanimalsowingtotheirparticularemoti
Visitorstothezoousuallypitytheanimalsowingtotheirparticularemoti
Visitorstothezoousuallypitytheanimalsowingtotheirparticularemoti
Visitorstothezoousuallypitytheanimalsowingtotheirparticularemoti
随机试题
HarlemRenaissance(文艺复兴)—ABriefIntroductionImpo
Overthepastcentury,thecompositionofthehumangenepool______appreciably
加速器应安装在靠近干式报警阀位置,且应有便于水进入加速器的措施
施工成本包括()。A.消耗的原材料、辅助材料、外购件等的费用 B.施工机械的
芍药汤的配伍特点是A.辛开苦降B.通因通用C.表里双解D.寒热共投E.气血并调
处方:植物油2g,硬脂醇25g,白凡士林25g,丙二醇12g,吐温-202g,蒸
患者,女,52岁,因患宫颈癌需行子宫切除术。术前准备做青霉素皮试时,稀释溶液用A
邓小平明确概括社会主义本质是在()。 A.党的十二大B.党的十二届三中全会
行车路单是由车辆运行调度机构根据车辆运行作业计划和()签发给驾乘人员的行车命令,
采用蓄电池组的直流系统,蓄电池的下列哪些电压不是直流系统正常运行时的母线电压?(
最新回复
(
0
)