问题 选择题

在“绿叶在光下制造有机物”的实验中,把天竺葵提前放到黑暗处一昼夜的主要目的是(  )

A.使叶片中的淀粉运走、消耗干净

B.提高光合作用的效率

C.使叶片中的无机物运走、消耗干净

D.提高蒸腾作用的效率

答案

实验步骤:

a.暗处理:把盆栽的天竺葵放到黑暗处一昼夜,目的是把叶片中的淀粉全部转运和消耗.

b.部分遮光:用黑纸片把叶片的一部分从上下两面遮盖起来,然后移到阳光下照射. 

c.几小时后,摘下叶片,去掉遮光的纸片.

d.脱色:把叶片放入盛有酒精的小烧杯中,隔水加热,使叶片中的叶绿素溶解到酒精中,叶片变成黄白色.

e.漂洗、染色:用清水漂洗叶片,再把叶片放到培养皿里,向叶片滴加碘液.

f.观察:稍停片刻,用清水冲掉碘液,观察叶片颜色发生了什么变化.

暗处理:把盆栽的天竺葵放到黑暗处一昼夜,目的是让天竺葵在黑暗中把叶片中的淀粉全部运走和消耗干净,样实验中用碘液检验的淀粉只可能是叶片在实验过程中制造的,而不能是叶片在实验前贮存.作对照实验(遮光处没有光合作用,不遮光生成淀粉),减少其他因素对探究的干扰使探究具有可信度,否则无法确认后来叶片中淀粉是否是光合作用制造的.

故选A

单项选择题
单项选择题

When European Union (EU) leaders took delivery of Europe’s first draft of a constitution at a summit in Greece last June, it was with almost universal praise.

There was wide agreement that the text could save the EU from paralysis once it expands from 15 to 25 members next year. It would give Europe a more stable leadership and greater clout on the world stage, said the chairman of the Convention which drafted the agreement, former French President Valery Giscard d’Estaing.

Such praise was too good to last. As the product of a unique 16-month public debate, the draft has become a battleground. Less than four months after it was delivered, the same leaders who accepted it opened the second round of talks on its content this week by trading veiled threats to block agreement or cut off funds if they don’t get their way.

The tone was polite, but unyielding. In a bland joint statement issued when the talks opened on October 4, the leaders stressed the constitution, "represents a vital step in the process aimed at making Europe more cohesive, more democratic and closer to its citizens. "Sharp differences remain, though, between member countries of the EU over voting rights, the size and composition of the executive European Commission, defense co-operation and the role of religion in the new constitution.

Italian Prime Minister Silvio Berlusconi’s hopes of wrapping up a deal on the constitution by Christmas seem far from being realized. While the six founding members of the EU--Germany, France, Italy, the Netherlands, Belgium and Luxembourg--plus Britain and Denmark, want as little change as possible to the draft, the 10 mainly central European countries due to join the 15-nation bloc next year want to alter the institution’s balance.

Such small states are afraid their views will be ignored under the constitution and are determined to defend the disproportionate voting rights they won at the 2000 Nice Summit. EU experts fear such sharp differences will create exactly the paralysis in the EU the Convention was established to avoid.

According to experts, the real thing that may paralyze EU is()

A. the bigger size that EU will have in the future

B. the disproportionate voting rights of some small states

C. the fierce disagreement among its member countries

D. the threat from some leaders who want their ways