问题 实验题

为验证pH对唾液淀粉酶活性的影响,实验如下:

(1)操作步骤:

①.在1~5号试管中分别加入0.5%的淀粉溶液2mL。

②.加完淀粉溶液后,向各试管中加入相应的缓冲液3mL,使各试管中反应液的pH依次稳定在5.00、6.20、6.80、7.40、8.00。

③.分别向1~5号试管中加入0.5%的唾液溶液1mL,然后进行37℃恒温水浴。

④.反应过程中,每隔1min从第3号试管中取出一滴反应液,滴在比色板上,加一滴碘液显色,待呈橙黄色时,立即取出5支试管,加碘液显色并比色,记录结果。

(2)结果见下表:(“+”表示蓝色程度)

请回答:

(1)实验过程为什么要选择都在37℃恒温水浴?____________________。

(2)3号试管加碘液后出现橙黄色,说明什么?____________________。

(3)如果反应速度过快,应当对唾液做怎样的调整?____________________。

(4)该实验得出的结论是什么?____________________。

答案

(1)①.在相同的温度条件下反应,排除温度对实验结果的影响。②.唾液淀粉酶的最适温度是37℃。

(2)此时淀粉被完全分解,加碘后呈现出碘液的颜色。

(3)加大唾液的稀释倍数。

(4)①.唾液淀粉酶的最适pH在6.2~7.4之间。②.pH过大或过小都会影响唾液淀粉酶的活性。

问答题 简答题
单项选择题

Shortly after September 11th, President Bush’s father observed that just as Pearl Harbor awakened this country from the notion that we could somehow avoid the call of duty to defend freedom in Europe and Asia in World War Two, so, too, should this most recent surprise attack erase the concept in some quarters that America can somehow go it alone in the fight against terrorism or in anything else for that matter.

But America’s allies have begun to wonder whether that is the lesson that has been learned--or whether the Afghanistan campaign’s apparent success shows that unilateralism works just fine. The United States, that argument goes, is so dominant that it can largely afford to go it alone.

It is true that no nation since Rome has loomed so large above the others, but even Rome eventually collapsed. Only a decade ago, the conventional wisdom lamented an America in decline. Bestseller lists featured books that described America’s fall. Japan would soon become "Number One". That view was wrong at the time, and when I wrote "Bound to Lead" in 1989, I, like others, predicted the continuing rise of American power. But the new conventional wisdom that America is invincible is equally dangerous if it leads to a foreign policy that combines unilateralism, arrogance and parochialism.

A number of advocates of "realist" international-relations theory have also expressed concern about America’s staying-power. Throughout history, coalitions of countries have arisen to balance dominant powers, and the search for traditional shifts in the balance of power and new state challengers is well under way. Some see China as the new enemy; others envisage a Russia-China-India coalition as the threat. But even if China maintains high growth rates of 6% while the United States achieves only 2%, it will not equal the United States in income per head until the last half of the century.

Still others see a uniting Europe as a potential federation that will challenge the United States for primacy. But this forecast depends on a high degree of European political unity, and a low state of transatlantic relations. Although realists raise an important point about the leveling of power in the international arena, their quest for new cold-war-style challengers is largely barking up the wrong tree. They are ignoring deeper changes in the distribution and nature of power in the contemporary world. The paradox of American power in the 21st century is that the largest power since Rome cannot achieve its objectives unilaterally in a global information age.

The author asserts that in dealing with world affairs the U. S should learn to()

A. combine unilateralism with nationalism

B. depend upon the conventional wisdom

C. draw a lesson from the collapse of Rome

D. revise its unilateral foreign policy wisely