问题 问答题 简答题

气孔开闭机理如何?植物气孔蒸腾是如何受光、温度、CO2浓度调节的?

答案

参考答案:关于气孔开闭机理主要有两种学说:

⑴无机离子泵学说又称K+泵假说。光下K+由表皮细胞和副卫细胞进入保卫细胞,保卫细胞中K+浓度显著增加,溶质势降低,引起水分进入保卫细胞,气孔就张开;暗中,K+由保卫细胞进入副卫细胞和表皮细胞,使保卫细胞水势升高而失水,造成气孔关闭。这是因为保卫细胞质膜上存在着H+_ATP酶,它被光激活后,能水解保卫细胞中由氧化磷酸化或光合磷酸化生成的ATP,产生的能量将H+从保卫细胞分泌到周围细胞中,使得保卫细胞的pH值升高,质膜内侧的电势变低,周围细胞的pH值降低,质膜外侧电势升高,膜内外的质子动力势驱动K+从周围细胞经过位于保卫细胞质膜上的内向K+通道进入保卫细胞,引发开孔。

⑵苹果酸代谢学说在光下,保卫细胞内的部分CO2被利用时,pH值上升至8.0~8.5,从而活化了PEP羧化酶,PEP羧化酶可催化由淀粉降解产生的PEP与HCO3-结合形成草酰乙酸,并进一步被NADPH还原为苹果酸。苹果酸解离为2H+和苹果酸根,在H+/K+泵的驱使下,H+与K+交换,保卫细胞内K+浓度增加,水势降低;苹果酸根进入液泡和Cl-共同与K+在电学上保持平衡。同时,苹果酸的存在还可降低水势,促使保卫细胞吸水,气孔张开。当叶片由光下转入暗处时,该过程逆转。

气孔蒸腾显著受光、温度和CO2等因素的调节。

⑴光光是气孔运动的主要调节因素。光促进气孔开启的效应有两种,一种是通过光合作用发生的间接效应;另一种是通过光受体感受光信号而发生的直接效应。光对蒸腾作用的影响首先是引起气孔的开放,减少内部阻力,从而增强蒸腾作用。其次,光可以提高大气与叶子温度,增加叶内外蒸气压差,加快蒸腾速率。

(2)温度气孔运动是与酶促反应有关的生理过程,因而温度对蒸腾速率影响很大。当大气温度升高时,叶温比气温高出2~10℃,因而,气孔下腔蒸气压的增加大于空气蒸气压的增加,这样叶内外蒸气压差加大,蒸腾加强。当气温过高时,叶片过度失水,气孔就会关闭,从而使蒸腾减弱。

⑶CO2CO2对气孔运动影响很大,低浓度CO2促进气孔张开,高浓度CO2能使气孔迅速关闭(无论光下或暗中都是如此)。在高浓度CO2下,气孔关闭可能的原因是:①高浓度CO2会使质膜透性增加,导致K+泄漏,消除质膜内外的溶质势梯度,②CO2使细胞内酸化,影响跨膜质子浓度差的建立。因此CO2浓度高时,会抑制气孔蒸腾。

阅读理解

A “lost tribe” that reached America from Australia may have been the first Native Americans, according to a new theory.

If proved by DNA evidence, the theory will break long established beliefs about the southerly migration of people who entered America across the Bering Strait, found it empty and occupied it.

On this theory rests the belief of Native Americans to have been the first true Americans. They would be classified to the ranks of escapee, beaten to the New World by Aboriginals (土著人) in boats.

To a European, this may seem like an academic argument, but to Americans it is a philosophical question about identity, Silvia Gonzales, of Liverpool University said .

Her claims are based on skeletons found in the California Peninsula of Mexico that have skulls quite unlike the broad Mongolian features of Native Americans. These narrow-skulled people have more in common with southern Asians, Aboriginal Australians and people of the South Pacific Region.

The bones, stored at the National Museum of Anthropology (人类学) in Mexico City, have been carbon-dated and one is 12,700 years old, which places it several thousand years before the arrival of people from the North. “We think there were several migration waves into the Americas at different times by different human groups,” Dr. Gonzales said. “The timing, route and point of origin of the first colonization of the Americas remains a most contentious topic in human evolution.”

But comparisons based on skull shape are not considered conclusive by anthropologists, so a team of Mexican and British scientists, backed by the Natural Environment Research Council, has also attempted to take out DNA from the bones. Dr. Gonzales declined yesterday to say exactly what the results were, as they need to be checked, but indicated that they were consistent(一致) with an Australian origin.

小题1:It is generally considered that the first Native Americans came from ________.

A.North Asia

B.Australia

C.South Pacific

D.South Asia小题2:The skeletons found in the California Peninsula of Mexico have ________.

A.the broad skull shape

B.the narrow skull shape

C.different features of Aboriginal Australians

D.the same features of Native Americans小题3:The underlined “contentious” is similar in meaning to “________”.

A.likely to cause great interest

B.difficult to solve

C.well-known to all

D.likely to cause argument小题4:Which of the following statements is true according to the text?

A.Research on skulls can draw an exact conclusion.

B.DNA tests have proved the fact that the first Native Americans came from Australian.

C.Scientists are still not sure about the origin of the Native Americans.

D.People began to enter America across the Bering Strait about 12,700 years ago.

填空题