Official 28 Set 5

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Plants' photoreceptors

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  • Q6
What is the lecture mainly about?
  • A. The differences in how humans and plants sense light

  • B. An explanation of an experiment on color and wavelength

  • C. How plants sense and respond to different wavelengths of light

  • D. The process by which photoreceptors distinguish wavelengths of light

显示答案 正确答案: C

我的笔记 编辑笔记

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    NARRATOR:Listen to part of a lecture in a botany class.

    FEMALE PROFESSOR:OK, last time we talked about photosynthesis, the process by which plants use light to convert carbon dioxide and water into food.Today, I want to talk about another way light affects plants.I'm sure you all know from physics class about how light moves in microscopic waves, and that we can only see light when the wavelength of that light is in a specific range; plus, depending on the wavelength, we see different colors.Well, plants are also capable of distinguishing between different wavelengths of light.Now I don't want to confuse you-it's not like plants have eyes-plants don't see in the sense that humans or animals do-but they do have photoreceptors.

    Photoreceptors are cells that respond to light by sending out a chemical signal.And the organism- the plant- reacts to this signal; in fact, the signals that plants get from their photoreceptors sometimes cause significant reactions.

    And many plants are seasonal, and one way they know when winter is ending and spring is beginning is by sensing the change in light.The time when an adult plant flowers is based on the amount of light the plant senses.Certain plant species won't flower if they sense too much light, and some plants will only flower if they sense a specific amount of light.Of course, these aren't conscious reactions-these plants just automatically respond to light in certain ways.

    Plants are also able to distinguish between specific wavelengths of light that the human eye cannot even see. Specifically, there's a wavelength called far-red.[realizing that the name is actually inappropriate] Although why they call it "far-red"... I mean, it's not really red at all. It lies in the infrared range of the spectrum. We can't see it, but plants can sense it as a different wavelength.

    OK, now I need to mention another thing about photosynthesis.I didn't explain how different wavelengths of light affect photosynthesis.When a plant absorbs light for performing photosynthesis, it only absorbs some wavelengths of light and reflects others.

    Plants absorb most of the red light that hits them. But plants only absorb some of the far-red light that hits them-they reflect the rest.Remember this, because it's going to be relevant in an experiment I want to discuss.

    This fascinating experiment showed that plants not only detect and react to specific wavelengths of light-plants can also detect and react to changes in the ratio of one wavelength to another.This experiment was called the Pampas experiment.

    The idea behind the Pampas experiment had to do with the response of plants to changes in the ratio of red light to far-red light that the plants sensed with their photoreceptors.Some biologists hypothesized that a plant will stop growing if it's in the shade of another plant-a reaction that's triggered when it senses an unusual ratio of red light to far-red light.OK, imagine there are two plants, one below the other.The plant on top would absorb most of the red light for photosynthesis but reflect most of the far-red light.That would lead to the plant in its shade sensing an unusual ratio-there would be less red light and more far-red light than normal.What that ratio signifies is important-a ratio of less red light to more far-red light would cause a reaction from the plant: it would stop growing taller-because that plant could sense it wasn't going to get enough sunlight to provide the energy to grow large.

    To test their hypothesis, researchers took some electrical lights-um, actually they were light-emitting diodes, or LEDs.

    These light-emitting diodes could simulate red light.So they put these LEDs around some plants that were in the shade.The LEDs produced light that the plants sensed as red-but, unlike sunlight, the light from these LEDs did not support photosynthesis.So the plants sensed a proper ratio of red light to far-red light and reacted by continuing to grow taller, while in reality, these plants were not getting enough energy from photosynthesis to support all of that growth.And because they weren't getting enough energy to support their growth, most of the shaded plants died after a short time.

  • 旁白:听一段植物学课程。

    教授:好了,上次我们讲了光合作用,即植物用光把二氧化碳和水转化成食物的过程。今天我想讲讲光影响植物的另一种方式。我相信你们从物理课上都知道了光是怎样做微观运动的,只有在光的波长达到一定范围时,我们才能看见光;还有,根据波长不同,我们看到的颜色也不同。植物也能分辨出不同波长的光。我不想把你们搞糊涂,不是说植物有眼睛—植物不像人或动物一样能看到光,但植物有光感受器。

    光感受器是通过发出化学信号对光线做出反应的细胞。生物体,或说植物,会对信号产生反应;事实上,光感受器发出的信号有时候会让植物产生剧烈反应。

    很多种植物是季节性的,它们感知冬去春始的一种方式就是感应光的变化。成年植物开花的时间基于植物感受到的光量而定。某些植物如果感受到过强的光时,是不会开花的,还有一些只会在感受到一定的光量时才会开花。当然,这些都不是有意识的反应——这些植物会以特定的方式自动对光线做出反应。

    植物也能分辨出人眼无法看到的特定波长的光,具体来说,一种称为远红的波长。尽管他们称其为“远红”……我是说,它根本不是红色的。这种光在光谱上处于红外谱段。我们看不见这种光,但植物能感受到这种不同的波长。

    好了,现在我要提到光合作用的另一方面。我没有解释不同波长的光是怎么影响光合作用的。植物吸收光进行光合作用时,它只吸收某些波长的光,反射别的光。

    植物吸收绝大多数接触到的红光,但只吸收了部分到达它们的远红光,它们反射了剩余部分。记住这点,这和我将要讲的实验有关。

    这个精彩的实验显示,植物不仅能感受到某些特定波长的光并作出反应,还能感应到不同光之间波长比的变化。这个实验称为潘帕斯实验。

    实验背后的思想是,植物的光感受器能感受到红光与远红光的比率变化,且植物会对此作出反应。一些生物学家提出假设,如果一种植物被另一种植物(的影子)遮住了,前者会停止生长——这是由植物感受到的红光与远红光的不寻常比率引起的反应。好了,想象有两种植物,一种在另一种下面。在高处的植物会吸收绝大部分的红光来进行光合作用,但反射绝大部分的远红光。这会让被影子遮挡住的植物感应到不寻常比率。红光比正常情况少了,而远红光多了。这个比率有重要意义—更少的红光与更多的远红光产生的比率会导致植物作出反应:植物会停止长高,因为会感觉到它没有足够的阳光来提供能量来生长。

    为了测试这个假说,研究人员用了一些电子灯,嗯……事实上他们用的是发光二极管,即LED灯。

    这些发光二极管能模拟红光。他们把这些LED灯放在被影子遮住的植物周围。LED灯发出的光会被植物感知成红光—和阳光不同的是,这些LED灯发出的光不能用作光合作用。那么这些植物感应到了红光和远红光的合适比率并作出反应,继续长高,但事实上,这些植物并没有从光合作用中得到足够的能量来维持这样的生长。因为植物没得到足够的能量来维持生长,大多数被影子遮挡的植物很快就死了。

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  • 本题对应音频:
    1 感谢 不懂
    音频1
    解析

    题型分析:主旨题

    原文定位

    Well, plants are also capable of distinguishing between different wavelengths of light.

    选项分析

    在文章开头部分先介绍相关的背景知识,提到之前讲到的观点和类比举例,第二段提到第一个关键词: distinguish between different length of light. 随后提到 sensing the change of light。最后提到photosynthesis.

    C选项sense respond是音频关键词的同义替换,是对全文主旨最准确的概括

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