Download Why are some Plants Green?

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Why Are Most Plants Green, But Some Are Not?
Introduction
A pigment is a molecule that absorbs light in the visible portion of the electromagnetic spectrum.
The leaves of most plants are rich in pigments. These pigments absorb light and convert it into
chemical energy to fuel the production of sugars. The primary photosynthetic pigment is
chlorophyll a. Other pigments such as chlorophyll b and carotenoids are referred to as accessory
pigments. These absorb light and funnel the energy to chlorophyll a.
Different pigments absorb different types (wavelengths) of light. Some pigments might
absorb blue light better than other wavelengths of light, for example. Others may absorb all the
colors well, or none.
A spectrophotometer is a machine used by scientists to measure the absorbance of light by
substances. The better a pigment absorbs a color (wavelength) of light, the higher it’s percent of
absorbance reading. The data in Table 6.1 give the spectrophotometer absorbance readings for
chlorophyll a and chlorophyll b.
Graphing
Create a line graph using the graph paper provided by your teacher. Graph the data for
chlorophyll a using a green colored pencil, the data for chlorophyll b using a blue colored pencil
and the data for the carotenoids using a red colored pencil. Put all three lines on the same graph.
Percent Absorption Readings For Chlorophyll a And Chlorophyll b
Wavelength
Chlorophyll a % Absorption
Chlorophyll b % Absorption
Carotenoid % Absorption
400 nm
32
8
425 nm
60
29
450 nm
10
62
475 nm
3
51
500 nm
0
8
525 nm
0
0
550 nm
4
3
575 nm
2
4
600 nm
4
2
625 nm
3
20
650 nm
21
29
675 nm
44
4
700 nm
12
0
Table 6.1: The percent absorption for plant pigments at various wavelengths.
22
23
49
43
55
34
0
0
0
0
0
0
0
Put the percent absorption on the Y-axis and the wavelengths on the X-axis. Make sure to give
your graph and both of the axes’ titles. Don’t forget to include the necessary units.
Use the long bar at the bottom of the graph to correctly color code the various wavelengths
correctly. Use your textbook to discover which color corresponds to each wavelength. Now use
your graph to answer the following questions.
Analysis
1. Based on the data and your graph, why is chlorophyll green?
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2. Why are most plants green?
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3. How can some plants perform photosynthesis if they are not green?
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4. In what way are chlorophyll a and chlorophyll b similar in their ability to absorb
wavelengths?
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5. In what way are chlorophyll a and chlorophyll b different in their ability to absorb
wavelengths?
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6. Which wavelengths can the carotenoids absorb well that both of the chlorophylls cannot?
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7. Which types of light are the most important to plants for photosynthesis?
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8. Design an experiment that would prove your hypothesis from question #7.
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9. What colors are carotenoids?
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10. What do carotenoids do for plants?
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11. Here in Michigan, the leaves of trees change color in the fall. Explain why and relate your
explanation to carotenoids.
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