Name
uuoBKslrEE:r 7,2
Why Are Plants Green? lntroduction A pigment is a molecule that absorbs light in the visible portion of tfte electromagnetic spectrum. The leaves of most plants are richin pigments. These pigments absorb light and con-
vert it into chemical energy to fuel the produ'ction of sugars. The prlmary photosynthetic pigment is chlorophyll a. Other pigments such as chlorophyll b and carotenoids are referred to as accessory p,igments. These absorb light and funnel the energy to chlorophyll a. Different pigrnentsabssrb differentty,pes{wavelengths} of light. Sorne pigments might absorb blue light better than other wavelengths of light for example. Others may absorb all of 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 its percent of absorbance reading. The data in the Table 7.1 give possible spectrophotometer absorbance readings for the two plant chlotophylls.
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Graphing
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Graph the data for chlorophylt a and chlorophyll b on,the same graph. The line for each is an approximation of the absorption spectrurn for that rnolec'ule.
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Table 7.1. Data
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Wavelength
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Ghlorophyll a 7o Absorption
Ghlorophyll b 9/o
400 nanometers
32
8
425 nanometers
60
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450 nanorneters
10
62
475 nanometers
3
5l
500 nanometers
0
8
525 nanometers
0
0
550 nanometers
4
3
575 nanometers
2
4
600 nanometers
4
2
625 nanometers
3
20
650 nanometers
21
29
675 nanometers
M
4
700 nanometers
12
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Date
Why Are PJants Green? , can{d. Analysis Using information provided by your teacher or other available resources, fifld the colors of
lightthat correspond to each wavelength in the data table. Ssme wavelengths mayfall in $e transition range between two colsrs. Color csde your graph in a waythat clearly shows the color range between fi() and 7fi) nanometers.
l. tsased on fie data and your graphs, what can you conclude aboutthe trruo chlorophylls and'their a,bsorption spectra? ln.lvhatways are the tws sirnilar? Differeat?
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Chloropfiylls are the predominant pigments in leaves. Based on the.6ata and your graph, give a possible explanation for,why plants are green.'
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lf some wavelengths (colors) of light are absorbed by chlorophylls, what hqppgns to the sther wavelengths that are not absorbed? Give any possibilities you cari think of.
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Date
Why Are Plants Gree fi?,
cont
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Background lnformation Find out more about how color is created by light and pigments. Use the lnternet or resources provided by your teacher.
Follow-Up 1. Explain why leaves are green. Begin your explanation with white light coming from the su1 and ending in your eye.
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Based on the above data and your graph, which type of light is most importantto plants for photosynthesis? Explain.
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3.
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Design an experimentto collect evidence that supports your answerfor 0uestion 2 above.
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WORKSHEET 7.3
Why Are Plants Not Always Green? lntroduction Suppose a researcher tested the effect of light wavelength on photosynthetic activity. ln the experiment photosynthesis rate was measured by 0.production. A graph of the results, called an action spectrum, would look something like Figure 7.1. L,
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The graph resembles the absorption spectra for chlorophyll a and chlorophyll b, but there are some subtle but irnportant differences. Compare the graph to the absorption spectra for the chlorophylls.
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Wavelength Figure 7.1. Action Spectrum
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Discussion Ouestions 1. What observations can you make about similarities and differences?
Now focus on the left half of the graphs. Notice that the photosynthesis graph has a wider left peak than the chlorophyll absorption graphs. lt doesn't bottom out until 550 to 600 nm. What does this mean?
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Why Are Plants Not Always Greefi?,
conrd.
3. The action spectrum shows that photosynthesis occurs even in light that is not absorbed by chlorophyll a or b. Propose an explanation for this.
4.
How could you collect evidence to support your hypothesis?
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Graphirg Use ihe
dita from Tabte 7.2 to make an absorption spectrurn, grapJt,for carotensids, as 1rou
have done prev.iously for chforophylf a and b. Your teacher might hqve yoll put it sn,the sarne
graphs as those.
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Table 7.2. Data Wbvelength 400 nanometers
22
425 nanometers
23
450 nanometers
49
475.na.norneters 500 nanometers 525 nanometers
550 nanometers 575 nanometers
6fi) nanometers 625 nanometers 650 nanorneters
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Carotenoids %
675 nanometers
7fi) nanometers
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Date
Why Are Plants Not Always Green?,
cont'd.
Analysis
l. By looking at your graph, explain why the action spectrum for photosynthesis
shows
wider activity than the absorption spectra for the chlorophylls.
2. What color are carotenoids? Explain.
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What is the adaptive value of accessory pigments like carotenoids? That is, what advanta:ge do they provide plants?
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Web/Lihrary Besearch Fi$.informgtion !o explain the following:
1.
Leaves of trees in many parts of North America change color in the fall or before a dry season. Explain why and relate your explanation to carotenoids.
What is beta carotene? Where is it found? What does it do for plants? Why is it beneficialin a human diet?
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