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Mrs. Fenech........Biology………Ch 8: The Cellular Basis of Activity
Optimizing photosynthesis & why are
plants green?
Can you increase the rate of Photosynthesis? Will stronger light lead to more
growth? Are certain wavelengths of light more effective? And why are plants green??
1. Examine the complete set of data for an experiment testing light intensity as a variable in
photosynthesis. Graph the data, draw a conclusion and explain why this would happen.
Light intensity % 0
5
10
15
20
25
30
35
40
45
O2 produced mL 0
4
8.2
12.6
17.1
21.6
26.1
30.5
31.3
31.3
Conclusion and explanation:
2. Examine the complete set of data for an experiment which tests wavelength as a variable in
photosynthesis. Graph the data, draw a conclusion and explain why this would happen.
Wavelength
375
400
425
450
500
525
550
575
600
625
650
nm
O2 produced
32
36
44
36
32
20
12
14
16.8
24
34
mL
Conclusion and explanation:
50
31.3
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 a chlorophyll b and carotenoids are referred to as accessory pigments.
Different pigments absorb different type (wavelengths) of light. Some pigments might absorb blue
light better that 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 absorbance of light by substances.
The better pigment absorbs a color (wavelength) of light, the higher its percent of absorbance
reading. The data table below gives possible spectrophotometer absorbance readings for the two plant
pigments.
Process & Procedure
1. Graph the data for chlorophyll a and chlorophyll b on the same graph. The line for each is an
approximation of the absorption spectrum for that pigment. As always, be sure to SPACE OUT your
graph, write a descriptive TITLE, label the AXES and make a KEY!
Wavelength of Visible
Chlorophyll a
Chlorophyll b
Light
% absorption
% absorption
400 nanometers
32
8
425 nanometers
60
29
450 nanometers
10
62
475 nanometers
3
51
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
44
4
700 nanometers
12
0
Process & Procedure….
2. Use the EM spectrum to find the colors of light that correspond to each wavelength on your graph.
Some wavelengths may fall in the transition range between two colors. Color-code your graph in a
way that clearly shows the color range between 400 and 700 nanometers.
Analysis
1. At what wavelength does chlorphyll a absorb the most light?
a. What color does this correspond to?
2. At what wavelength does chlorphyll b absorb the most light?
a. What color does this correspond to?
3. What wavelength of light is NOT absorbed by chlorophyll a or b?
a. What color does this correspond to?
4. Based on the data and your graphs, what can you conclude about the two chlorophylls and their
absorption spectra? In what ways are the two similar? Different?
5. Chlorophylls are the predominant pigments in leaves. Based on the data and your graph, give a
possible explanation for why plants are green.
6. If some wavelengths (colors) of light are absorbed by chlorophylls, what happens to the wavelengths
that are not absorbed? Give any possibilities you can think of.
7. Based on the data and your graph, which type of light is most important to plants for photosynthesis?
Explain.
8. Design an experiment to collect evidence that supports you answer for question 7 above.