PHOTOSYNTHESIS: CHLOROPHYLL DRY LAB ASSIGNMENT
ANSWER KEY
PART A: Relationship Between Light Energy, Wavelength and Light Absorption by Chlorophyll
Use one piece of graph paper. Both graphs in Part A are done on the same graph paper with a common
scale used for wavelength (nm) on the horizontal axis for both graphs.
1. Plot these data on a graph (3 marks):
Energy (kJ)
17
14
11
10
Wavelength (nm)
400
500
600
700
Energy (kJ)
Wavelength vs. Energy
18
16
14
12
10
8
6
4
2
0
350
400
450
500
550
600
650
700
750
Wavelength (nm)
2. Can you find any relationship between light energy and wavelength? (1 mark)
There is a negative correlation; inverse correlation.
As wavelength increases, light energy decreases.
Short wavelengths = high frequency = high energy.
Long wavelengths = low frequency = low energy
3. Plot these data on a second graph (3 marks):
Wavelength (nm)
400
450
500
550
Light Absorption by Chlorophyll a (%)
67
72
39
23
600
650
700
30
70
7
Wavelength vs. Light Absorption by
Chlorophyll a
Light Absorption by Chlorophyll a (%)
80
70
60
50
40
30
20
10
0
350
400
450
500
550
600
650
700
750
Wavelength (nm)
4. Which wavelength(s) does chlorophyll a absorb the most? The least? (2 marks)
Most: 450 nm and 650 nm
Least: 700 nm
5. What light energy does chlorophyll a absorb? (2 marks)
450 nm: 400 nm corresponds with 17 kJ and 500 nm corresponds with 14 kJ.
Average energy: (17 kJ + 14 kJ)/2 = 15.5 kJ
15.5 kJ x 72% = 11.16 kJ
650 nm: 600 nm corresponds with 11 kJ and 700 nm corresponds with 10 kJ.
Average energy: (11 kJ + 10 kJ)/2 = 10.5 kJ
10.5 kJ x 70% = 7.35 kJ
Therefore chlorophyll a absorb at light energies of 11.16 kJ (450 nm) and 7.35 kJ (650 nm)
6. What colour(s) of artificial light would you see used to grow indoor plants successfully? (2
marks)
Violet, blue, or red light; these colours are absorbed the most by the plant (chlorophyll) for
photosynthesis.
PART B: Absorption Spectrum of Spinach Leaf Pigments
Wavelength (nm)
400
420
440
460
480
500
520
540
Absorption Spectrum of Spinach Leaf Pigments
Absorbance (%)
Wavelength (nm)
0.42
560
0.68
580
0.60
600
0.58
620
0.83
640
0.23
660
0.11
670
0.12
680
Absorbance (%)
0.12
0.15
0.17
0.25
0.40
0.32
0.56
0.24
1. Construct a line graph of the data with percent abundance along the vertical axis and
wavelength along the horizontal axis. Indicate the colours of the visible spectrum corresponding
to the wavelengths along the horizontal axis. (5 marks)
Absorption Spectrum of Spinach Leaf
Pigments
0.9
0.8
Absor bance (%)
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
350
400
450
500
550
600
650
Wavelenth (nm)
2. Which colours of an intact spinach leaf would be least visible? Why? (2 marks)
Blue because the leaf absorb blue wavelengths the most.
3. What colours is absorbed the least by the pigment extract? Why? (2 marks)
Green because it is the most reflected; spinach are green in colour.
700
4. Compare the graph to the absorption spectrum found below. Which pigment is most likely
responsible for the peak at 670 nm? (1 mark)
Chlorophyll a is responsible for 670 nm peak.
5. Why are there no peaks in the range of 500 nm-620 nm? (1 mark)
There are no pigments that absorb light at those wavelengths. % absorbance is closer to 0.
To absorb light at these wavelengths, accessory pigment phycoerythrin has to be present.
6. Which pigments are primarily responsible for absorption in the range of 400 nm-480 nm? (2
marks)
Chlorophyll a, b, and carotenoids.