Investigation: Absorption Spectrum & Action Spectrum
Introduction:
Light is a form of electromagnetic (EM) radiation, which travels in waves. More specifically, light
is a discrete packet of EM radiation called a photon. All types of electromagnetic radiation travel in
the form of waves at the speed of about 300,000 km per second – the speed of light. However, each
type of EM radiation has a unique wavelength. The different wavelengths determine the
characteristics of each form of EM radiation. The various types of EM radiation can be arranged in a
continuum, with the longest wavelength (radio waves) at one end and the shortest (gamma rays) at
the other. This continuum is called the electromagnetic spectrum.
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 as you will see in in the Chromatography Lab. 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.
Pre-Task Questions:
1. What are photons?
2. What is the electromagnetic spectrum?
3. What part of the electromagnetic spectrum should be emphasized in the analysis of plant
pigments?
Different pigments absorb different wavelengths of light. Some pigments might absorb blue light
better than other wavelengths of light, for example. Others may absorb all of the colors well. A
spectrophotometer is a machine used by scientists to measure the absorbance of light by substances,
like plant leaf pigments. The better a pigment absorbs a color (wavelength) of light, the higher its
percent of absorbance reading. The data in Table 1 give possible spectrophotometer absorbance
readings for the two major plant chlorophylls.
Graphing
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 molecule.
Table 1: Absorption Data by Chlorophyll pigments (a and b)
Wavelength of
Chlorophyll a
Chlorophyll b
electromagnetic radiation
% absorption
% absorption
400
32
8
425
60
29
450
10
62
475
3
51
500
0
8
525
0
0
550
4
3
575
2
4
600
4
2
625
3
20
650
21
29
675
44
4
700
12
0
Figure 1:
Analysis
Using Figure 1, find the colors of light that correspond to each wavelength in the data table. Some
wavelengths may fall in the transition between two colors. Color code your graph in a way that
clearly shows the color range between 400 and 700 nanometers.
1. Based on the data and your graph, what can you conclude about the two chlorophylls and their
absorption spectra? In what ways are the two similar? Different?
2. If some wavelengths (colors) of light are absorbed by chlorophylls, what happens to the other
wavelengths that not absorbed? Think of as many possibilities as you can think of.
Follow Up
3. Explain why leaves are green. Begin your explanation with white light coming from the sun and
ending in your eye.
4. Based on the above data and your graph, which type of light is most important to plants for
photosynthesis? Explain.
Action Spectrum
How do we link exposure to different wavelengths of light to the rate of photosynthesis in plants?
We use an action spectrum. Below, you will see a graph of oxygen production (i.e. photosynthesis
effectiveness) versus wavelength of incoming light.
Notice the graph of the action spectrum has some similarities & differences in structure to the
absorption spectrum of chlorophyll molecules.
Give one similarity:
Give one difference:
How do we account for these differences?
Now go back to your original graph…and add another graph for the carotenoid pigment.
Wavelength of
Carotenoid
electromagnetic radiation
% absorption
400
22
425
23
450
49
475
43
500
55
525
34
550
0
575
0
600
0
625
0
650
0
675
0
700
0
Analysis
1. By looking at your new graph, explain why the action spectrum for photosynthesis shows wider
activity than the absorption spectrum for chlorophyll.
2. What colour are carotenoids?
3. What is the adaptive value of accessory pigments like carotenoids, xanthophylls, and
anthocyanins?