Loudoun County Public Schools / Overview



Investigation: Absorption Spectrum & Action SpectrumIntroduction: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:What are photons? What is the electromagnetic spectrum? 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.GraphingGraph 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 ofChlorophyll aChlorophyll belectromagnetic radiation% absorption% absorption4003284256029450106247535150008525005504357524600426253206502129675444700120Figure 1:AnalysisUsing 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 Up3. 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 SpectrumHow 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 ofCarotenoidelectromagnetic radiation% absorption4002242523450494754350055525345500575060006250650067507000Analysis1. 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? ................
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