Lab 10 Activities Continued

The Role of Light in Carbohydrate Synthesis

Overview

In this activity, you will extract the photosynthetic pigments from geranium leaves. You will then use the SpectroVis to analyze the pigments and generate an absorbance spectrum. Your instructor will provide you with an absorbance spectrum for the pigments found in spirulina so you can compare the two.

Materials for Pigment Extraction and Starch Detection

Each lab group will need the following materials:

• 1 geranium leaf with the stem that was kept in the light
• 1 geranium leaf without the stem that was kept in the dark
• 1 large beaker
• 1 250 ml beaker
• water
• alcohol
• iodine
• hot plate
• petri dish with lid
• test tube clamp or tongs
• hot glove

Method for Pigment Extraction and Starch Detection

1. Set up a hot water bath using the large beaker. The water needs to be boiling.
2. Pick a leaf from a geranium exposed to light and one kept in the dark for 48 hours.
   • Keep the stem on the leaf grown in light.
   • Remove the stem from the leaf grown in the dark.
3. Hydrolyze the cell walls of the geranium leaves by boiling in a hot water bath for 5 minutes or until it looks like over-cooked vegetables.
4. While the leaves are boiling, fill the smaller beaker about a quarter full with alcohol. Make room on the hotplate to heat the alcohol.
   • Heat the alcohol for 2-3 minutes and remove from the hotplate using the hot glove.
5. Carefully remove the leaves from the boiling water using the test tube clamp or tongs. Place the leaves in the hot alcohol beaker for 7 minutes or until they turn white.
   • Save the green solution for the Absorbance Spectrum exercise.
6. Remove the leaves from the alcohol and place one in the petri dish and the other in the lid.
7. Add iodine to the dish. If starch is present, the leaf will turn a deep bluish-black color.
8. Photograph the leaf with your phone to document the effects of light on carbohydrate storage.

Materials for Absorbance Spectrum

Each lab group will need the following materials:

• green pigment extract from the previous activity
• pipette
• cuvette
• SpectroVis Plus spectrophotometer
• lap top

Method for Absorbance Spectrum

1. Connect the SpectroVis to the lap top using the USB cable.
2. Launch the Vernier Spectral Analysis program.
3. Select “Absorbance vs. Wavelength (Full Spectrum)”.
4. Wait 90 seconds while the lamp warms up.
5. When the warm up is complete, insert a blank cuvette.
6. Choose “Finish calibration” and remove the blank cuvette once calibration is complete.
7. Use the pipette to fill the cuvette with the geranium pigment.
   • Do NOT use acetone in these plastic cuvettes since it will frost over the plastic.
8. Insert the cuvette into the spectrophotometer and press “Collect” to start data collection.
9. Collect data between 380 nm-700 nm. Click “Stop” once 700 nm is reached.
10. Your instructor will prepare Spirulina extract diluted in ethanol in a cuvette and obtain the continuous absorbance spectrum.
11. Plot relative absorbance against wavelength using a line graph and compare the absorption spectrum of the extracts.
    • Relative absorbance sets the maximum value in each dataset as a denominator.
    • Every value is divided by this maximum value.

Print this page