Sunlight is white light that is actually a mixture of different wavelengths of light from the visible light spectrum. Photosynthetic plants gather energy from sunlight through the use of light-absorbing pigments. The plants use this energy to power the synthesis of organic molecules.
Chlorophyll is the green pigment in the chloroplasts of most plants. Chlorophyll absorbs light very well in the red and blue-violet regions of the visible spectrum, but it does not absorb green light very well. Green light is instead reflected, which is why chlorophyll, and the leaves of plants where it is found, appear green.
When chlorophyll absorbs energy in the form of light, most of that energy transfers directly to the electrons in the chlorophyll molecule. This raises the electrons to higher energy levels. These high-energy electrons that do the work of photosynthesis pass on to carrier molecules such as NADP+ to form NADPH in the electron transport chain. Thus the light energy is trapped in a chemical form.
In this demonstration we remove chlorophyll from the chloroplasts of plant cells and place it in a solution of ethanol. When we excite the electrons of the chlorophyll molecules with the black light (ultraviolet light), in the absence of the electron transport chain the electrons release their energy in the form of red light as they return to their ground state.
Ethanol is flammable; keep it away from flames. Avoid contact with the chlorophyll extract because it can stain your skin and clothing. Do not look directly at the black light.
- Spinach Leaves
- Mortar and Pestle
- 95% Ethanol, 100 mL
- 125-mL Erlenmeyer Flask
- Coffee Filter or Filter Paper
- Black Light
- Paper Towels
- Safety Goggles
- Lab Apron
On the day of and prior to the demonstration, prepare the chlorophyll extract.
- Tear apart several leaves and place in the mortar.
- Add ethanol to the mortar to just cover the leaves.
- Use the pestle to crush the leaves and release the chlorophyll from the thylakoid of the chloroplast.
- Line the funnel with filter paper and place the funnel in the Erlenmeyer flask.
- Pour the contents of the mortar through the filter paper into the Erlenmeyer flask.
- Show students the Erlenmeyer flask of chlorophyll and explain how you prepared the solution.
- Darken the lights in the room.
- Hold the black light up to the flask of chlorophyll. The chlorophyll should appear red.
- Turn the lights back on to show students that the color of the solution did not change.
- Ask your students why the chlorophyll appears red when exposed to the black light.
This activity can be used to teach a number of different science concepts about:
- The electromagnetic spectrum
- Fluorescence and fluorescence spectroscopy
- Electron energy levels
Beyond being a cool demonstration, this activity showing chlorophyll fluorescence is a useful tool for studying plant physiology. You can use chlorophyll fluorescence to measure photosynthesis, specifically the efficiency of photosystem II. You can also use chlorophyll fluorescence to measure plant stress, since these stresses typically impact the plant’s metabolism and create an imbalance between the amount of energy absorbed by the chlorophyll and the use of this energy in photosynthesis. Have your students research these concepts.
Students are often intrigued by fluorescence. If you have time, you can show students how some other common materials—such as petroleum jelly, white paper, a strip in a $20 bill, and tonic water—fluoresce under a black light. Just as with chlorophyll, fluorescence occurs because compounds in these objects absorb the ultraviolet light from a black light and emit light with a longer wavelength that is in the visible range. In the case of the US currency, a fluorescent strip is added as a security feature. Challenge students to find other items that exhibit fluorescence under a black light.
You can separate any remaining spinach extract by paper chromatography to demonstrate the different forms of chlorophyll, chlorophyll a and chlorophyll b, and other compounds such as carotenoids that are within the chloroplasts.