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Plants Exhale: Does Light Help Plants Produce Oxygen?

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Author: Bob Bonnet and Dan Keen

Purpose or Problem

The purpose is to determine if the intensity of light on a plant's leaves affects the plant's oxygen output.

Overview

Chlorophyll (a chemical found in the green leaves of plants) and light collected by the leaves of plants combine with water and carbon dioxide to start a process that causes the leaves to release oxygen.

How critical is light to this process? If less light reduces the amount of oxygen produced, you might conclude that plants living underneath a thick canopy layer in a forest do not produce as much oxygen as those in direct sunlight.

Hypothesis

Hypothesize that elodea, a common aquarium plant, will produce more oxygen when subjected to a higher intensity of light.

Materials' List

  • Two small aquarium tanks
  • Two test tubes
  • Two elodea aquarium plants (available at pet shops)
  • Two table lamps
  • One 15-watt light bulb
  • One 100-watt light bulb
  • Two dark areas (for example, closets)
  • Possible adult supervision needed

Procedure

Fill two small aquariums (or very large, tall, clear glass jars) with water. Place an elodea plant in each one. Elodea is a common underwater plant that many people who have pet fish use in their aquariums.

Fill a small test tube with water. Place your finger tightly over the top to prevent the water from escaping and air from entering. Turn the test tube upside down and lower it into the water, capturing as much foliage from the elodea plant inside the tube as possible. All the plant's leaves do not have to fit inside the tube. Also, it is not necessary for the test tube to remain completely vertical. It can tilt a little if it needs to rest against the side of the tank for support.

Similarly, place a water-filled test tube over elodea foliage in the second aquarium. The elodea, aquariums, and size of the test tubes are constant. The variable in this project is the amount of light each plant receives.

Place each aquarium in a dark area of your house, for example, a closet or an unused room with the drapes closed.

Find two table lamps, and remove the lamp shades. Position one lamp next to each aquarium. In one lamp, place a 15-watt light bulb; use a 100-watt bulb in the second lamp.

 

Be sure the electric cord is not plugged into the wall when you are changing light bulbs. Also, make certain the lamps are not touching any curtains, drapes, or any paper or cloth material that could pose a fire hazard. Keep the lamps alongside the aquariums. Do not attempt to put them on the top, where they could come in contact with the water and create an electrical hazard.

Make daily observations to see if any oxygen is accumulating in the test tubes.

After observing for many days, is the quantity of oxygen different in the test tubes? Which one has accumulated the most?

Results

Write down the results of your experiment. Document all observations and data collected.

Conclusion

Come to a conclusion as to whether or not your hypothesis was correct.

Something More
  1. Is there an intensity of light beyond which there is no advantage increasing it?
  2. In school science labs, a burning splint is often used to detect the presence of oxygen in a small test tube. A "pop" or tiny explosion occurs if oxygen is present. Use the burning splint technique to confirm that the gas collected from the elodea is oxygen. Do this under the supervision of a school science instructor with proper safety precautions in place.
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