How to Use Elodea Plants to Teach Photosynthesis

The common water plant elodea (Elodea canadensis) is frequently used in a classroom setting to allow biology students to observe the process of photosynthesis. There are a number of ways to conduct lab experiments testing the effects of light and darkness on photosynthesis, and some require more sophisticated equipment than others. One way of conducting an elodea photosynthesis lab requires very little equipment and is suitable for teaching photosynthesis at the 9th grade level. While the steps below are written for a single student, the activity is equally suited for a small team of students.

Things You'll Need

• 2 test tubes
• Post-it note
• Goggles
• Bromothymol Blue
• Tape
• 2 rubber stoppers
• 50 milliliter graduated cylinder
• Drinking straw
• Eyedropper
• Scissors
• Aged tap water
• Fluorescent grow lamp
• 2 Elodea sprigs

Instructions

1. Have your student put on safety goggles and stress the importance of wearing goggles when handling any chemicals. Next have him measure 40 millilitersl of aged water and pour it into one of the test tubes.

2. Have the student use an eyedropper to add 100 drops of bromothymol blue (BTB) to the test tube. Explain that BTB is an indicator of the presence of carbon dioxide.

3. Instruct your student to mix the BTB into the water with a straw. Next, have the student use his fingers to block splashing from the top of the test tube while gently blowing air through the straw just until the solution turns yellow. Ask the student to hypothesize why this change might be taking place.

4. Have the student cut the stem end of the elodea sprig on the diagonal with a clean pair of scissors and then insert the sprig into the test tube. Ask him why he might be making the cut.

5. Instruct the student to stopper the test tube and label it with the time and date that the experiment was begun by inserting the plant in the water. Have the student insert the test tube into a rack next to the grow light.

6. Supervise the same preparation of a second test tube, except this time have him cover the test tube with black paper to block out light. Have the student place the second test tube in the same rack with the first, in front of the grow light. Explain that part of the reason for doing this is to assure that both test tubes are exposed to the same amount of heat. Only the light conditions differ.

7. Several hours later, have the student check both test tubes to see whether the BTB solution has changed color. Instruct the student to record changes in a lab notebook and explain the results

8. Ask the student to list the conclusions he has been able to draw from the experiment and to explain why elodea might have been chosen for the experiment.

9. Ask the student to identify the processes that took place in each test tube. Ask the student to explain the difference between respiration and photosynthesis based on what he has observed.

Tips & Warnings

• Aged water is tap water that has been allowed to stand in an open container for 24 hours or more to allow chlorine and other impurities to evaporate from it.

• Science labs performed by teams of students are the best way to demonstrate scientific method, since the team will naturally build upon small insights by one or more team member to develop or take new meaning from their own observations. In this way the lessons taught by the lab will develop organically and each student will come away with a better understanding of the importance of scientific observation.

References

• ForestInfo.org: Lesson Plan Elodea Plant
• EduWebLabs.com: Elodea Photosynthesis
• Teacher Vision: Teacher Notes - Demonstrating Respiration in Elodea