Grade Level: 6 (5-8)
Group Size: 3
Time Required: 1 hours

Summary: Students learn a simple technique for quantifying the amount of photosynthesis that occurs in a given period of time, using a common water plant (Elodea). They can use this technique to compare the amounts of photosynthesis that occur under conditions of low and high light levels. Before they begin the experiment, however, students must come up with a well-worded hypothesis to be tested. After running the experiment, students pool their data to get a large sample size, determine the measures of central tendency of the class data, and then graph and interpret the results.

Engineering Connection: Students perform data analysis and reverse engineering to understand how photosynthesis works. Both are important parts of being an engineer.

Materials List

-5 liters (about 1 gallons) of aged tap water (tap water in an open container that has been allowed to sit for 36-48 hours to eliminate any chlorine used in municipal water supplies)
-15-20 total of Elodea plants. These are hardy freshwater aquarium plants, sold in bunches at pet stores and suppliers such as Carolina Biological Supply Company (www.carolina.com)
-string, yarn, or twist ties for tying Elodea plants into bunches
-small rocks or similar objects to serve as weights to hold the Elodea plants underwater
-500-ml beakers, enough for one per team
-a few tablespoons of sodium bicarbonate (baking soda)
-timers or watches with second hands, enough for one per team
-small adjustable desk lamps that can be set up so that their light bulbs are a few inches above the beakers and shine vertically down onto them; flashlights with strong beams that are mounted on ring stands will also work. You will need one light source per team.

Introduction/Motivation: Ask your students to each raise one hand high in the air. Then ask them to take a deep breath and hold it for as long as they can. Tell the students to lower their raised hands when they can’t hold their breath any longer. After no one is left holding their breath, ask them why they needed to start breathing again. From their elementary school studies, they should be able to tell you that their bodies need air in order to survive.

Then, ask if they know exactly what is in air. They may not know that air isn’t just oxygen. Explain that most of the atmosphere consists of nitrogen gas (about 78%). Oxygen is the next largest component (about 21%), and a tiny part (1%) is made up of argon (an inert gas), water vapor, and carbon dioxide. If you then ask students what part of the air it is that our bodies need, they should be able to answer that it is oxygen. They probably will also be able to explain that oxygen from the air is picked up in the lungs by the blood and carried to all parts of the body, where it is needed by muscles and the brain and all the other organs and tissues of the body.

Finally, ask students where the oxygen in the atmosphere came from. They may know or be able to reason that it is the result of all the plants that have lived on the earth and have been doing photosynthesis for many millions of years. Then let them know that they can do an experiment to see if the amount of light plants receive can affect this production of oxygen.

Vocabulary/Definitions
Mean: the sum of all the values in a set of data, divided by the number of values in the data set; also known as the average. For example, in a set of five temperature measurements consisting of 22° C, 25° C, 18° C, 22 ° C, and 19° C, the mean temperature is 106° divided by 5, or 21.2° C.
Median: the middle value in a set of data, obtained by organizing the data values in an ordered list from smallest to largest, and then finding the value that is at the half-way point in the list. For example, in a set of five temperature measurements consisting of 22° C, 25° C, 18° C, 22 ° C, and 19° C, the ordered list of temperatures would be 18° C, 19° C, 22° C, 22° C, and 25° C. The middle value is the third value, 22° C. If the data set consists of an even number of values, the median is determined by averaging the two middle values. For example, in a set of six temperature measurements consisting of 20° C, 22° C, 25° C, 18° C, 24° C, and 19° C, the middle values are 20° C and 22° C. Therefore, the median value is the average of 20° C and 22° C, which is 21° C.
Mode : the value in a set of data that occurs most frequently. For example, in a set of five temperature measurements consisting of 22° C, 25° C, 18° C, 22 ° C, and 19° C, the measurement of 22° C occurs most frequently, so it is the mode. It is possible to have two or more modes in a set of data, if two or more values occur with equal frequency.

Procedure

1. In a class discussion format, students will establish a hypothesis to be tested by the class in the experiment.
2. Working in teams, students will set up and conduct the experiment. Each team will conduct two trials: one with the plants lit only by the ambient light available in the classroom when some or all of the room lights are turned off, and one with the plants receiving bright light from the desk lamps. The data collected will be the number of bubbles of oxygen that are given off by the plants in a five-minute period, first at low light levels, and then at high light levels.
3. The teams will come together and pool their data from each of the two trials. From these data, students will individually determine the mean, median, and modes for the numbers of bubbles produced during the two different light conditions.
4. Students will then individually graph the data, using bar graphs that show the mean numbers of bubbles and the ranges for each test condition.

Part 1: Generating a hypothesis

Explain to the class that before a scientist starts an experiment, he or she must first have a prediction about what the outcome of the experiment will be. This prediction is known as a hypothesis. A hypothesis is not simply a guess, however. Instead it is a prediction based on prior knowledge of or experience with the subject. For example, if a gardener wanted to find out if it was really necessary to fertilize his zucchini plants, he or she might grow twelve zucchini plants, but fertilize only half of them. In this case, the hypothesis being tested might be, “Fertilized zucchini plants will produce more zucchinis than unfertilized zucchini plants.” The data collected to support or refute the hypothesis would be the total number of zucchinis produced by the fertilized plants, compared to the total number produced by the unfertilized plants.

Point out to students that in the zucchini experiment, the gardener collected data that involved numbers. In science this is usually the case, because numbers can easily be compared and they are based on things that actually happened, as opposed to things that the experimenter thought happened.

Then, explain briefly how the photosynthesis experiment will be set up, and ask the class what hypothesis will be tested. It shouldn’t take them long before they come up with a statement such as, “The plants that receive more light will produce more bubbles than the plants that receive less light.”

2) Setting up the experiment

These steps should be performed with some or all of the room lights turned off. The room should not be dark, and there should be adequate light for students to see easily, but the room should not be brightly lit.

1. Each team needs to fill a beaker with about 500 ml of aged water for the Elodea. To this water they should add a scant teaspoon of sodium bicarbonate (baking soda). This provides a source of carbon dioxide for the plants, since they can’t get it from the atmosphere like terrestrial plants do. Students should stir the water until the sodium bicarbonate is dissolved and the water looks clear.
2. Each team should obtain enough sections of Elodea plants so that it has about 18-24 inches of total plant length. These should be arranged so that all of the plants will be at least 1″ under the water in the beaker. String or twist ties can be used to hold them together, and then a small rock should be added to keep the plants from floating to the surface. Point out to students that the more area exposed to the light above the plant, the more photosynthesis can occur within the leaves. If students form clumps of Elodea, many of the leaves will be shaded by those above, and thus may not be able to conduct as much photosynthesis. It would be better to form the plants into loops that cover the entire bottom of the beaker, instead of a single clump in the middle of the beaker.

3) Running the experiment

1. As soon as the plants are arranged in the beaker, the team should start timing for five minutes. Two team members should have their eyes glued to the beaker for those five minutes, watching for bubbles to rise to the water surface. Any bubbles that rise should be announced to the third team member, who will keep count (tally marks will help with this) as well as monitor the time until the five minutes are up. The bubbles are fairly large, about 2 mm in diameter, and so are easily seen when they rise to the surface.
2. When all the teams have counted bubbles for five minutes (it is quite possible that some teams will see no bubbles at all), turn on the room lights and have students position the desk lamps directly above the beakers. The light bulbs should only be a few inches above the beakers. Once the lights are in place, the teams should again begin timing and counting bubbles for five minutes.

4) Pooling and analyzing the data

1. Make a large chart on the board in which the teams can fill in the number of bubbles they observed in each of the two light conditions.
2. Once it is filled in, have students work individually to determine the mean, median, mode, and range of each of the two sets of data. Allow enough time so that all students arrive at the same answers.
3. Provide students with grid paper and ask them to make a vertical bar graph that compares the mean number of bubbles in the two light conditions. Be sure that students include a title, labels on the axes, and a legend if different colors were used for the two bars of the graph. Then show them how they can indicate the ranges of the data by adding a vertical line segment to the center top of each bar, with the lower end of the line segment situated at the lowest number of bubbles observed by a team, and the upper end of the line segment at the highest number of bubbles observed by a team.

Part 5: Interpreting the data

Ask students what these numbers tell them about the amount of photosynthesis that occurred in each of the two light conditions. In other words, was the hypothesis the class tested supported or not?

Then ask students how they know that the bubbles they saw rise to the surface were bubbles of oxygen. They may answer that they know photosynthesis produces oxygen, so the bubbles must have been oxygen. However, without a way to determine the chemical composition of the bubbles, it is only an assumption that the bubbles contain oxygen. They might just as well have been bubbles of nitrogen, or carbon dioxide, or some other gas from some other process that was occurring in the plants instead of photosynthesis. Nevertheless, since the plants were exposed to light, the bubbles were most likely made up of oxygen. Point out that it is important for scientists to make sure they recognize the difference between what they know about an experiment and what they assume about it.

Investigating Questions

-What do you think would happen if you left some plants in a completely dark closet for two or three weeks? Why do you think that?
-Why is it important for crop plants to receive enough rainfall?
-The earth’s atmosphere did not always contain as much oxygen as it does now. In fact, there was a time when it probably contained no oxygen at all. How do you think the oxygen in the earth’s atmosphere got there? Why do you think that?

Assessment

Ask students questions such as:

-What things are needed in order for photosynthesis to occur?
-What are the products of photosynthesis?
-Where in the plant does photosynthesis occur?
-Why do plants need water in order to survive?

Provide a graph of data from an experiment similar to the one they performed, and ask them to draw a conclusion from it. For example, the data could represent the heights of corn plants, half of which were grown in the shade of a forest and half of which were grown in an open field.

Activity Extensions

The light that comes from the sun consists of light waves of many different wavelengths. In the visible spectrum of light, these range from red with the longest wavelength, to violet with the shortest wavelength. Chlorophyll does not respond equally to all wavelengths, or colors of light. Students can use the same experimental set-up to determine what color or colors of light result in the most photosynthetic activity. The only modification they need to make is to loosely cover the beaker with colored plastic wrap or cellophane during the five minutes of bubble counting. Since blue wavelengths are the best for most plants, be sure that this is one of the colors available. If possible, have red and one other color available as well.

Owner: Engineering K-Ph.D. Program, Pratt School of Engineering, Duke University

Contributors: Mary R. Hebrank, Project and Lesson/Activity Consultant, Pratt School of Engineering, Duke University

Copyright: 2004 by Engineering K-Ph.D. Program, Pratt School of Engineering, Duke University
including copyrighted works from other educational institutions and U.S. government agencies; all rights reserved.

Filed under: Grades 6-8, Grades K-5, Lesson Plans