Materials Included In Kit

Additional Materials Required

Prelab Preparation

1. Use distilled water to make the solution of sodium bicarbonate. Add 3.4 g to 1000 mL of water. Dissolve thoroughly. Note: At least 4 liters will be needed to accommodate 10 groups of students. Do not reuse the sodium bicarbonate solutions from a previous class.
2. From cardboard, make frames with open cutouts for light to shine through. Tape the colored filter gels to the cardboard and over the hole but make several open cutouts for the white light control.
3. Arrange stations around the room and assign one group to test a specific variable at each station:

Temperature increase
Temperature decrease
Sodium bicarbonate concentration increase
Sodium bicarbonate concentration decrease
Light wavelengths
Light distance

Safety Precautions

Although materials in this activity are considered non-hazardous, goggles should be worn and all normal laboratory safety procedures followed. Students should wash their hands thoroughly with soap and water before leaving the laboratory. Please consult current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. The used sodium bicarbonate solutions from the test tubes and beakers may be poured down the drain according to Flinn Suggested Disposal Method #26b.

Lab Hints

• Enough materials are provided for 30 students working in groups of three or for 10 groups. The laboratory work and data collection can reasonably be completed in one 50-minute class period. Ideally the student experiment works well with a minimum of three students per group. One person would have a job as the timer/recorder and two as bubble counters.
• Answering the Post-Lab Questions will require the use of pooled class data. These data may be written down in a class data table as it accumulates or compiled by the teacher for later distribution and discussion.
• As timing is crucial to this lab, ensure that stopwatches and/or a clock with a second hand are available to each lab group.
• If time is short, have student groups collect data from only two trials, not three.
• The stem of the Hornwort should be cut with a razor blade or very sharp scissors. If not, the stem may become pinched and no oxygen bubbles will rise.
• A piece of dark or white paper placed behind the beaker and test tube setup, supported by a larger beaker or flask, may make it easier to see and count the rising bubbles.
• After the Hornwort has been ordered but before it arrives, set up a large container containing aged tap water or spring water. For best results, place the container in continuous light while the Hornwort is being used.
• A hotplate should be available at the station testing the effect of temperature increase on the rate of photosynthesis.
• Either a refrigerator or an ice bath should be available for the station testing the effect of temperature decrease on the rate of photosynthesis. Note: Ice should not be added to the beaker as it will dilute the concentration of sodium bicarbonate solution. Place the beaker on ice to cool down the solution to a predetermined temperature before beginning the experiment. Between trials, keep the setup on ice while the black paper is wrapped around it.
• The station testing sodium bicarbonate concentration could have pre-made molar concentrations, more or less than the control, available for student use. However, depending on the skill of your students, you may want to provide the station with a balance, a mixing container, NaHCO₃ powder and plenty of distilled water so students can make up their own molar concentrations. The formula for calculating molarity is:

Teacher Tips

• If a dissolved oxygen probe from a computer-based lab is available, it could be used to verify that the gas being given off during photosynthesis is oxygen. The sodium bicarbonate solution inside the test tube could be tested before and after the experiment to see if the level of DO increased.

• If a more complete discussion of color and the visible light spectrum is needed, consider the Color and Light—Spectrum Demonstrations Kit, Flinn Catalog No. AP6172.
• To make this activity more inquiry-based, after assigning groups to test various environmental conditions (e.g., temperature, light distance, NaHCO₃ concentration) allow each student group to determine how the condition will be changed from the control conditions, without input from the teacher.

Further Extensions

If testing for changes in pH during photosynthesis is desired, consider the following procedures for sampling the water inside the test tube. Be sure and note the pH of the solution at the very beginning of the experiment.

• Insert the shorter, flexible end of the straw included with the kit, into the test tube.
• Firmly insert a wide-barrel, disposable pipet into the other end of the straw and withdraw a sample of the water.
• Place one drop on a piece of filter paper and compare the color to the chart included with the pH paper.
• Leave the straw inside the test tube and at 3–5 minute intervals, remove another sample, test it and note any changes in color.

Since this method may not be very sensitive to small changes in pH, here is an alternative:

• Remove a water sample from the test tube at the designated intervals.
• Place inside a small beaker, add one drop of universal indicator solution, compare to the included color chart and note any color changes as the experiment progresses.

Answers to Prelab Questions

1. Propose a hypothesis for the effect of different colors (wavelengths) of light on the rate of photosynthesis by completing the following statement: If plants are exposed to ___green___ light, then ___green___ light should ___decrease___ the rate of photosynthesis.
2. Propose a hypothesis for the effect that changing an environmental condition—temperature, NaHCO₃ concentration or light source distance (intensity)—will have on the rate of photosynthesis. Write a statement similar to the statement completed in step 1.

Student answers will vary.

Sample Data

Rate of Photosynthesis Data Table

These data were collected in the Flinn Scientific, Inc., lab under the conditions indicated in the table.

Answers to Questions

1. Use the combined class data to construct two graphs. One graph should compare the rate of photosynthesis of a control variable—temperature, light intensity or concentration of NaCHO₃ to the corresponding experimental environmental condition that was tested. The other graph should compare the rate of photosynthesis in white light (control) with all the other wavelengths (colors) that were tested in the class.

2. Briefly describe the control set of conditions used throughout these experiments.

The control conditions used were: white light, water room temperature, 0.04% sodium bicarbonate solution, blowing into the solution for 20 seconds and having the light source 10 cm away.

3. In your group’s experimental setup, what was done to change one of the control conditions?

The wavelengths of light were changed from white to blue, green and red.

4. Were the individual light wavelengths that were tested equally preferred by the plants for photosynthesis? (Support your answer using the combined class data.)

Based on the data, the blue wavelengths light appeared to increase the rate of photosynthesis much more than either red or green wavelengths.

5. 1. Was your light wavelengths hypothesis (Prelab Activity Question 1) supported? Explain your answer.

      Answers will differ depending on the hypothesis.

   2. Was your environmental conditions hypothesis supported? Explain your answer.

      Changes in temperature, light intensity (distance), and concentration of NaHO₃ were all tested. The control conditions, as given in the write-up, were determined from the results of the changes made to those parameters.

6. What was the purpose of the sodium bicarbonate solution in this experiment?

It provided a source of carbon dioxide necessary for photosynthesis.

7. Why were you instructed to blow 20 seconds worth of air into the solution?

To add an additional amount of CO₂ to the solution in an attempt to saturate the sodium bicarbonate solution

8. 1. Since air is really a mixture of many different gases, which kind of “air” (gas) bubbles do you think were being released by the plant?

      Oxygen gas

   2. How could the gas released from the plant be identified?

      Student answers will vary but give credit for any reasonable, thoughtful answer.
      One possibility is to use a dissolved oxygen probe or conduct a dissolved oxygen test on the solution inside the test tube before and after the experiment to see if the amount of DO increased.

9. 1. List all the variables that were tested by various groups in the class (including the control variables/conditions).

      Variables tested:

b. Calculate the effect each variable had on photosynthesis using the formula:

Write the answer (it will be a percent) next to each listed variable in step 9a.

10. Use the calculated percentages from 9b to briefly explain the effect each variable had on the rate of photosynthesis. Hint: The higher the percentage, the greater the effect.

From the calculations, the blue wavelength’s effect on the rate of photosynthesis was significant (97%). It was 18% greater than the rate of photosynthesis for red wavelengths and was 24% greater than green wavelengths. As may be seen from the other calculations, the rate of photosynthesis for all of the tested distances were significantly lower. The temperature data seem to indicate a definite temperature effect on the rate of photosynthesis.

11. 1. Looking at all the variables tested and the calculated percentages from each test, briefly describe the ideal conditions for Hornwort photosynthesis.

       Answers will vary depending on the conditions tested.

    2. Would these conditions be the same for other plants? Why or why not?

       No. Plants live in many kinds of habitats and the optimum conditions for photosynthesis within their habitat will vary.

12. List at least three possible errors that did or could have occurred during the experiment that may have affected the results.

Miscounting bubbles
Timing—allowing too little or too much time for counting bubbles
Forgetting to turn the light off and cover the setup between trials
Accidentally testing more than one variable

References

Flinn Scientific, Inc., would like to thank Bridget Thuente of Roncalli High School in Indiana for submitting the basic idea for this activity.

Introduction

Which biochemical process is the most important to all life on Earth? Here are three clues: (1) It provides food directly or indirectly for nearly every living organism on Earth. (2) It generates, as a by-product, a gas critical to the survival of aerobic (“air-breathing”) organisms. (3) The name of the process literally means, “putting together with light.”

Concepts

• Photosynthesis

• Visible light spectrum

Background

The answer to the riddle is photosynthesis and the history behind the discovery of its fundamental parts is well-documented. It begins in the 1600s with the “potted willow tree” experiment of Jan Baptista van Helmont (1580-1644), a Flemish (Belgian) physician and chemist. Van Helmont planted a small willow in a large pot. Prior to planting, he weighed the tree and the soil and hypothesized that the soil would lose mass due to the uptake of nutrients by the tree’s roots. After 5 years of growth outdoors, the soil’s mass had only decreased about 60 grams (2 oz), but the tree’s mass had increased by 74 kg. He concluded that the addition of rainwater during that time must have been solely responsible for the plant’s extra mass. However, in 1699 a Cambridge University professor named John Woodward (1665–1728) conducted experiments showing that even after 77 days of growth and the addition of 76,000 grams of water, a plant’s mass had only increased about 1 gram. Therefore, water was rejected as a specific nutrient used by plants for growth but was still recognized as necessary component for keeping plants alive.

It took 72 years (1771) for another major piece of the photosynthesis puzzle to be discovered. In 1771, the English chemist Joseph Priestley (1733–1804) placed a cutting from a mint plant inside a closed container with a candle that had burned out. After 27 days, the candle could be re-lit. Further experiments using both candles and mice led him to hypothesize that plants “repair” the air “injured” (depleted of oxygen) by burning candles and the breathing of animals. A few years after Priestley’s work, Jan Ingenhousz (1730–1799), a Dutch physician, demonstrated that both light and the green parts of plants are necessary for producing oxygen (O₂). A French scientist, Jean Senebier (1742–1809), found that plants require carbon dioxide (CO₂) from their surroundings, which stimulates them to produce oxygen in the light. With this, the fundamentals of the photosynthetic process were established:

CO₂ + H₂O → C₆H₁₂O₆ + O₂.

Still, it was not until the 1882–1883 experiments of Theodor Engelmann (1843–1909) that plants were found to prefer specific wavelengths of the visible light spectrum for photosynthesis.

Experiment Overview

This two-part laboratory activity has two purposes. The first purpose is to discover which wavelength(s) of visible light are preferred by plants for photosynthesis. The second portion of this activity studies the effect of specific environmental factors on the rate of photosynthesis.

Materials

Prelab Questions

1. Propose a hypothesis for the effect of different colors (wavelengths) of light on the rate of photosynthesis by completing the following statement: If plants are exposed to ___________ light, then ___________ light should ___________ the rate of photosynthesis.
2. Propose a hypothesis for the effect that changing an environmental condition—temperature, sodium bicarbonate (NaHCO₃) concentration or light source distance (intensity)—will have on the rate of photosynthesis. Write a statement similar to the statement completed in step 1.

Safety Precautions

Although materials in this activity are considered nonhazardous, goggles should be worn and all normal laboratory safety procedures followed. Care should be taken when using the razor blade and/or scissors. Wash hands thoroughly with soap and water before leaving the laboratory.

Procedure

(Please read carefully before beginning to ensure greater accuracy in the collecting of data.)

1. Go to the station to which your group has been assigned. Each group will first prepare and collect data from a control setup. Then, one variable will be changed for the experimental setup, data recollected, and recorded in the accompanying table.
2. Get two 400-mL beakers. Pour 300 mL of sodium bicarbonate solution into each.
3. Fill the test tube about ¾ full with sodium bicarbonate solution.
4. Use a thermometer to determine the temperature of the solution in the beaker. Record in the data table.
5. Obtain one sprig of fresh Hornwort. Use a ruler to measure a 9- to 12-cm length and use a razor blade to cut off, at an angle, a ½-cm piece from the stem. Insert the entire sprig, stem-end first, into the test tube.
6. Using a 50-mL beaker or disposable pipet, add additional sodium bicarbonate solution to the tube until it is filled. Note: There should be a bubble of liquid above the rim of the tube.
7. Place a thumb or index finger over the opening of the test tube and invert it, upside down, into one of the beakers. Note: If done correctly, no air bubbles should be visible in the tip of the tube.
8. Use a straw to blow bubbles of air into the beaker for 20 seconds. Be careful not to inhale.
9. Tape a cardboard frame to the front of the light source—gooseneck or clamp lamp. If your group has been assigned to test a color filter, record the color of the filter in the data table.
10. Place the beaker containing the test tube 10 cm away from the light source.
11. Let the setup sit for 3–5 minutes without turning on the light to become acclimated to the surrounding conditions.
12. Turn on the light and allow the setup to sit in the light for one minute or until bubbles begin rising steadily. As photosynthesis occurs, bubbles will become visible and begin rising from the cut end of the Hornwort stem.
13. Two group members should count the rising bubbles in the tube silently to themselves while the third group member times each trial and records the data. Note: Differences in the number of bubbles between the two counters should be averaged to the next whole number.
14. After one minute of counting, turn off the lamp, completely wrap the setup with a piece of black construction paper and fold over the top to keep out the light. Everything should remain covered for two minutes before beginning the next trial.
15. Repeat steps 12–14 two more times with the control conditions and record the data.
16. Discard the sodium bicarbonate solution in the test tube and then repeat steps 3–15 using the experimental conditions. Note: Reuse the sprig of hornwort.
17. When finished collecting data, remove the Hornwort from the test tube and put it back into the aquarium or supply container.
18. Use the data from the three trials to determine the Average Rate of Photosynthesis for both the control and the experimental conditions that were tested. Record the information in your group’s data table.

Rate of Photosynthesis Data Table

19. Pour the used sodium bicarbonate solutions down the drain with running water and thoroughly rinse the beakers and test tubes.

Student Worksheet PDF

10704_Student1.pdf