Science

• MS-LS1-6. Photosynthesis – Matter Cycling and Energy Flow. Construct a scientific explanation based on evidence for the role of photosynthesis in the cycling of matter and flow of energy into and out of organisms.
• PS3.D. Energy in Chemical Processes and Everyday Life. Sunlight is captured by plants and used in a reaction to produce sugar molecules, which can be reversed by burning those molecules to release energy.
• LS1.C. Organization for Matter and Energy Flow in Organisms x Plants, algae (including phytoplankton), and many microorganisms use the energy from light to make sugars (food) from carbon dioxide from the atmosphere and water through the process of photosynthesis, which also releases oxygen. These sugars can be used immediately or stored for growth or later use.

Learning Objectives

After completing the activity, the student will be able to:

• Explain why photosynthesis is an important chemical process in cycling energy and matter.
• Describe the anatomy of a leaf relative to gas exchange in photosynthesis.
• Conduct analyses necessary to determine rate of photosynthesis.
• Identify and test different variables to determine the effect on photosynthesis.

Materials

• Sodium bicarbonate (baking soda)
• Water (gallon jug)
• Liquid dishwashing soap
• Plastic syringe (20 cc or larger). Remove any needle!
• Leaf material (spinach or ivy work well)
• Hole punch
• Plastic cups or petri dishes
• Timer
• Light source (LED floodlights work well)
• Student Worksheets (pages S1-S6)

Procedures for Instruction

Length of Time for Preparation: Minimal

Preparing to have students run the leaf disk assay requires making a gallon or two of sodium bicarbonate solution. This can be done quickly.

• Add approximately 8 grams of baking soda, NaHCO3, to a gallon of water.
• Add approximately 1 mL of dishwashing liquid to the gallon of solution.
• Replace the lid and mix the solution gently.

Length of Time for Classroom Teaching: 1-2 class periods

Classroom Discussion

Introduce the topic and assess students for prior understanding.

Procedure for Lab

1. Aliquot 100 ml of baking soda, detergent solution into each plastic cup.
2. Cut leaf disks using the hole punch, avoiding veiny leaf material.
3. Remove the plunger from the syringe and add leaf disks.
4. Disks stuck to the sides may be rinsed to the bottom of the syringe.

5. Replace plunger and draw up approximately 10 ml (half full) of solution.
6. Point syringe up and gently press plunger to remove all air and a small amount of the solution is pushed out.

7. Place finger over opening and pull a vacuum on the solution containing the disks.
8. Hold the disks in this vacuum for at least 10 seconds.
9. Gently allow the plunger to go back and observe disks.
10. Repeat steps 7 and 9 until all disks sink in the syringe.

11. Remove the plunger over a cup.
12. Dispense disks evenly among the cups for each trial.

13. Start a timer and place cups under the light sources (be careful to control distance from light as this has a significant impact on the rate of photosynthesis).
14. Count and record on the provided data table the number of floating disks in each cup at 1-minute intervals.
15. Gently swirl cups at this interval to dislodge any disks stuck to the sides or bottom of the cup.
16. Record the time when the 5th disk floats in each sample as the ET50. This is the time required to float 50% of the disks.
17. Graph the data of floating disks vs time for each trial.

Experimenting
Now that students have observed the assay used to measure the rate of photosynthesis, they can design experiments to test many different variables to see their effect on the rate of photosynthesis.
Hand out the student worksheets. Have students determine variables they want to test. Possible ideas include, but are not limited to:

• Light source: LED, incandescent, CFL bulbs
• Light intensity: Vary the distance from the light source
• Color of light: Use light color filters over each cup
• Carbon amounts: Different concentrations of baking soda solutions

Have students graph number of floating disks vs time. This allows students to get an idea of the rate of photosynthesis for various trials.

Optional: Have students use whiteboards to graph and present their findings to the class.

The time for the 5th disk to float, ET50, is a more reliable and repeatable data point.

• It is important to remember that, like in a race, the lower the ET50, the faster the rate of photosynthesis.

Science and Agriculture Careers

To learn more about agriculture careers, visit https://agexplorer.ffa.org/.

Sources

https://www.bradwilliamson.net/the-floating-disk-assay-to-study-photosynthesis-an-update/

Wickliff, J.L. and Chasson, R.M. 1964. Measurement of photosynthesis in plant tissues using bicarbonate solutions. Bioscience, 14: 32-33.

Steucek, Guy L. Robert J. Hill and Class/Summer 1982. 1985. Photosynthesis I: An Assay Utilizing Leaf Disks.
The American Biology Teacher, 47(2):96-99.
Tatina, Robert E. 1986. Improvements to the Steucek and Hill Assay of Photosynthesis. The American Biology Teacher, 48(6): 364-366.

Juliao, Fernando and Henry C. Butcher IV. 1989. Further Improvements to the Steucek and Hill Assay of Photosynthesis. The American Biology Teacher, 51(3): 174-176.

Rukes, Kari L. and Timothy J.Mulkey. 1994. Measurement on the Effects of Light Quality and Other Factors on the Rate of Photosynthesis. Bioscene, 20(3): 7-11. http://www.acube.org/volume_20/v20-3p7-11.pdf

Greenler, John. 1990. Exploring Photosynthesis with Fast Plants. Wisconsin Fast Plant Notes, 4(1): 4-5. http://www.fastplants.org/pdf/activities/exploring_photosynthesis.pdf
Anderson, Paul. 2011. Photosynthesis Lab Walkthrough. video: https://www.youtube.com/watch?v=ZnY9_wMZZWI