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photosynthesis lab

Photosynthesis Lab


33 points totalTitle: The Floating Leaf Disk Assay For Investigating Photosynthesis 1 pt Purpose: Will there be a difference in the rate of disks floating in water with carbon dioxide and water without over a 10 minute period of time? 2 ptsHypothesis: Null hypothesis: IF10 disks are placed in water with carbon dioxide and 10 in water without, then there will be no difference in the two group’s rate of disks floating after 10 minutes. 1 pt

  • Leaf disks float, normally.  When the air spaces are infiltrated with solution the overall density of the leaf disk increases and the disk sinks.  The infiltration solution includes a small amount of soap.  Bicarbonate serves as the carbon source for photosynthesis.  As photosynthesis proceeds, oxygen is released into the interior of the leaf which changes the buoyancy--causing the disks to rise.  Since cellular respiration is taking place at the same time, consuming oxygen, the rate at which the disks rise is an indirect measurement of the net rate of photosynthesis.
  • To determine the net rate of photosynthesis, one could measure one of the following:
  • • Production of O2
  • • Consumption of CO2
  • We will indirectly measure the production of oxygen!

In the first part of the lab, students learn how to measure the rate of photosynthesis indirectly by using the floating leaf disk procedure to measure oxygen production.


In the floating leaf disk procedure, a vacuum is used to remove trapped air and infiltrate the interior of plant (leaf) disk samples with a solution containing bicarbonate ions that serve as a carbon source for photosynthesis. The infiltrated leaves sink in the bicarbonate solution. When placed in sufficient light, the photosynthetic processes then produce oxygen bubbles that change the buoyancy of the disk, eventually causing them to rise.

  • Baking soda (sodium bicarbonate)
  • • Liquid soap (approximately 5 mL of dishwashing liquid or similar soap in 250 mL of water)
  • • 2 plastic syringes without needles (10 mL or larger)
  • • Living leaves [spinach, especially baby
  • spinach from the produce section of
  • the grocery story, or ivy (Hedera helix)
  • • Hole punch
  • • 2 clear plastic cups
  • • Timer
  • • Light source
  • The primary safety issues in this lab have to do with solutions near electric lights.
  • The light get very hot!!! Be careful not to touch them!
  • 1. Use the hole-punch to cut 20 uniform disks from the leaf samples. Make sure the disks are the same size and a complete circle. Avoid major veins. Try not to damage the leaves!
  • 2. Label and fill 2 cups with 100ml of each solution. Be careful not to mix the solutions or graduated cylinders. Add 1 drop of dilute soap to each cup! Not a dropper full!!

3.Infiltrate the leaf disks with water solution.

  • a. Remove the piston or plunger and place 10 leaf disks into the syringe barrel.
  • b. Replace the plunger being careful not to crush the leaf disks.
  • c. Push on the plunger until only a small volume of air and leaf disk remain in the barrel (<10%).
  • d. Pull a small volume of water into the syringe from the water cup. Tap the syringe to suspend the leaf disks in the solution. Use the purple for CO2 and clear for Water
  • e. Holding a finger over the syringe-opening, draw back on the plunger to create a vacuum. Hold this vacuum for about 10 seconds.
  • f. While holding the vacuum, swirl the leaf disks to suspend them in the solution. Let off the vacuum. The water will infiltrate the air spaces in the leaf causing the disks to sink. YOU WILL PROBABLY have to REPEAT this procedure several times in order to get the disks to sink.
  • g. You may have difficulty getting the disks to sink even after applying a vacuum 3 or 4 times. Keep trying!

4. Pour the disks and solution into a clear plastic cup. Labeled water Use the clear syringe for the water!

  • 5. Repeat step #2 (a-g) but use sodium bicarbonate water (CO2 ) instead of the water.
  • Use the purple syringe for the CO2 solution in the flask!
  • 6. Place the cups under the light source and start the timer. At the end of each minute, record the number of floating disks on the chart. Then swirl the disks to dislodge any that are stuck against the sides of the cups. Only swirl at each minute, to make it “fair”, do this with each cup.
  • 7. Continue until all the disks are floating or at least 10 minutes!
graph the results on graph paper 9 points
graph the results on graph paper. 9points
  • Title:
  • Label each axis
  • Plot points show where 50% would be floating
  • Key for tap water and CO2 water
  • X time
  • Y # of disks floating
analysis 7 pts
  • 1. What causes the disks to rise?
  • 2. What is the dependent variable?
  • 3. What is the independent variable?
  • 4. By looking at your graph (yes, you need a graph) at what point would 50% of the disks float?
  • 5. What purpose does sodium bicarbonate serve?
  • 6. Relate the equation of photosynthesis to this lab.
conclusion 8 pts
  • State whether the hypothesis was supported by the data( support or reject hypothesis 1 pt) data to support this statement ( 1 pt) AND answer the question(1 pt)in the purpose (no personal pronouns).
  • Errors? What are they? How can you fix them and how do they alter the results? ( 3 pts)
  • What other factors could be changed and tested to see if they alter the rate of photosynthesis? You need at least 2, extra points for more
  • ( max 5 total )
et 50 estimated time for 50
ET 50estimated time for 50%
  • Calculate the estimated time for 50 % of the disks to rise. This is a good rate indicator to use to compare experimental groups.
  • Graph the raw data from the original experiment (water and bicarbonate water), show the ET50 and then the ET50 for your different variable compared to the control …
design your own lab
Design your own lab
  • 1. Once you have mastered the floating disk technique, you will design an experiment to test another variable that might affect the rate of photosynthesis. Some ideas include the following, but don’t limit yourself to just these:
  • • What environmental variables might affect the net rate of photosynthesis? Why do you think they would affect it? How do you predict they would affect it?
  • • What features or variables of the plant leaves might affect the net rate of photosynthesis? How and why?
  • • Could the way you perform the procedure affect the outcome? If the outcome changes, does it mean the net rate of photosynthesis has changed? Why do you think that?
video for finding stomata
Video for finding stomata
viewing stomata
Viewing stomata!
  • Get leaves
  • Paint part of the under side with clear polish
  • Let it dry completely
  • put a piece of tape over the dry polish and press down firmly
  • Gently pull off the tape
  • Place the tape on a clean slide and view the stomata!
  • Draw what you see, label the power.