NOTE: In this experiment you will indirectly observe photosynthesis and cellular respiration using a floating leaf disc in a solution. During photosynthesis, air bubbles will cause the leaves to float, and during respiration, the discs will sink. HYPOTHESES: In this exercise, the experimental hypothesis is that the leaf discs will have a greater rate of photosynthesis in the bicarbonate solution, because bicarbonate provides added CO2 to fuel photosynthesis, causing more leaf discs to float. Additionally, all of the discs will sink in dark conditions as they perform cellular respiration. The null hypothesis is that there will be no difference in the rate of photosynthesis, and therefore the number of floating discs, between the bicarbonate and water, or light and dark treatments.
To place leaf discs under vacuum, first remove the plungers from two 20 mL syringes, and then place 10 leaf discs inside each syringe tube. Label one syringe “bicarbonate”, and label the other syringe “water”.
Replace the plungers and push the plunger until only a small amount of air remains in the syringe. Take care not to damage the leaf discs.
Pull 5 mL of the bicarbonate solution into one of the syringes. Invert and swirl the syringe to suspend the leaf discs in solution.
Push as much air out as possible without expelling the solution or damaging the leaf discs.
Then pull 5 mL of the water solution into the other syringe and swirl it as previously described (step 3).
To create a vacuum, hold one finger over the tip of the syringe while pulling back on the plunger. Hold this for 10 seconds while swirling the syringe to keep the leaf discs in suspension.
Then, release the vacuum. NOTE: The discs should have absorbed the solution into the air spaces in their tissues and you should see them sink. If the discs don't sink, you can repeat the vacuum creation up to three times.
Next, add 50 mL of bicarbonate solution to a plastic cup or a glass beaker, and then gently add the discs from the bicarbonate vacuum syringe.
For the control, add the same amount of water to an identical cup, and then add the leaf discs from the water vacuum syringe. Label the containers appropriately.
Place both cups under a light source.
Every five minutes record the number of discs floating on the surface of the cup in Table 3 until 20 minutes have passed. Click Here to download Table 3
Next, remove the cups from the light source and then swirl them so that the discs at the surface intermix with any gases also at the surface.
Move the cups to a dark place. Every five minutes record the number of leaf discs floating at the surface until 20 minutes have passed. Swirl the cup each time before placing it back in the dark.
To clean up, dispose of the leaf discs in the trash, and pour the bicarbonate solution down the drain. Wash the syringes and cups thoroughly.