IB Photosynthesis Leaf Exploration Activity

Photosynthesis converts some of the energy absorbed from sunlight into the chemical energy of sugars.  The process is also the major source of oxygen in Earth’s atmosphere.  By measuring oxygen production indirectly, you can measure the rate of photosynthesis.  Remember that a rate describes how one quantity changes compared with another.  In this lab you will conduct an experiment to determine the effect of a light source on the rate of photosynthesis in leaves.

Materials:

Procedure:

  1. Prepare 0.2% 300 ml of bicarbonate solution for each trial
  1. Add 3.5g to 300 ml of water; stir well until bicarbonate is dissolved
  2. Add 1 drop of liquid soap to this solution
  1. Cut 12 uniform disks from leaf material for each trial
  2. Remove the plunger from the syringe and place the leaf disks into the syringe barrel.
  3. Replace the plunger being careful not to crush the leaf disks.  
  4. Holding the plunger tip upwards, push on the plunger until only a small volume of air and leaf disk remain in the barrel (less than 10%).
  5. Pull 50 mL of sodium bicarbonate solution into the syringe.  Tap the syringe to suspend the leaf disks in the solution.
  6. Holding a finger over the syringe-opening, draw back on the plunger to create a vacuum (Be careful to not pull the plunger all the way out of the syringe).  Hold this vacuum for about 10 seconds while swirling the leaf disks to suspend them in the solution.
  7. Remove finger over syringe-openings.
  8. Repeat steps 2-3 times in order to get all disks to sink. (If the disks don’t sink after three evacuations it may be due to not enough soap in the solution; add a few more drops.
  9. Fill the 250mL beaker with 150mL bicarbonate solution.
  10. Remove the plunger, place your finger over the tip of the syringe and pour the disks and solution into a 250 mL beaker; use forceps to remove any leaf disks sticking to the walls of the syringe.
  11. Place the beaker containing the leaf disks so that the light source shines on it horizontally at a distance of 15 cm from the light source and immediately start the timer.
  12. As the leaf disks begin to photosynthesize, the production of oxygen replaces the carbonate solution in the air spaces, the leaf disks become less dense and float to the top of the water.
  13. Record the number of disks that have floated to the top of the beaker each minute for a duration of 10 minutes as an indirect means of measuring photosynthesis rate.  
  14. Repeat steps 2-9 with a new set of Leaf disks.
  15. Repeat steps 10-14 by using distilled water rather than bicarbonate solution.

Data:

Table 1. The Effect of Light Source on the Rate of Photosynthesis

Time (seconds)

Trial 1: Number of floating leaf disks

Control: Number of floating leaf disks

60

120

180

240

300

360

420

480

540

600

Analysis:

  1. Share your data with the class by adding it here (and on website), don’t add units to your numbers).

  1. What evidence do you have the photosynthesis occurred in the leaf disks?  Explain.

  1. Why is sodium bicarbonate used in this investigation (think about the overall equation for the process of photosynthesis)?

  1. What are possible sources of procedural error in the design of this experiment?  How might they be improved?

  1. Calculate the rate of respiration for both Trial 1 and control:

  1. Using all classes data and averages, found here (and on website), create a graph to show average rate of photosynthesis for both Trial 1 & Control.  Be sure to include error bars and to complete a t-test to compare the results of the CO2 and one for O2 results.  Upload your graphs below.

Assessment:

Lab Standard: Analysis

Exceeds

Meets

Nearly Meets

Beginning

A full explanation of the lab standard rubric can be found here