Lab: Investigating Photosynthesis

Introduction

You will be using a leaf disk assay to investigate photosynthesis. You will use a hole punch to make leaf disks and put these leaf disks in a sodium bicarbonate (baking soda) solution. In water, the bicarbonate solution is a carbon source that the plant can use to undergo photosynthesis (akin to CO2 in air). Initially when you submerge the leaf dish in the bicarbonate solution, the air spaces in the leaf should fill with the bicarbonate solution causing the leaf disks to sink. As photosynthesis proceeds, oxygen gas will be produced and build up in the leaf. As this happens the density of the leaf should become lower than water and the leaf disks should begin to rise. Since plants also use oxygen gas during cellular respiration, this assay is only a relative measurement of photosynthesis. The rate of photosynthesis can be measured by how quickly the leaf disks rise. The faster they rise, the higher the net rate of photosynthesis. 

Materials

  • Sodium bicarbonate (Baking Soda)

  • Liquid soap

  • Plastic syringe (10cc or larger) – remove any needle

  • Leaf (spinach or ivy are known to work very well

  • Hole punch

  • Clear plastic cups

  • Timer

  • Light source

This is a short instructional video on the procedure for sinking leaf disks in a vacuum using a 10 ml syringe. The Floating Leaf disk assay is an excellent protocol for students investigating photosynthesis. The written protocol is available at http://www.elbiology.com/labtools/Leafdisk.html

Protocol (Experimental Group) 

  1. Prepare 0.2% bicarbonate solution. Add 1/8 of a teaspoon of sodium bicarbonate to 300ml of water. Stir until the sodium bicarbonate is completely dissolved. Add 1 drop of  liquid soap to the solution and mix. This allows the solution to be absorbed by the leaf. If you  acquire suds , you will need to add more bicarbonate solution until the suds disappear.

  2. From your plant material, cut out 10 leaf disks with a hole punch. Be careful to avoid major plant veins.

  3. Remove the plunger from your syringe.

  4. Place 10 leaf disks in plunger.

  5. Place plunger back in the syringe and depress it, being extremely careful not to apply any pressure to the leaf disks. If you crush the leaf disks, the experiment will not work. If this happens, simply cut out more leaf disks.

  6. Place the syringe in the bicarbonate solution, and draw a small amount (2-3cc) of bicarbonate solution. Tap the syringe to suspend the leaf disks.

  7. While holding your finger over the opening of the syringe (make sure you removed the needle), draw the plunger back in order to create a vacuum. Hold the vacuum for 10 seconds and swirl the leaf disks to suspend them in the solution.

  8. After 10 seconds, remove your finger releasing the vacuum. This will allow the sodium  bicarbonate solution to infiltrate the air spaces in the leaf. Once this occurs, the leaves will sink. You will likely need to repeat this procedure several times to get the  leaf disks to sink. If you can not get your leaf disks to sink after three evacuations, add one more drop of soap to your bicarbonate solution and repeat the procedure. Continue adding soap one drop at a time and repeating the procedure until the leaf disks sink.

  9. Carefully remove the plunger from your syringe. Pour the contents (leaf disks and bicarbonate solution) in to a transparent glass. Add more bicarbonate solution to the cup to a depth of 3cm. 

Protocol (Control Group) 

  1. Repeat the procedure for the Experimental Group, substituting water for the bicarbonate solution. 





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