The Floating Leaf Disk Assay for Investigating Photosynthesis

AP Biology2015-2016

Cerveny

Floating Leaf Disk Assay for Investigating Photosynthesis

Pre-Lab questions: answer these in your lab notebook before starting the experiment

1. Name at least four parameters that will affect the rate of photosynthesis. How does each parameter have bearing on the reactions of photosynthesis?
2. How do plant pigments and the absorption spectrum relate to photosynthesis?
3. What is the purpose of the soap in the solution used to infiltrate the leaves?

Materials:

Procedure:

1. Obtain a couple well-hydrated spinach leaves.
2. Using a single-whole punch, cut 20 or more uniform leaf discs for each trial. Avoid major veins.
3. Infiltrate the leaf disks with sodium bicarbonate solution.

a.Remove the piston or plunger and place the 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 sodium bicarbonate solution into the syringe. Tap the syringe to suspend the leaf disks in the solution.

e.Holding a finger over the syringe-opening, draw back on the plunger to create a vacuum. Hold this vacuum for about 10 seconds. While holding the vacuum, swirl the leaf disks to suspend them in the solution. Let off the vacuum.

• The bicarbonate solution 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. You may have difficulty getting the disks to sink even after applying a vacuum three or four times. Generally, this is usually an indication that you need more soap in the bicarbonate solution.
• Some leaf surfaces are more water repellent than others are. Adding a bit more soap usually solves the problem.
• Do not use leaf discs that do not sink.

1. Repeat the procedure in #3 using 10 of the discs and distilled H2O + soap. This group of discs will serve as your control.
2. Pour the discs and solution intoclean plastic cups.
3. Add bicarbonate solution to a depth of about 3 centimeters to one cup. To the other add an equivalent amount of distilled H2O.
4. Place under the light source (growing rack or sunlight) and start the timer. Record the number of floating discs at 30 second intervals. Then swirl the discs to dislodge any that are stuck against the sides of the cups. Continue until all of the discs in the bicarbonate cup are floating.
5. Dump all materials into the lab sink. Rinse the cups and syringes and place on paper towels to dry.
6. Conclusion:

For each of your cups, make a graph of number of discs floating vs. time and use this graph to find ET50 for each graph (point at which 50% of the discs would be floating). Make sure the graphs are in your notebooks and correctly labeled. List the ET50 values in a summary table.

The point at which 50% of the leaf discs are floating is the point of reference for this procedure. By interpolating from the graph, the 50% floating point is determined using the graphed data. Using the 50% point provides a greater degree of reliability and repeatability for this procedure.

Add your ED50 values to the table of class data. Using this data create a dot-plot for each period. Describe how your data agrees or disagrees with the larger data set. List sources of error that could have impacted your assay and how you could control this error. Remember, these are non-human based errors.

Post-lab Activity:

1. Now that you are familiar with a photosynthesis rate assay, design an experiment to investigate a unique question.

• List the question you want to answer. Be sure to include any background information that leads to your question.
• Present a hypothesis. This should be in the if-then-because format. The ‘because’ is important in this class because it shows that you are able to apply the theoretical knowledge to new ideas.
• What will be the controls? Variables? What observations will you take and how will you analyze them?
• Describe possible results and how you would interpret them (mechanisms)
• List materials and methods to be used. Be specific. Write out solution preparations.
• This should be all written out in your lab notebook at the end of the leaf disk assay experiment.

1