2.3.4 Floating Leaf Disk Lab

Introduction:

Photosynthesis is a process that converts light energy into chemical energy.

Light from the sun is absorbed by the chloroplasts in the plants leaves. Carbon dioxide (CO2) is absorbed through holes in the leaf. The chloroplasts convert the light energy and CO2 into sugars such as glucose. 

The chemical equation of photosynthesis looks like this:

6 CO2                 +         6 H2O                          C6H12O6         +         6 O2

Carbon dioxide         +         Water                  Glucose         +         Oxygen

In this lab, you will be using small disks of spinach leaves to test the rate of photosynthesis under various conditions. The four conditions we are testing are low light, low Carbon Dioxide, low temperature, and a control with a normal level of CO2, temperature, and light. You will be given one of these conditions to test. 

The small leaf disks normally float, however when the air spaces are filled with carbon dioxide, the overall density of the leaf disk increases and the leaf disk sinks. When sodium bicarbonate (baking soda) is added to the water, it fills the air spaces in the leaf and causes the disks to sink. It is also the source of carbon dioxide. As photosynthesis occurs, carbon dioxide is used up and oxygen is released into the inside of the leaf. This changes its buoyancy causing the disk to rise.

1.  What is the purpose of the lab?_______________________________________

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2.  How will the three conditions affect the speed that the leaf disks rise? 

Make a hypothesis (If, then, because) for each condition.

Low Temperature: ________________________________________________

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Low Light: _______________________________________________________

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Low Carbon Dioxide: _______________________________________________

________________________________________________________________

Materials:

• Sodium Bicarbonate (Baking soda)         
• Liquid Soap                                         
• Spinach Leaf                                        
• Timer                                                
• Light Source
• Hole Punch or Drinking Straw
• Syringe (20 ml)
• Ice Bath
• Beaker
• Electronic Balance
• rule

Procedure: 

1. Add the required amount of sodium bicarbonate (see your groups amount below) into 300 ml of water in a beaker.
2. Add 1 drop of soap to the bicarbonate solution. (Soap makes the air spaces easier to fill) 
3. Use a straw to punch 10 disks from the spinach leaves avoiding major leaf veins.
4. Remove the plunger of the syringe and place 10 leaf disks in the syringe barrel.
5. Replace the plunger being careful not to crush the leaf disks. Push on the plunger until only a small volume of air remains in the barrel
6. Draw a small volume of the sodium bicarbonate solution into the syringe. Invert the syringe and tap the syringe to suspend the leaf disks in the solution.
7. Push the plunger removing as much air as possible from the syringe.
8. Close the valve on end of the syringe and draw back on the plunger to create a vacuum. Hold this for 10 seconds.
9. Let off the vacuum and repeat step 8 if needed 2-3 more times until all leaf disks sink. Pour the disks and solution into the beaker.
10. Place the beaker the required distance from the light and begin timing.
11.  Record the number of floating disks at the end of each minute in your table.
12. Dispose of the leaf disks in the trash when you are finished taking data. The bicarbonate solution can be dumped down the sink.

Group 1: Control

• Room temperature                                
• 3g baking soda                                  
• 10 cm from the light                         

Group 2: Low Temperature        

• Cold temperature: (Place your beaker containing the Baking soda solution in an ice bath for 3 minutes)

• 3 g baking soda
• 10 cm from the light

Group 3: Low CO2                        

• Room Temperature

• 1 gram baking soda                        
• 10 cm from the light        

Group 4: Low Light

• Room Temperature        
• 3 g baking soda
• 25 cm from the light        

Data:

Draw a table and use it to record your data. You will need to include space in your table to record time and the number of disks that are floating at each minute of your data collection time. There should be space to record the other 3 conditions.

Analysis:

1. Compare your results to the other group who tested your same condition. How do they compare? Why might they be different? List some specifics. 

2.  Talk to the three groups you are paired with to get their data and write it in your table on the previous page. Make a line graph on this page showing the results of all four conditions. Give the graph a title, label both axes with units, and provide a legend to distinguish each trial. 

3.  When the temperature, CO2, or light were decreased, what happened to the rate of photosynthesis? How do you know this? Explain.