Lucy StanleyAP BiologyGiannou12/14/2014Testing the Rate of Photosynthesis in Leaf DisksIntroductionPurposeThe purpose of this lab is first to test the rate of photosynthesis in spinach leaves in different solutions, the to test the rate of photosynthesis in various species of leaves.BackgroundThe process of photosynthesis consumes carbon dioxide in order to produce sugar. As sugar is produced, oxygen is produced as well. The rate of photosynthesis can be measured by the accumulation of oxygen in the leaf. In this lab, the rate at which leaf disks float to the surface of a cup of bicarbonate solution will indirectly determine the rate of photosynthesis in the leaf tissue. Leaves will usually float in water due to the spongy mesophyll cells within the leaf being infused with CO2 and O2. In order to make them sink, these gases must be drawn out by a vacuum and replaced with water. The leaf disks can then be placed in a bicarbonate solution. Because they have an alternate form of CO2 , they can still perform photosynthesis. Cellular respiration also continues in the cells of the leaf disks, so some of the O2 in the leaves will be consumed through aerobic respiration. Even so, the leaves will float to the surface once they have accumulated enough oxygen, providing a method of measuring the net rate of photosynthesis in the leaves. (AP Biology 62)HypothesisFor the first part of the lab, the leaf disks in the water will not float up because there is no source of carbon dioxide, and the leaf disks in the bicarbonate solution will float. In the second part of the lab, the tropical plant will have the fastest rate of photosynthesis because of the climate it normally grows in, followed by turnip green, red chard, kale, mustard green and finally the astro lily. VariablesIn the first part of the lab, the independent variable was the solution the leaf disks were placed in. The dependent variable was the rate of photosynthesis, and the constants included the type of leaf, the number of disks, the light source, the amount of solution in each cup, and the size of the leaf disks. The control was the cup with the plain water. In the second part of the lab, the independent variable was the species of leaf. The dependent variable was the rate of photosynthesis, and the constants included the number of leaf disks in each cup, the number of cups under a light source, the type of light source, the size of the leaf disks, and the solution that was inside the cups.MethodsMaterialsFirst TestSecond Test

Baking soda Liquid soap Plastic syringes without needle (2) Living spinach leaves Hole punch Clear plastic cups (2) Timer Light source Pipette

Baking soda Liquid soap Plastic syringe without needle Living tropical, mustard green, turnip green, red chard, kale, and astro lily leaves Hole punch Clear plastic cups (3) Timer Light source Pipette

ProceduresTest 11. Create 300 mL of 0.2% bicarbonate solution. Pour bicarbonate solution into a clear cup up to a depth of 3cm. Label this cup With CO2. Fill another cup with water to a depth of 3cm. Label this cup Without CO2.2. Use a pipette to add one drop of diluted liquid soap in each cup, so that the hydrophobic surface of the leaf is wetted and can sink in the fluid.3. Use the hole punch to cut 10 uniform leaf disks per cup. 4. Remove plunger from both syringes. add the 10 leaf disks to each syringe barrel. Replace the plunger. Add a small volume of bicarbonate solution to one syringe, and a small volume of water to the other. Tap the syringes to be sure that all the leaf disks are floating, then make sure that there is no air left in the syringe.5. Create a vacuum by holing placing a finger over the opening of the syringe and drawing the plunger back. Hold the vacuum for about 10 seconds. Swirl the leaf disks, then let the plunger spring back. Repeat two or three times if necessary until all leaf disks have sunk in the solution.6. Remove the plunger and pour leaf disks and solution into the appropriate cups. Place both cups under the light source and begin timing. Record how many leaves have floated to the surface of each cup every minute.Test 2 7. Create 300 mL of 0.2% bicarbonate solution. Pour bicarbonate solution into three clear cups up to a depth of 3cm.8. Use a pipette to add one drop of diluted liquid soap in each cup, so that the hydrophobic surface of the leaf is wetted and can sink in the fluid.9. Each group picks a different leaf to test. Use the hole punch to cut 10 uniform leaf disks per cup. 10. Remove plunger from syringe. Add the 10 leaf disks to syringe barrel. Replace the plunger. Add a small volume of bicarbonate solution to syringe. Tap the syringe to be sure that all the leaf disks are floating, then make sure that there is no air left in the syringe.11. Create a vacuum by holing placing a finger over the opening of the syringe and drawing the plunger back. Hold the vacuum for about 10 seconds. Swirl the leaf disks, then let the plunger spring back. Repeat two or three times if necessary, until all leaf disks have sunk in the solution.12. Remove the plunger and pour leaf disks and solution into the appropriate cup. Repeat process twice more for the remaining cups. Place all three cups under the light source and begin timing. Turn cups and record how many leaves have floated to the surface of each cup every minute.

Data & Calculations1. Amount of Time for Leaf Disks to Float When Treated with CO2 (Group Data)Time (minutes)Leaf disks floating in bicarbonate solutionLeaf disks floating in water

100

200

300

400

500

610

710

820

920

1030

1150

1290

1390

1490

1590

1690

1790

1890

1990

2090

21100

2. Amount of Time for Leaf Disks to Float When Treated with CO2 (Class Data)Time (minutes) Group 1 Group 2Group 3Group 4Group 5 Group 6Average

10101000.33

20101000.33

30101000.33

40101000.33

50101000.33

60101110.67

72113111.5

84215222.67

95316423.5

107328434.5

1110328455.3

12103610897.7

13103610897.7

14103710897.8

15103710897.8

16105910898.5

17106910898.7

18107910898.8

19109910899.2

20109910999.5

2110101010101010

3. Amount of Time it Takes Red Chard to Float to the Surface of a Bicarbonate SolutionTime (minutes)Cup 1Cup 2Cup 3 Average

10000

20000

30000

40000

50000

6101.67

7101.67

82222

92232.33

103353.67

113374.35

123675.33

133675.33

144675.67

155676

166797.33

177797.67

187797.67

199798.33

209798.33

2198109

2299109.33

23910109.67

24910109.67

2510101010

4. Amount of Time it Takes Various Plants to Float to the Surface of a Bicarbonate SolutionTime (minutes)Tropical PlantTurnip GreenRed ChardKaleMustard GreenAstro Lily

1000000

2000000

3000000

4.67.33001.330

51.32.670031

62.33.33.67.674.331.67

72.33.67.67.675.332.67

82.352.676.332.67

92.65.672.3316.673.33

103.373.671.384.67

114.678.334.33386.33

1258.335.333.678.337

135.38.675.335.6798

145.39.335.676.339.338.33

155.39.6766.339.338.67

165.3107.336.339.679.33

175.3107.676.339.6710

185.67107.676.331010

197108.336.671010

207108.336.671010

2171097.331010

237109.337.671010

247109.338.331010

258109.339.331010

269.3310109.331010

279.3310109.331010

281010109.331010

291010109.331010

301010109.331010

31101010101010

CalculationsAverage: ni=1 xi /n or Sum of data points divided by number of data points, Ex. 6+7+9= 22 22/3= 7.33Conclusion & EvaluationConclusionIn the first part of the lab, the spinach leaves in the water did not float at all, while the spinach leaf disks in the bicarbonate solution began floating after several minutes. This is consistent with the hypothesis, and shows that the leaf disks needed a source of carbon dioxide in the water to photosynthesize. In the second part of the lab, the turnip greens had the highest rate of photosynthesis, followed by astro lily, mustard greens, red chard, tropical plant, and finally kale.This did not match the hypothesis at all. The thickness of the leaf disks may have prevented some from becoming buoyant quickly because they were too heavy. The different types of leaves may also fare better under different wavelengths of light. Evaluating ProceduresIn both procedures, the leaf disks may not have gotten the same amount of light because the cups shared a light source. The leaf disks may also have been different thicknesses due to being punched out of different locations on the leaf. There also could have been disks with veins. These could both cause leaf disk to be heavier than others, causing them to float up more slowly. The leaf disks may have been subjected to the vacuum for too long, making them take longer to rise to the surface of the solution. Some disks became stuck to the sides of the cup, and therefore would not float, causing outliers.Improving the Investigation The thicknesses of the leaf disks could be improved by making sure to punch all of the disks out of the same area of the leaf, avoiding veins. Each group should be sure to create the vacuum only two or three times. The procedures in the lab packet say to swirl the disks at the end of each minute to dislodge stuck disks. The groups conducting the lab should read more carefully and follow the procedures as they are written in the packet. Each cup should be provided with its own light source. The lab should be conducted again with different wavelengths of light and different colored leaves, such as poinsettias, to see how the type of light affects the rate of photosynthesis.

Works CitedAP Biology Investigative Labs: An Inquiry-Based Approach. New York: College Board, 2012: 61-69. Print.