13 Jun BIO1408 Photosynthesis is dependent on light for energy
Question
Experiment 2: Measuring the Rate of Photosynthesis
Photosynthesis is dependent on light for energy. However, only certain light wavelengths are suitable for this process. Wavelengths which are too short possess high energy which can easily destroy vital biomolecules such as DNA and essential proteins. Wavelengths which are too long possess low energy which may not be energetic enough to power photosynthesis.
When wavelengths which carry the correct amount of energy are absorbed by plant pigment, the electrons inside absorb that energy are move up to higher energy levels. Photosynthesis is thus engaged and ATP and NADPH are produced.
In this experiment, you will use plant leaf disks to view the net photosynthetic rate of the plant pigments. When a normal air-based environment is infiltrated with a new solution, the density of the leaf disks increases and they sink to the bottom of the solution. However, as photosynthesis proceeds, oxygen is produced, the density decreases, and the leaf disks rise again.
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Materials
(1) 250 mL Beaker
(1) 600 mL Beaker
10 mL Dishwashing Soap (Liquid)
1-Hole Puncher
Measuring Spoon
Permanent Marker
5 Pipettes
4 Plastic Cups
Ruler
2 g. Sodium Bicarbonate (Baking Soda), NaHCO3
Stir Rod
Stopwatch
(1) 10 mL Syringe
*2 different environments which might affect photosynthesis rate.
*Fresh plant leaves. Spinach or pokeweed are recommended, although most leaves with a smooth, thin surface are sufficient.
*Water
*You Must Provide
Procedure
Part A:
Use a permanent marker to label two plastic cups as “CO2, 30 cm Light” and “Soapy Water, 30 cm Light”. Leave the remaining cups unlabeled and set aside. Label the 600 mL beaker as “1”, and the 250 mL beaker as “2”.
Use a measuring spoon to measure¼ tsp. of sodium bicarbonate and transfer it into Beaker 1.
Fill Beaker 1 approximately 599 mL of water. Stir to mix.
Pour approximately two to three mL (approximately one squirt!) of dishwashing soap into Beaker 2.
Fill Beaker 2 with 200 mL of water. Gently stir to mix; avoid creating soap suds.
Use a pipette to transfer approximately one drop of the solution from Beaker 2 into Beaker 1. Again, avoid creating soap suds.
Note: It is critical not to create suds during this step. If suds are created, add a small amount of sodium bicarbonate or water to Beaker 1 to dilute the soap.
Pour the solution from Beaker 1 into the plastic cups labeled “CO2, 30 cm Light” until there is approximately three cm of solution in the cup. Pour the solution from Beaker 2 into the cup labeled “Soapy Water, 30 cm Light” until there is approximately three cm of solution in the cup.
Use the 1-hole punch to punch out 40 leaf disks. Try to avoid punching through any major veins in the leaves. Separate these disks into four piles of 10 disks.
Note: It is advisable to punch out extra leaf disks should you need to repeat part of the procedure.
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Figure 7: Step 12 reference.
Pull the piston out of the syringe and transfer 10 leaf disks into the bottom of the syringe.
Replace the piston in the syringe and push it down until there is only a small amount of air remaining in the syringe (less than 10% of the total syringe volume). Be careful not to smash the leaf disks.
Place the bottom/tip of the syringe into Beaker 1. Pull up on the piston to transfer a small volume of the sodium bicarbonate/soap solution into the syringe. Take the syringe out of Beaker 1 and place your finger on the open tip of the syringe.
Rotate the syringe so that the tip is facing up, and the piston handle is directed towards the ground. Use your finger to gently tap the syringe near the leaf disks to move the leaf disks into solution.
Keeping your finger on the syringe tip, pull back on the piston to create a vacuum environment within the syringe. Hold this vacuum in place for approximately 10 seconds, continuously swirling the syringe barrel to further encourage the leaf disks into suspension. The leaves should move away from the tip and towards the rubber end of the piston during this step.
Remove your finger from the syringe tip. If all of the leaf disks are suspended in the solution, proceed to Step 15. If some disks still remain near the syringe tip (at the top of the solution) repeat Steps 11 – 13. See Figure 7 for reference.
Note: If the disks do not suspend in the solution after three attempts of Steps 11 – 13, add a few more drops of the soapy water from Beaker 2 to Beaker 1 and repeat Steps 9 – 14. Repeating the vacuum procedure more than three times will damage the leaf disks!
Remove the piston from the syringe to pour the solution and disks from the syringe into the cup labeled “CO2, 30 cm Light”. Position the plastic cup approximately 30 cm below a light source and begin timing.
Record the number of floating disks in one minute intervals in a clear, logical format in Table 3. Be sure to indicate the time interval and trial to which each measurement corresponds. Continue timing until all leaves have floated to the top of the solution (approximately 15 minutes).
Note: It is important to closely observe the leaf disks to ensure that none of them become stuck to each other or to the plastic cup. If any disks become stuck, gently swirl the cup or use a pipette to disturb and separate them.
Indicate whether this part of the experiment, Part A, demonstrates a positive control or a negative control in Table 3.
Part B:
Empty the syringe contents and repeat Steps 9 – 10.
Place the bottom/tip of the syringe into Beaker 2. Pull up on the piston to transfer a small volume of the soap solution into the syringe. Remove the syringe from Beaker 2 and place your finger on the open tip of the syringe.
Rotate the syringe so that the tip is facing up, and the piston handle is facing down towards the ground. Gently tap the syringe near the leaf disks to encourage the leaf disks into solution.
Keeping your finger on the syringe tip, pull back on the piston to create a vacuum environment within the syringe. Hold this vacuum in place for approximately 10 seconds, swirling the syringe barrel to further encourage the leaf disks into suspension. The leaves should move away from the tip and towards the rubber end of the piston during this step.
Remove your finger from the syringe tip. If all of the disks are suspended in the solution, proceed to Step
If some disks still remain near the syringe tip (at the top of the solution) repeat Steps 20 – 22.
Note: If the disks do not suspend in the solution after three attempts, repeat Steps 29 – 22 with new leaf disks. Repeating the vacuum procedure more than 3 times will damage the leaf disks!
Remove the piston from the syringe to pour the solution and disks from the syringe into the cup labeled “Soapy Water, 30 cm Light”. Position the plastic cup approximately 30 cm below a light source and begin timing.
Record the number of floating disks in one minute intervals and record your results in Table 3. Continue timing for approximately 15 minutes.
Note: It is important to closely observe the leaf disks to ensure that none of them become stuck to each other or to the plastic cup. If any disks become stuck, gently swirl the cup or use a pipette to disturb and separate them.
Part C:
Select a new environmental variable in which to test the photosynthetic rate. Label one of the remaining cups with the variable you select (example, if you select a dark environment, label the cup as “CO2, Dark”).
Construct a hypothesis in the Data section at the end of this procedure. This should indicate how you expect the variable to impact photosynthetic rate, and why.
Repeat Steps 9 – 16, using the newly labeled plastic cup instead of the “CO2, 30 cm Light” cup. Position the plastic cup in the new environment and begin timing. Record your data in Table 3.
Part D:
Repeat Steps 27 – 28 to test a third environmental variable.
Repeat Steps 9 – 16, using the newly labeled plastic cup instead of the “CO2, 30 cm Light” cup. Position the plastic cup in the new environment and begin timing.
Table 3: Measuring Photosynthetic Rate
Time (minutes)
Part A
Part B
Part C
Part D
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Post-Lab Questions
Using your data from Part A – D, determine the most appropriate method to graph the information, and provide the graph(s) in the space below. Include a brief statement describing why you selected the graphing method you did. Be sure to clearly indicate the x and y axes, the units used, and the graph title (s). You may also use a graphing software program, if authorized by your instructor.
Identify the controls used in this experiment. Which control is positive, and which control is negative? How do you know? Did you receive the results you would expect from such a control?
What was the purpose of the sodium bicarbonate?
What was the purpose of the soap?
Describe why the leaves rise. Alternatively, describe why the leave sink.
How does darkness affect photosynthesis. Be specific and be sure to discuss biological processes and molecules involved.
Predict what might happen if the solutions and leaf disks were boiled prior to the photosynthesis. Alternatively, what might happen if the solutions and leaf disks were subjected to very cold temperatures?
Will the leaf disks float faster or slower if the experiment is adjusted to decrease the photosynthetic rate?
Picture of the set up:
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