Photosynthesis Lab

Using Leaf Disks to Measure the Rate of Photosynthesis

What can we measure to determine the rate of Photosynthesis?

What can we measure to determine the rate of Photosynthesis?

Consumption of:

Production of:

Baking Soda = sodium bicarbonate NaHCO_{3�Serving as a carbon source for photosynthesis}

• sodium cation (Na+) and a bicarbonate anion (HCO₃−)

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When Added to Water:

• NaHCO₃ + H₂O → H₂CO₃ + (OH-) + (Na+).

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When carbon dioxide dissolves in water, it exists in chemical equilibrium with carbonic acid:

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Materials

• Baking soda (sodium bicarbonate)
• 2 plastic 10mL syringes without needle
• Living leaves (spinach, ivy, etc.)
• Hole punch
• 2 clear plastic cups
• Timer
• Light source
• Stirring Rod

Step 1 - Fill 2 beakers ¾ish full of solutions and label appropriately

• One beaker should contain the CO₂ solution
• The other beaker should contain only water

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Step 2 – Punch out 10 leaf disks for each cup

• Punching the disks �onto the index card�will make it easier �to transfer the disks �to the syringe.

Step 3 – Remove the plunger and put the disks into the syringe and sink

Sinking The Disks

• Replace the plunger
• Draw up about 5cc’s of water from the corresponding cup
• Invert and push out excess air

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Sinking The Disks

• Place your thumb on the tip of the syringe wrapping your fingers around the syringe for support
• Place your other thumb on the base of the syringe and place the plunger between your pointer finger and middle finger
• Pull back on the plunger �to create a vacuum
• Shake

Note: You must pull and shake �at the same time

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Step 4 – Pour the Disks into a cup

• Remove the plunger holding the end over the respective cups
• Pour the contents into the cup
• If some of the disks don’t come out: replace the plunger, draw up more water, shake, remove the plunger and pour into the cup

Step 5 – Place both cups under a light source

• Swirl or stir the cups periodically to dislodge disks that may have stuck to the side of the cup �(Be sure that what you do to one cup you do to both cups)

• Record how many disks are floating at the end of every minute until all* disks are floating

Sample Data Table

Design Your Own Experiment

• Consider other factors that may affect photosynthesis.
• When you decide what your group is interested in measuring, let me know and I’ll help get your supplies.
• Record the number of leaf disks floating and record your data in an appropriately labeled data table.

Discussion Questions

1. How are we measuring the rate of photosynthesis in this experiment? How does cellular respiration factor into this?
2. Why is it important to add the baking soda to the solution? Use appropriate terminology.
3. Explain the experimental design for your experiment. What variables were you testing? This isn’t a description of the procedure, which you’ll include in the procedure section. Instead, you should discuss the chosen methods/variables and their relationship to photosynthetic rate.
4. What are some additional variables we could test in this photosynthesis experiment? ie Future directions?

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Lab Write-up Mini-Poster

• Title
• Null and alternate hypotheses for your experiment:
• Increased/decreased rate of photosynthesis with the variables you chose
• Experimental Procedure – Write out the steps you followed!
• Data Table(s)
• Graph
• Answers to the Discussion Questions (last slide)
• Conclusion – Provide the claim, evidence, and reasoning in your discussion of both parts of the experiment.
• A claim about the results of your designed experiment
• Evidence is primarily the data you collected
• The reasoning is where you dig into the science! The process of photosynthesis etc.