Floating Leaf Disk Photosynthesis Lab

I use this experiment to reinforce what we’ve been learning in the photosynthesis lessons. I will be using principles of experimental design, scientific method, and CER as well. The floating leaf disk photosynthesis lab is pretty accessible. The only special equipment is really some syringes to sink the floating disks (I got mine overnight from Amazon). That means that if you are like me and sometimes lose track of time, this is still a lab you can do relatively last minute.

Objectives:

• Understand the inputs and outputs of photosynthesis
• Investigate how the manipulation of variables like amount of sunlight and baking soda affects the photosynthetic rate in spinach leaves. (For my folks with NGSS standards this is modeling).

Materials:

• Fresh spinach leaves
• Baking soda
• Distilled water
• Plastic syringes (without needles)
• Transparent cups or beakers
• A light source (lamp or direct sunlight)
• Hole punch
• Timer
• Stirring rod or spoon

Hypothesis:

Students can hypothesize whether the presence of baking soda and light will increase, decrease, or have no effect on the rate of photosynthesis in spinach leaf disks. Review the inputs and outputs of photosynthesis before asking students to make a hypothesis. Students may need support like setting up and if/then statement for them. If I/we __________, then _____ will occur. I like to have the I/we because that helps them identify the dependent and independent variable later.

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• Prepare two solutions: one with baking soda and one without.
• For the baking soda solution, I usually dissolve about 1 grams of baking soda in 100ml of water. You could also use different concentrations of baking soda solutions.
• Use plain distilled water for the control solution.

• Use the hole punch to cut out uniform disks from spinach leaves. Aim for 10 disks for each trial. Try to avoid the veins in the leaf because the vein tissue doesn’t have chloroplasts.

• This is the hardest part.
• Label syringes according to instructions.
• Pull apart syringe and place leaf disks inside.
• Shake them down toward the tip.  Put two pieces back together and gently push air out of syringe.  Be careful not to crush the leaf disks.
• Fill a syringe halfway with the provided solutions (one syringe for each solution), by pulling back and pulling the liquid into the syringe.
• The leaf disks will be floating.  Make sure they are all in the water.
• Close the tip and pull back on the syringe plunger to create a vacuum. Hold for 10 seconds.
• This process helps remove air from the spongy mesophyll layer of the leaf disks, causing them to sink in the solution.
• Continue until all of the disks have sunk in the syringe.
• Remove plunger over container with the solution where you will conduct the experiment.
• Check out this video to see how it is done.

• This is where you can have a lot of fun.
• I make sure each group has a control (which depends on what they are testing). You can easily test amount of light or color of light if you have grow lamps. You can also test different concentrations of baking soda solution (I do a combination of both)
• Place an equal number of infiltrated leaf disks (about 10) at the bottom of two separate clear containers (I usually use beakers because I have them)—make sure that students have them labelled.
• Ensure that all disks are fully submerged and lying flat at the bottom.

• Position the cups under a light source. Make sure both cups receive equal light intensity and duration (if light isn’t one of your variables).
• Start a timer when you expose them to light.

• Observe the time it takes for leaf disks to rise to the surface. As photosynthesis occurs, oxygen accumulates in the disks, causing them to float.
• Record the time it takes for each disk to float to the top.

Repeat the Trials:

• Conduct at least two trials for each setup to ensure reliability of the data. Do more if time allows. I usually do have to do the data analysis the next class period.

• Compare the average time it takes for disks in each solution to float.
• Graph the data. This is a skill that students continuously need to work on. Have them identify what will go on the x and y axis.
• Discuss the results in relation to the hypothesis. Did baking soda enhance the rate of photosynthesis?

• Discuss the role of carbon dioxide in photosynthesis and how baking soda (sodium bicarbonate) may act as a source of carbon dioxide.
• Explore the importance of controlled experiments and variables. How do we eliminate other variables as possible causes for the results?
• Have students write up their conclusions using a claim-evidence-reasoning format. Their claim should either support or not support their hypothesis.

This lab provides a hands-on way for students to visualize and measure photosynthesis, and also use scientific methods and data analysis.