How to Show Oxygen Is a By Product of Photosynthesis

Co-authored by Meredith Juncker, PhD

Last Updated: November 11, 2020

Photosynthesis is the process plants (or photoautotrophs) use to convert energy from the sun, carbon dioxide, and water into food (carbohydrates). The byproduct of that process is oxygen, and you can prove it using a simple science experiment! This project is best started in the morning on a sunny day. Elodea is the chosen plant because it gives off oxygen in the form of bubbles, which you'll be able to see, but any kind of aquatic plant (or hydrophyte) will work.^{[1]
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Part 1

Part 1 of 2:

Setting up the Experiment

1
Gather the necessary materials. To prove that oxygen is a by-product of photosynthesis, you can set up an experiment with an aquatic plant and capture the gas it emits in response to photosynthesis. You will also set up a control condition where the plant will not be exposed to light, preventing photosynthesis. To perform this experiment you will need the following items:^{[2]
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- 8 sprigs of an elodea plant, an aquatic plant available at your local pet or garden store
- 2 large, clear containers
- Sodium bicarbonate (baking soda), no more than 1 tablespoon
- 2 large funnels (must be small enough to fit inside the large container)
- 2 test tubes (must fit over the end of the funnel)
- a light source: sunlight or at least a 40-watt bulb
- dechlorinated tap water (chlorine in the water will kill the plant), enough to fill both containers about ¾ full
- Gloves
For each sprig, remove several leaves at the base of the stem and cut the end at an angle. Gently crush the end of the stem with your fingers.^{[3]
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Research source} You will use four sprigs for each experimental setup.
3
Make the sodium bicarbonate solution. Fill the large container about ¾ full of room temperature dechlorinated water and add a few spoonfuls of the baking soda. Stir the solution until it is fully dissolved. This bicarbonate solution will provide the elodea with the carbon dioxide it needs to use photosynthesis.^{[4]
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- Add the same amount of baking soda and water to both of the large containers.
4
Place the elodea sprigs in the bottom of the container with the funnel inverted over them. Add four sprigs to the bottom of each container. Invert the funnel and place in the container so that it covers all of the sprigs of elodea and the end of the funnel points towards the top.
- Make sure there is enough water so that the end of the funnel is still submerged. Add more room temperature water, if necessary.^{[5]
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Either submerge the test tube in water until there are no bubbles left in the tube or simply fill the tube all the way to the top. It is crucial to the experiment that there is no air trapped anywhere in the test tube.^{[6]
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6
Invert the test tubes and place over the stems of each funnel. Hold your thumb over the top of the test tube, gently turn the test tube over and lower the end into the water. Remove your thumb and check for any bubbles in the tube. If there are bubbles, tilt the tube until the bubbles escape to the surface. When all bubbles are gone, set the test tube over the stalk of the glass funnel.^{[7]
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- Ensure that the setup is stable and the test tube sits snugly on top of the funnel.
- At this stage there should be no air bubbles anywhere in either of the test tubes. If there is air, remove the test tube, refill with water, and try again.
7
Store one jar in direct light and one jar in darkness. For the jar in direct light, place it near a window or next to a lamp. For the jar in darkness, store it in a dark room or closet with no windows. Leave a note on the door that says “Do Not Open” so your family will know an experiment is in progress.
- The jar stored in the darkness is the “control” for photosynthesis. Since we are trying to prove that photosynthesis produces the oxygen, we want to make sure that no oxygen is produced in the absence of light.
- Leave both jars alone for at least 8, but up to 24 hrs.^{[8]
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- After just a few hours, you should start to see air bubbles appear in the test tube of the elodea in the light.
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Part 2

Part 2 of 2:

Showing that Oxygen is the Gas Produced

1
Check on the experiment after 24 hours. Take the jar that has been sitting in the sun at the end of the day. You should see an air bubble at the top of the test tube. For the experiment that was in darkness, there should be no air bubbles at the top of the test tube.^{[9]
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- If you don’t see air in the setup that was stored in light, try the experiment again with some fresher elodea sprigs.
- If you see air in the setup that was stored in darkness, you may have accidentally let in air while you were setting up the experiment. Try it again, making sure there is no air when you place it in the dark.
2
Lift the test tube from the funnel of the “light” experiment. Put gloves on to avoid contaminating your test tube. Keeping the test tube upside down and submerged in the water, remove it from the funnel. Place your thumb over the end of the tube and take it completely out of the water.
- Be careful not to tilt the tube at all because the oxygen bubble at the top will escape and you will have to start the experiment again.
3
Burn the oxygen. Light a match, but blow out the flame quickly so it is still glowing. You might need a second pair of hands for this step: someone to light the match and someone to hold the test tube. Invert the tube so it is now upright and the trapped gas is at the top of the tube. Remove your finger and place the glowing match into the gas of the test tube.
- Parental supervision is recommended when lighting matches.
- Don’t let the match touch the water, only the gas.
- You should see the match reignite, bursting into flame. Because oxygen is highly flammable, when the smoldering match comes into contact with it, it bursts back into flame.
4
Repeat this with the control elodea that was kept in the dark. Again, remove the test tube from the funnel and place your thumb over the top. Although you can’t see any air at this point, you want to perform all the steps the same way you did previously. Remove the test tube from the container, light the match, and remove your thumb. Place the match into the top of the test tube and observe the results.
- Because there is no gas in the tube, the match will touch the water and be quenched. This part of the experiment proves that the oxygen in the “light” condition was produced by photosynthesis.
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Why is it important for plants to release oxygen as a byproduct?

Meredith Juncker, PhD
Scientific Researcher
Meredith Juncker is a PhD candidate in Biochemistry and Molecular Biology at Louisiana State University Health Sciences Center. Her studies are focused on proteins and neurodegenerative diseases.

Meredith Juncker, PhD

Scientific Researcher

Expert Answer

The oxygen byproduct of photosynthesis is utilized in another process in the plant: cellular respiration. This is how the plant generates energy, or ATP, to survive. If oxygen isn't released during photosynthesis, its counter process, cellular respiration, cannot occur, and there will be a lack of ATP generated and the plant will die.

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What happens when extra sodium bicarbonate is added?

Meredith Juncker, PhD
Scientific Researcher
Meredith Juncker is a PhD candidate in Biochemistry and Molecular Biology at Louisiana State University Health Sciences Center. Her studies are focused on proteins and neurodegenerative diseases.

Meredith Juncker, PhD

Scientific Researcher

Expert Answer

Sodium bicarbonate is the source of CO2 in this experiment, so adding more sodium bicarbonate would increase the amount of CO2 exposed to the plant for use in photosynthesis. With more CO2, the rate of oxygen bubble formation will increase since photosynthesis uses CO2 and water to produce glucose and oxygen.

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What are the dependent and independent variables of this experiment?

Meredith Juncker, PhD
Scientific Researcher
Meredith Juncker is a PhD candidate in Biochemistry and Molecular Biology at Louisiana State University Health Sciences Center. Her studies are focused on proteins and neurodegenerative diseases.

Meredith Juncker, PhD

Scientific Researcher

Expert Answer

The independent variable would be the light, and the dependent variable would be the oxygen produced (the amount of oxygen produced depends on the presence or absence of the light).

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Warnings

Be careful when using the match.

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Things You'll Need

2 small elodea plants per student or group - available from shops that stock supplies for aquariums (they are water plants)
2 wide-mouthed jars or beakers
2 test tubes
2 glass funnels
Matches