Leaf Bubbles

Oxygen bubbles on a leaf (click image to zoom in)

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​Did you know that plants and trees breathe air, much like we do?  It's part of photosynthesis, the process plants use to convert sunlight into the energy they need to grow.  In this simple experiment you will actually observe tiny bubbles of oxygen gas that the leaves of green plants "exhale" as photosynthesis takes place.

​Click and expand the tabs below to get started.

what you'll need

• A few freshly-picked green leaves, you may want to collect multiple leaves from 2 or 3 different trees or plants
• A few rocks or other small objects for weights
• 2 Pie pans or baking dishes  (a Pyrex glass dish works well)
• A magnifying glass (optional, if you have one)

Experimental procedure

1. Place 2 or 3 different leaves in one of your pans or dishes.  Make sure they don't overlap or touch each other.
2. Place a rock or other small weight on each leaf.  You may want to clean any dirt or sand off your rock first.
3. Repeat Steps 1 and 2 to prepare a second identical pan or dish.
4. Carefully pour enough water into each pan to completely cover all of the leaves.  Make sure the rocks keep the leaves from floating to the surface.
5. Place one of your leaf pans outside or next to a window where it will receive direct sunlight.  
6. Place your other leaf pan someplace completely dark, or cover the pan with a box or cloth that will completely block all sunlight from the leaves.  
7. Wait at least one hour, then closely observe the surface of all the leaves while they are still under water.

Initial setup in sunlight (click image to zoom in)

After 1 hour in sunlight (click image to zoom in)

what's happening

Trees and other plants need to "breathe" air and "drink" water in order to grow, just as we do, but when the sun shines on them they can do something that we can't do.  It's an amazing chemical reaction called photosynthesis.   Cells in the plant's leaves get carbon dioxide gas molecules, or CO2, from the air, and water molecules, or H2O, from their roots.  These cells also contain special structures called chloroplasts which in turn contain a chemical called chlorophyll, which is the pigment that gives the leaves their green color.  

The chlorophyll in chloroplasts can absorb sunlight and use its energy to split apart  the hydrogen (H) and oxygen (O) atoms of H2O molecules (just like you might use your energy to break apart the individual pieces of a Lego car).  Another part of the cell breaks apart the carbon (C) and oxygen (O) atoms of CO2.  Now the cell can use some of these H, C and O atoms to rebuild new molecules called carbohydrates (just like you could build a new boat or plane with the Lego pieces from your car).  Some common carbohydrates made by plants include glucose (used for energy), cellulose (used for building leaves and stems) and starch (used to store extra energy).

After building these new carbohydrate molecules it turns out that there are some extra oxygen atoms left over, which simply escape through the leaves, just like we exhale when we breathe.  We breathe in oxygen and exhale carbon dioxide, while plants breathe in carbon dioxide and exhale oxygen during photosynthesis, thus we need each other to survive.  [Plants can also breathe in oxygen and exhale carbon dioxide and water, just like we do. This process is called plant or cellular respiration.]  If you look closely at your leaves that were in the sunlight, you should see lots of very tiny bubbles.  These are the escaping oxygen atoms which form bubbles since they are under water.  If you look closely at the leaves that were kept in the dark, however, you won't see any bubbles.  This shows that without sunlight the plant does not have the energy it needs for photosynthesis, thus it can't split apart the atoms of the  H2O molecules and there are no oxygen atoms to escape through the leaves.

variations and related activities

• Repeat the experiment with different types of leaves.  Do some plants seem to be better than others at producing oxygen?
• Compare sunlight to light from different types of light bulbs, including different colors.  How does this affect photosynthesis?
• In the fall try comparing leaves that are still green with those that have changed color to brown, red or yellow.​
• Are you surprised that this experiment works even though the leaves are under water where they are not in contact with air?  That's because the only gas from the air that the leaves need for photosynthesis is carbon dioxide (CO2), and there is actually enough CO2 gas dissolved in the water you used for the leaves to use.  You can prove this by carefully boiling (to remove the dissolved CO2) and then cooling your water before the experiment, as described in this reference link below.

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references and links to more information

These links will get you started on your own internet search to learn more

Learn more about photosynthesis and chlorophyll:

• https://en.wikipedia.org/wiki/Photosynthesis
• https://ssec.si.edu/stemvisions-blog/what-photosynthesis
• https://en.wikipedia.org/wiki/Chlorophyll
• https://byjus.com/biology/difference-between-photosynthesis-and-respiration/

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Learn more about plant and leaf science (botany):

• https://gobotany.nativeplanttrust.org
• https://www.thoughtco.com/plant-leaves-and-leaf-anatomy-373618
• https://en.wikipedia.org/wiki/Botany

Learn about plant cells:

• http://www.sciencewithme.com/learn-about-the-plantcell/
• https://www.science-sparks.com/plant-cell-model/
• https://www.thoughtco.com/what-is-a-plant-cell-373384
• https://en.wikipedia.org/wiki/Plant_cell
• https://byjus.com/biology/plant-respiration/

More experiments with plants and cells:

• https://www.sciencebuddies.org/science-fair-projects/project-ideas/experiment-with-plant-cells
• https://sciencing.com/happens-hypertonic-hypotonic-isotonic-environments-8624599.html
• https://www.biologydiscussion.com/experiments/photosynthesis-experiments/top-11-experiments-on-photosynthesis-in-plants/21976
  

Identifying plants and trees by their leaves:

• https://www.woodlandtrust.org.uk/blog/2020/03/tree-id-kids/
• https://www.arborday.org/trees/whattree/
• https://www.thoughtco.com/trees-with-leaves-a-deciduous-tree-leaf-key-1343475
• https://www.plantsnap.com

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Learn more about carbohydrates:

• https://en.wikipedia.org/wiki/Carbohydrate
• https://www.visionlearning.com/en/library/Biology/2/Carbohydrates/61
• https://www.thoughtco.com/chemistry-of-carbohydrates-603878

​Subject Tags
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• Photosynthesis
• Plants

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