Photosynthesis: Watch It Happen

All these busy plants around us, producing what we need to live, and they don't look like they're doing anything. How can we tell if they are performing photosynthesis? One way is to see if they are giving off oxygen, the most important byproduct of photosynthesis. Of course, we can't usually see leaves producing oxygen, but watch what happens when you use an underwater plant!

Materials

• Elodea (also called pondweed, available at pet stores)
• Glass jar or beaker
• Funnel that fits inside the jar
• Test tube
• Matches and a wooden splint
• Safety goggles

What To Do:

(Adult supervision recommended, especially with step 5.)

1. Fill a sink with water and set the beaker in it. Put some elodea in the beaker and cover it with the funnel.
2. Now submerge the test tube in the water so that there is no air inside it. While holding it under the water, carefully place it over the neck of the funnel. Don't let its mouth break the surface of the water.
3. Lift the whole apparatus out of the water. You can tip a little water out of the jar so it won't spill. Set the jar on a sunny windowsill. As soon as the elodea begins to photosynthesize, you will see tiny bubbles appearing on its leaves and then floating upwards into the test tube. These bubbles are oxygen produced by photosynthesis!
4. Leave the jar on the windowsill for several hours. The rate of photosynthesis will vary depending on the intensity of the sunlight and other factors, but slowly the oxygen will collect in the test tube.
5. When the test tube is about half full of gas, put on safety goggles and use a match to light the wooden splint. Gently blow it out again and then immediately lift the test tube straight up and insert the splint up into it, without touching it to the sides of the test tube. The splint should glow brightly, or even burst back into flame! This is proof that the gas you collected is oxygen, which is flammable.

What's Happening?

Normally we can't see the oxygen produced by photosynthesis, but when it is produced underwater it appears as bubbles in the water. These float up through the funnel and displace the water in the test tube. Fire needs oxygen to burn, so when you insert the splint, the pure oxygen in the test tube causes it to glow brighter or produce a flame.

The rate of photosynthesis varies with several factors, including the intensity of sunlight and the temperature of the plant. (Other factors include the amount of water and the concentration of carbon dioxide (CO₂) in the air.) You can design an experiment to test some of these variables: for example, will photosynthesis happen faster or slower if you put the elodea in warm water? Collect data by measuring how much oxygen is produced in a given amount of time when the elodea is submerged in warm water vs. cold water. You can either mark the level of oxygen on the glass with a wax pencil, or you can use a graduated cylinder instead of a test tube and measure more precisely in milliliters. Try to keep the other variables constant €”it's best if you can run two jars simultaneously so you know they are getting the same intensity of sunlight. If you only have one jar, how else could you make sure the light intensity is constant? Could you use a light source other than the sun?

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