Teaching Photosynthesis

Instructor: Amanda Robb

Amanda holds a Masters in Science from Tufts Medical School in Cellular and Molecular Physiology. She has taught high school Biology and Physics for 8 years.

This lesson will help you teach yourself or others about photosynthesis. We'll go over the basics of photosynthesis, including the two main steps. Then, we'll learn two different experiments you can try at home to demonstrate photosynthesis.

Engaging Students in Photosynthesis

Although students may not be familiar with the exact details of the photosynthesis reaction, most likely all of them can tell you trees give us oxygen. This is a great start to engaging students in the technical information of photosynthesis. You might even ask them where trees get their food? Most might suggest water, or the sun, but this is only part of the equation.

The real mass of trees comes from carbon dioxide in the air, which is combined with sunlight and water to make glucose and oxygen during photosynthesis. Students will be amazed at the idea that massive 200 foot tall redwood trees arise from nothing but air!

To keep students engaged though, we need to add some hands on experiments. Students will manipulate the reactants of photosynthesis, sunlight or carbon dioxide, and measure the products to make inferences about the rate of photosynthesis.

Sunlight and Photosynthesis Experiment

To look at how light affects the production of oxygen in photosynthesis, you'll need some plants that grow in water, like elodea (you can get this at a local pet store or online), a straw, a chemical called bromothymol blue (BTB), and a bright light. Student can preform this experiment in groups, or you could do it as a demo for the whole class depending on your class size. If you want to have students try it on their own in groups, you'll need the supplies mentioned above for each group. About 50mL of BTB will be enough for a class of thirty students working in pairs.

First, students will take two test tubes filled with water and about 10 drops of BTB. In one tube, have your students place a sprig of elodea. Next, have the students observe the color of the tubes, which should be blue, and record this in their notebook. However, when carbon dioxide is added to the water, the BTB will turn yellow. Now, have your students use the straw and blow some carbon dioxide into the water. The water should turn yellow if your BTB is working.

Next have your students compare this process in both the light and the dark. Ask your students to observe your water in the dark, away from a light source. Does the water stay yellow? It should. Since there is no light, the plant can't do photosynthesis. Without photosynthesis, the plant can't use up the carbon dioxide in the water and the water stays yellow.

Now have everyone try put the plant under a bright light after blowing into the tube. Your students should observe the water turning blue again as the plant uses up the carbon dioxide during photosynthesis.

During this lab carbon dioxide turns the water yellow which is reversed by photosynthesis which takes in the carbon dioxide
elodea lab setup

You also might notice tiny bubbles on the plant leaves. Ask your students what they think the bubbles are. One of the products of photosynthesis is oxygen, a gas. The bubbles are another sign your plant is doing photosynthesis!

Oxygen bubbles on the leaves of an aquatic plant

Carbon Dioxide and Photosynthesis Experiment

Students can also test the relationship of carbon dioxide to oxygen production using spinach leaf discs. Students should do this experiment in groups or pairs. The experiment takes at least forty-five minutes from start to finish, so be sure to leave enough time. If your classes are shorter, you can do the first steps of preparing the spinach discs on your own, and then let your students record the results.

To begin, you or your students will need to prepare some plant cells that have carbon dioxide in the leaves, and some that don't. Mix 1/8 a teaspoon of baking soda, which contains carbon dioxide, 300mL of water and one drop of dish soap in a beaker. This is your carbon dioxide solution. Your students will also need to cut about 50 small circles of spinach by punching a straw into a spinach leaf. Once you've done this, you're going to put half your spinach disks in a large, plastic syringe labeled carbon dioxide. Suck up a small amount of your carbon dioxide solution in the syringe.

Set up your spinach disc experiment by sucking up some of the solution into a syringe with your spinach discs
spinach disc setup

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