Photosynthesis Lessons


All the energy on the earth comes from the sun, in one way or another. The energy that plants use is one of the simpler paths for solar energy, since plants are able to take the electromagnetic energy of the sun and use it for fuel. They use that energy to produce things like fruit that we can use for energy ourselves.

The process plants use to make fuel from solar energy is called photosynthesis. “Photo” means light and synthesis is making something, so photosynthesis is the process of making something from light. Often this is as much as students understand about photosynthesis. Get some hands-on practice to get a fuller grasp of the process.

  1. Light is made up of different colors which have different wavelengths. Green plants absorb red and blue light and reflect green light, so they look green to us. Use a prism to divide light into its component colors. Then learn more about colors and light:
    • Rainbows are formed when water in the air acts as a prism. Learn more about rainbows.
    • Older students may be interested to know about the RGB system. Whereas we normally think of red, yellow, and blue as the the primary colors, light actually divides best into red, green, and blue. A basic explanation of the system can be followed by an exploration of light with the Hubble telescope.
  2. Green leaves have chloroplasts in their cells. These chloroplasts contain chlorophyll, a pigment that is very good at trapping the energy of red and blue lightwaves. These chloroplasts makes leaves into little solar cells which trap the energy of sunlight just the way that a solar power array does. Learn more about solar energy.
  3. Plants use the energy to power chemical reactions. A reaction is what happens when chemicals get together. A great example of a chemical reaction is a glowstick. Inside a glowstick there is a liquid called phenyl oxalate ester, plus dyes for colored glowsticks. Floating in the phenyl oxalate ester is a small glass vial that contains hydrogen peroxide. When you bend and snap a glowstick, the vial breaks, the hydrogen peroxide and the phenyl oxalate ester get together, and you get a chemical reaction. You can — carefully following all lab safety procedures — open up a glowstick so you can see the process. The leaves of a plant pull in carbon dioxide from the air and water from the soil. They divide up the molecules of water into hydrogen and oxygen, adding hydrogen to the carbon dioxide molecules to create glucose for their food. The leftover oxygen is sent back into the air. See the chemical details and equations on UC Clermont’s Photosynthesis page.
  4. Having combined hydrogen (from water) and carbon dyoxide (from air) to make glucose, the plant now has food. Glucose is a carbohydrate, a sugar in fact. Plants can use this energy to grow and to make flowers, fruit, and seeds. People also use food to grow and develop. Grow beans in your classroom to get a full understanding of how plants grow. When you have a big, strong plant, put a small Post-It note on one of the large leaves and leave it for a week. When you remove it, you’ll see what happens when a leaf is deprived of sunlight.
  5. Oxygen is a waste product for plants. They take the hydrogen from water (H2O) and the don’t need the leftover oxygen, so they let it go from pores (stomata) on the bottoms of their leaves. This is breathing for plants. We breathe in oxygen and breathe out carbon dioxide — which plants need. Plants and animals swap carbon dioxide and oxygen, making a very elegant system. Put an underwater plant into a dish of water in your classroom and put a test tube over it. Put the plant on a sunny windowsill. Oxygen bubbles will come up from the plant into the test tube. If you’re lucky, you’ll see them!
  6. From the point of view of a plant, fruit is a way to make seeds. If the fruit is not eaten, the seeds will fall to the ground or perhaps be carried by the wind for a little distance. Getting some creature to eat the fruit and take the seeds to another place allows the plant to spread out and make new plants without getting too crowded. In order to convince animals (including people) to eat the fruit, plants use the glucose they create from sun and water to make their fruits sweet. In some classrooms, there may be students who have not eaten fresh vegetables or fruits. If your class is like this, think about growing some sugar snap peas or strawberries in the classroom window or in a school garden. If that’s not practical, bring some fresh carrots or grapes to class and enjoy them together.

Your students should really understand photosynthesis now!


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