Saturday, May 6, 2017

Hands-On Photosynthesis Lesson

I absolutely love teaching photosynthesis!

Once you understand something, it's so super-fun to explain it to someone else. Here was our lesson from yesterday. This lesson can be used in

I've written up this lesson in handy PDF format for a classroom, but homeschoolers often don't have a big enough group to act it out. So this is how I modified it. I had only one child with me. We had a great time!

I actually think it might work best in this version because everyone is able to have an opportunity to be hands-on building the molecules.

Hands-On Lesson
this lesson is meant to be impressionistic *
its focus is on the concepts of raw materials, product, and by-product

What is the chemical formula for water? He will probably promptly say H2O.

Give the child a pile of 3 inch square pieces of grey paper and a marker. Don't have the "right" number of squares of paper in advance; keep it open ended.

Write H on one of the squares to represent an atom of Hydrogen. Then ask him to make a molecule of H2O. How many H? How many O?

Write H2O on the board. Explain subscript. Note that if there's only one atom of an element in a molecule, we don't bother to write the 1.

Now ask the child what is the chemical formula for carbon dioxide? Having "di" as a prefix is a hint. Write CO2 on the board.

Ask the child to make a molecule of CO2 with the paper squares. How many C? How many O?

Now we need a leaf. Spread out a large green silk on the floor. Note that photosynthesis only happens in the green leaves, which have chlorophyll (found in the chloroplasts).

Place the water and carbon dioxide molecules on the silk. Here's our leaf. How does water get into the leaf (in through the roots, then up the stem)? How does carbon dioxide get into the leaf? It gets in differently because it's not a liquid like water (the underside of the leaf has stomata which are like little mouths that take in the gas).

Ok, so now we have a leaf with H2O and CO2 in it. But what the plant wants is food, which is sugar. It wants to make a molecule of glucose. Write the formula for glucose on the board: C6 H12 O6. Does our leaf have enough carbon dioxide and water to make a molecule of sugar right now? No.

Give the child time to figure out how many molecules of each are needed, make them, and add them to the leaf. The grey atoms which make up each molecule should be touching, to show that the atoms are joined together by molecular bonds.

While the child is making the molecules, point out that Hydrogen is part of glucose. Without the water (H2O), the plant can't make sugar. A plant dies when it isn't watered, not because it is thirsty but because it is hungry. It literally can't make itself any food.

Now the leaf has what it needs but the molecules are still joined together as they were when they came into the leaf. The leaf can't unlock the elements it needs to make glucose. It needs the energy from sunlight to do this.

One of the ways to think about what a plant needs to survive is to think of all the ways to kill a plant. We already talked about the first one... which is to not water it. But isn't it true that a plant with plenty of water and plenty of carbon dioxide will still die if it is left in a dark closet? Yep. That's because it can't make food out of those ingredients all by itself. It needs the sunlight. It's like if you had a pantry full of canned food but no can opener. The sunlight is the can opener.

The sunlight rushes in and breaks those bonds. Have the child break apart all the molecules and then combine the C6 H12 O6 to make a molecule of glucose. He can just put those pieces of paper in a little pile on the leaf.

What is left in the leaf? Oxygen.

The oxygen wants to be joined up in pairs. Put all the remaining oxygen into pairs so that every one has a buddy. Then what happens to that oxygen? The plant doesn't need it so it releases the oxygen back into the air. Oxygen is a by-product of photosynthesis. This is the oxygen we breathe. We need it to survive. We, in turn, give carbon dioxide back to the plant every time we exhale. Children already know this is a cycle but that's different from having them understand WHY and understanding how photosynthesis works.

Remind the child that we can't make our own food, and that we get our energy from eating plants, animals that ate plants, and animals that ate other animals that ate plants. Ultimately, all of our food depends on sunlight.

Thinking Question
note: we did this in the Spring

What do you think would happen if we put stickers on part of a leaf, so that sunlight couldn't get to that part of the leaf? Write your hypothesis down.

In a moment we are going to do that, but first I want to read you something.


Share the cool facts about photosynthesis at the bottom of page 138 of David Mitchell's book, then read page 140 (this is a wonderful excerpt from "The Great American Forest" by Rutherford Platt).

This book is available for free (above), or for purchase (below).

Experiment (with two variations)

Now, we are going to see what happens when we shut that sunlight packaging factory down.

(This is a great use for the circular stickers that are sold for yard sale pricing... or the stickers that Highlights Magazine keeps sending to your house... or any other stickers which you have lying around.)

Walk outside. Place the stickers on a healthy green leaf. Do this with several different types of plants in your yard. Leave them for several days, a week, a month. Then take the stickers off. What do you think will happen? Why?

You can also do a version of this experiment by putting the stickers on in early Autumn before the chlorophyll starts to leave the leaves. Find some beautiful images from this experiment on Pinterest.

Here's another related experiment: Find Out Why Leaves Change Color Activity. For this experiment you will need the following supplies:

  • leaves
  • small jar (a baby food jar work well)
  • cover for jars or aluminum foil or plastic wrap
  • rubbing alcohol
  • paper coffee filter
  • shallow pan
  • hot tap water
  • plastic knife or spoon

* "For the botany in fifth grade is not yet the botany of the high school. Of course there must be enough science in the lessons to satisfy the fifth graders’ growing capacities, but wonder is still the key to learning here [emphases added]."

Dorit Winter, from p.83 of her contributing essay, "The Chariot of Michael," in And Who Shall Teach the Teachers, available to download for FREE at the Online Waldorf Library

This post contains affiliate links to the materials I actually use for homeschooling. I hope you find them helpful. Thank you for your support!

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