STEM Mom

Why Do The Leaves Change Color?

Photo by Autumn Mott Rodeheaver on Unsplash

If you live in the northern temperate regions around the world the leaves are changing colors and falling. This causes many people to put away their lawn mowers and break out their rakes and leaf blowers. I remember jumping into freshly made leaf piles as a kid, much to my father’s dismay, spreading all the leaves we had just raked into a neat mountain, so we’d have to rake them again. But what is the scientific process happening that’s resulting in leaves changing color and falling off the trees? Why does it happen during “Fall y’all”? If you, or your child(ren), have ever wondered why we suddenly have this beautiful array of colors during this season read on and I’ll try to explain it.

If you remember there are types of trees; we could classify them in lots of different ways but for the purpose of this explanation we’re just thinking about trees that lose their leaves (deciduous) and trees that don’t (coniferous). The deciduous trees are trees like oak, maple, elm, and linden trees; these turn a beautiful mix of colors during Autumn and coniferous trees are needle leaved trees that make cones and are typically green all year round like pine, fir, spruce, and cedar trees.

Trees are green because of a chemical called chlorophyll. Chlorophyll is the pigment that absorbs the energy from the sun. I’m not going to get too involved in photosynthesis, but chlorophyll is the molecule that can absorb the light energy and use it to push photosynthesis along. Chlorophyll is what makes plants and leaves green. It is also the pigment that works the best for harnessing sunlight so during the spring and summer when sun is plentiful plants and trees make a lot of it.

Photo by Jeremy Bishop on Unsplash

Chlorophyll is very good at trapping the energy from the sun to use it but it’s also a very expensive molecule for the plants to make; it takes a lot of resources: you need a lot of sun and photosynthesis happening to make chlorophyll. Once the seasons shift and the amount of sun is starting to wane deciduous trees stop making chlorophyll because they must put energy into preparing for the upcoming winter when photosynthesis isn’t going to be happening as much. That means the trees will stop putting the energy into making the expensive pigments. Once they use up the pigments that means it won’t be replaced with new pigments and the leaves are slowly going to lose their green color.

But the magic of beauty of Fall reminds us of something, the trees are also making other pigments at the same time as chlorophyll — they’re just not nearly as abundant. It would pretty short sighted for a plant to only make one color pigment because that doesn’t give it full access to the colors of the rainbow, so plants make other pigments that help them get energy from all the colors in the rainbow. While we only see the green during spring and summer, the beautiful colors of Fall that have been capturing our imagination for generations remind us of all the other pigments that work with chlorophyll to feed plants.

The yellow leaves are caused by a group of pigments called xanthophylls. The orange leaves are caused by the carotenoid pigments first identified in carrots and anthocyanins are the beautiful reds of the maple leaves. All these molecules work together to help plants get the most bang for their buck from the sunlight. 

As the sunlight fades, so does the chlorophyll and we’re reminded of all these other players behind the scenes.

Here is a simple experiment you can do with your child(ren) to see the different pigments in plant leaves:

Materials:

1. A fresh, washed spinach leaf per child. I don’t recommend baby spinach from a bag

2. A small glass or container like the size of a yogurt cup per child

3. Rubbing alcohol

4. Knife to chop the spinach with (I recommend these as kid safe knives or make sure the adult does this part)

5. Spoon or something to gently stir the leaves once they’re in the container

6. White coffee filter that is cut into a long enough strip to hang out of the container

Steps:

1. Chop up the spinach (make sure to supervise this part) and layer it in the bottom of the container. You don’t need very much; you’ll need less than a quarter of an inch in the bottom of the container

2. Pour the rubbing alcohol on the leaves and make sure you cover the leaves entirely

3. Stir the mixture gently, just enough to get the leaves in contact with the alcohol

4. Place the coffee filter in the alcohol-spinach solution and drape it over the side of the container

5. Let it sit for at least an hour, the alcohol will travel up the coffee filter using some of the magical properties of water

6. During this hour would be a great opportunity to ask your child(ren) to make some predictions. What do they think is going to happen? What are they expecting to see?

7. After an hour you should see bands of color on the filter paper and your child(ren) can see the different pigments in the spinach leaf.

If you have a particularly curious kid interested in learning more about pigments, you can try these other activities:

1. You can pick up leaves from outside that have fallen and do the same prep to see if the color bands are the same.

2. You can also draw a line on the filter paper with a marker (one color per strip of paper) and just put them in the rubbing alcohol to see what pigments make up your child(ren)’s favorite colors.

With some simple household supplies your child(ren) can investigate the natural phenomena that makes Fall such a beautiful time of year.

Have fun!

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