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Kid Questions: What's With Fall Leaves and Glow-in-the-Dark Toys?

Why do leaves change color? Why do things glow? Ask Science answers science questions from his kids. 

By
Lee Falin, PhD
3-minute read
Episode #118

While I enjoy getting questions from readers and listeners of the Ask Science show, some of my best science questions come from my own kids.

In fact, Mrs. Ask Science and I hung a big white board next to our dinner table just so we can illustrate answers to questions they ask. We've used that board to explain everything from why germs make us sick to how Washington liberated Boston in the Revolutionary War.;

This week, I wanted to share some of their more recent science-related questions with you.

Question #1: Why Do Leaves Change Color?

Sometimes we go on nature walks around our yard with the youngins’. Usually this involves my son trying to catch pill bugs or ladybugs so that he can build houses for them. But this past week the leaves had started to change color, so he and one of my daughters were wondering why that happens.

fallleavesMost of you probably know that the green leaves are green because of a chemical called chlorophyll contained inside of plant cells.  As the weather cools off, the chlorophyll starts to degrade, making it possible to see the carotenoids that most leaves contain.

Carotenoids are orange-yellow colored chemicals that play various roles inside of plants. One of those roles is to assist with photosynthesis, which explains what they’re doing hanging around inside of leaves. Carotenoids are found in many orange and yellow things, including carrots, bananas, and even some bird feathers.

At the same time that these yellowy-orange colors are showing up, the leaves also start to synthesize another chemical called anthocyanins, which are usually deep red in color. Anthocyanins are the same chemicals that give apples and cranberries their red color, and they also give blueberries and plums their colors.

The reason for this range of colors is because the pigment color of anthocyanin is based on pH. In fact, if you’ve ever heard of hydrangeas, you might know that they can change color based on the acidity of the soil they’re planted in. That color change is due to the presence of anthocyanins in their sepals.

The amount of anthocyanin a leaf produces (and therefore how deeply red it turns) is based on how much light the plant receives in the late fall and winter, as well as how cool it gets. Lots of cool (but not freezing) nights, plus plenty of exposure to sunlight, will produce more anthocyanin, and therefore, more red leaves.

The final color that most leaves turn is brown, which is what the dried out cell walls look like once most of the other chemicals have degraded away.

Up next, why do glow-in-the-dark toys glow?

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Please note that archive episodes of this podcast may include references to Ask Science. Rights of Albert Einstein are used with permission of The Hebrew University of Jerusalem. Represented exclusively by Greenlight.

About the Author

Lee Falin, PhD

Dr. Lee Falin earned a B.S. in Computer Science from the University of Illinois, then went on to obtain a Ph.D. in Genetics, Bioinformatics, and Computational Biology from Virginia Tech. 

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