The Science of Fall!

As the days start to cool down here in the desert, we celebrate the beginning of Fall by taking daily walks around the neighborhood. We look forward to watching the beautiful colors sweep through the trees and we collect bits of nature to make our Fall collages. This year, we wanted to try something new. We wanted to learn more about Fall and how it happens. Why do the leaves change color? What makes Fall so brilliant and beautiful?

Photo taken at the Boyce Thompson Arboretum Used with permission: Kimberly Hosey of Arizona Writer

It all starts with Chlorophyll! Chlorophyll is the stuff that helps plants make food and makes leaves green. It absorbs energy from sunlight and Carbon Dioxide from the atmosphere and uses it to make carbohydrates (like sugar!) to feed the plants. The leaves then release waste energy in the form of Oxygen. We know this process as photosynthesis! (There’s even a catchy song about it, by They Might Be Giants!)

Photosynthesis! Image Credit: CC user At09kg Wikipedia

There are also molecules called carotenoids and flavonoids that work with the chlorophyll to make food for the plants. Carotenoids absorb sunlight and protect the chlorophyll from photodamage (sun damage). They also assist in converting carbon dioxide energy into sugars! Flavonoids help with pigments of flowers and also act as chemical messengers for the plant.

During the months of Spring and Summer, there is a lot of sunlight available for the plants to take in. Leaves unfurl and open wide, capturing the maximum amount of sunlight and carbon dioxide. This costs a lot in energy from the plants, as water evaporates through the pores in the leaves. Plants spend a lot of energy maintaining healthy leaves, but due to the amount of food these leaves can make, it is well worth the trouble for the plants.

Plants do very well photosynthesizing in Spring and Summer, but what about in Fall? As the seasons change, the temperatures drop and the days grow shorter. There is less sunlight available for the chlorophyll to use in photosynthesis. Suddenly, it becomes much more expensive for a tree to maintain its leaves. It spends more energy than it’s taking in, as the leaves can’t make enough food during the shorter sunlight hours. (This only applies to deciduous trees with broad leaves. Evergreens typically have long needle-like leaves that produce smaller amounts of food, year round.)

The tree responds to this by tapping into its reserves and going dormant for the winter. It stops producing chlorophyll and as a result, the green begins to fade from the leaves. Now we get to see some of the other colors that are present! Remember those carotenoids and flavonoids? Those reflect colors too! Carotenoids reflect yellow and orange, while flavonoids typically reflect reds. We just couldn’t see them before because the abundance of chlorophyll reflected so much green light that it dominated the color of the leaf.

Soon, the leaves start to dry out, becoming brittle, before a slight breeze finally knocks the leaves off the tree, leaving them to flutter gracefully to the ground.

Now that we knew why the leaves change colors during the Fall, it was time for us to take this a step further. Sure, we know all about chlorophyll in the plant cells, but what does that actually mean? Is that something we can see? To answer these question, we would need to explore the science of fall through the sciences of biology and chemistry!

The Biology of Fall

On the left:

This is a small pile of crushed Ficus tree leaves. These leaves are bright green and still contain a lot of chlorophyll! This leaf was ripped, crushed, and pulverized with a mortar and pestle.
Bottom: This is the same leaf, this time ground to a pulp with water to obtain a wet sample.

On the right:

This is a small pile of crushed Birch leaves. These leaves were yellow and brown and contained hardly any chlorophyll. This leaf was ripped, crushed, and pulverized with a mortar and pestle.
Bottom: This is the same leaf, this time ground to a pulp with water to obtain a wet sample.

What’s the difference?

On the left, you’ll notice that the leaf has a lot of great structure! The plant cells are all healthy and intact, and you can actually see the green chloroplasts that contain all of the chlorophyll! The leaf on the right looks a lot like algae. As Kat said, it looks “mushy” and not very healthy. There isn’t as much structure in the leaf itself, the chlorophyll has all but disappeared as the plant cells begin to fall apart.

Now that we had taken a look at the biology of Fall, it was time for us to take a closer look at the chemistry! We had learned all about the photosynthesizing leaf molecules, but we wanted to see them in person. Their presence can be seen as different colors in the leaves, but we could only see one or two at a time! How could these leaves have other colors in them?

The Chemistry of Fall

Materials Needed:

An assortment of leaves
3 glass heat-resistant jars
Plastic wrap
White coffee filters
Shallow baking pan
91 %Isopropyl Alcohol
Hot water

Paper and pen


Go on a nature walk! Collect leaves from a variety of trees and get as many colors as you can! When you get home, tear and chop the leaves into tiny pieces. Put them into small jars or glasses and label them with the name of the tree you collected them from.

Put 3-4 leaves from one type of tree into your glass jar. Using your scissors, roughly chop and tear your leaves into very small pieces.

Cover the leaves with Isopropyl alcohol. Then use your knife to briskly stir and chop them into the solution. You want to break up the leaves as roughly as possible, so that the pigments can be drawn out by the alcohol.

Loosely cover your jars with saran wrap, and place them into a shallow tray. Slowly pour about 1” of hot water into the bottom of your tray. Keep the jars in the water for 30 minutes. Every 5-10 minutes, use your knife to roughly swirl the leaves around in the alcohol.

Cut your coffee filters into long strips, about ½ inch wide. After 30 minutes, the alcohol will have changed color. Remove the saran wrap, and add your filters. Tape one end of the strip over the edge of your jar, and place the other end of the strip into the solution, so that it is just barely touching the solution.

Leave your jars alone and let chromatography do its job! Wait approximately 90 minutes. During this time, the alcohol will start traveling up the coffee filter, carrying the molecules of the leaves with it. As it evaporates, these molecules, and their colors will be deposited at various points along the filter. You’ll end up with rainbows of yellows, reds, oranges, golds, browns, and greens, depending on the leaves you use!

Optional: Leave it overnight! We checked on our leaves the next morning and we saw that our colors had seeped in even more! It was beautiful!

What’s Happening?

Chromatography is a process that is used to take pigments and colors, and separate them along a filter. You can think of the alcohol as a river, and the filter paper as a river bed. The pigments within the leaves get carried along the riverbed, but stop off along the way. When they temporarily stop along the filter paper, we can see the separate colors that were in the solution!

When we broke up the leaves and soaked them in alcohol, this allowed the cells within the leaves to break open, and the molecules that reflect these different pigments to seep out. As we heated up our mixture, this caused the molecules to move up onto our filter at a faster rate, carrying more of them along with it! Finally, as the alcohol evaporated on the filter paper, what was left behind was a colorful fingerprint of the chemical makeup of our leaves!

Take it further:

Collect samples of a variety of green leaves. Test your leaves to see if they have the same kind of chemical fingerprints! Do they all have the same colors locked within the leaves? You can also explore the temperature of the water. Does this experiment work as well if you have cold water instead of the hot water you used into this experiment?

Exploring the hows and why behind Fall was an awesome experience! We got to walk through our neighborhoods, collecting leaves and meeting neighbors (it’s always a good idea to ask before walking through someone’s yard)! By really exploring how Fall works, we are better able to appreciate the beauty we see all around us as we admire the colors of the changing foliage.

Of course, perhaps the best part about Fall is that as the trees stop producing chlorophyll and the leaves drop to the ground, we get to rake them up into HUGE piles and then jump into them! Just remember to give a thought as to the biological and chemical processes behind the fun you get to have as you play with your Fall leaves!

Happy Exploring!

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