Thursday, November 7, 2013

The Science of Fall!

It's Fall here in Arizona!
Photo taken at the Boyce Thompson Arboretum
Printed with permission: Kimberly Hosey of Arizona Writer

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?

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!)

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 photo damage (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

This is what Fall looks like under the microscope!

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 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?

To answer this question, we turned to the study of chromatography! Chromatography is a term that describes the separation of mixtures, by attaching one element of the mixture to a liquid that moves it to another place. In this case, we were going to be using this fantastic chromatography experiment by Science Made Simple, to pull the color out of the leaves by attaching it to rubbing alcohol!

The Chemistry of Fall

Materials Needed:

1. Leaves! Go for a walk and collect leaves in a variety of colors
2. Jars or clear glass cups
3. Aluminum foil, plastic wrap, or jar lids - something to cover your containers
4. Paper coffee filters (white is the best for seeing all of the colors)
5. Shallow baking pan
6. Isopropyl Alcohol (rubbing alcohol)
7. Hot tap water
8. Tape
9. Bowl, knife and spoon
10. Paper and Pen


1. 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.

Identifying the trees where your leaves came from can be a fun lesson itself!
You can practice scientific illustration by drawing your leaves on your cards too!

2. Cover the leaves with Isopropyl alcohol. Then use your knife to stir and chop them into the solution.

3. Cover the jars very loosely and place them into a shallow tray. Pour 1" of very hot tap water into the tray.

4. Keep the jars in the water for at least a half hour. Every five minutes or so, go outside and swirl your jars. You'll also want to make sure that the water remains hot, replace with fresh hot water if necessary. We need to get those molecules moving!

5. Once the Isopropyl alcohol has darkened with color, cut some coffee filters into thin strips. Take the jars out of the water and set them aside. Now you're going to add your filters! Put one end of the strip into the alcohol solution, and tape the other end of the strip to the top of the glass.

6. Wait for about an hour and a half. The alcohol will start travelling up the coffee filter, carrying the molecules of the leaves with it. As it evaporates, the colors will stay on the filter. You'll end up with rainbows of yellows, reds, oranges, golds, browns, and greens, depending on the leaves you use!

7. 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!

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!

Let the epic leaf fights commence!

1 comment:

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