Plants have chemicals in their leaves that can convert sunlight, water, and carbon dioxide to food in the form of sugars. The process is called photosynthesis. The chemicals in the leaves that absorb light are pigments. The most common pigments that convert sunlight energy are green pigments called chlorophyll. Other pigments found in leaves include xanthophylls, carotenes, and anthocyanins (see below for definitions).
How do scientists study the colors in plants? One way is to perform a simple color separation or “chromatography.” Let's look for the pigments found in growing plant leaves, in this case fresh spinach which is full of pigments.
The chromatography requires time, at least an hour, so prepare your children in advance. It also requires rubbing alcohol. Read and follow the warnings on the alcohol bottle label carefully.
You will need:
- A fresh spinach leaf per child
- Small clean containers, like a yogurt cups or jars, one per child
- Tool to chop the spinach leaves (requires adult supervision)
- Rubbing alcohol
- Stirring utensil that will fit in containers
- Coffee filter cut into about ¾-inch-wide strips that will fit into the container from top to bottom, one per container
Chop up the spinach leaves into tiny pieces. Put the chopped bits into the bottom of the containers. They should cover the bottom in a layer, up to ¼-inch deep. Pour in just enough rubbing alcohol to cover the leaf bits. Stir briefly. Slide the coffee filter strip into the container, so that the bottom touches the alcohol/leaf mixture. Rest it against the side. Now wait for about an hour.
The alcohol should slowly move up dry part of the coffee filter, carrying the pigments as it goes. The heavier pigments will travel more slowly, the lightest most quickly. Once the wet alcohol "front" nears the top of the strip of coffee filter, remove it. Check the colors. Lay it on a piece of paper towel or paper and let it dry a bit. Often the colors of the pigments will show more intensely once the alcohol has evaporated.
Your coffee filter might look like this:
The green bands at the bottom are chlorophyll pigments. The plant uses chlorophyll to convert the sun’s energy into food. If you look closely, there are two bands of green, with some yellow. Those are the two chlorophyll pigments, named chlorophyll a, and chlorophyll b. Chlorophyll a is the main type of chlorophyll. Chlorophyll b is an accessory pigment that captures light energy from a broader range of light and transfers it to chlorophyll a.
Just above the green is an intense yellow band. The yellow is a group of pigments called xanthophylls. Xanthophylls are what make corn yellow. When chickens eat plants with xanthophylls, they can use it in their eggs to make the yolk a deeper yellow.
At the very top, is a thin orange band. Those pigments are carotenes, named for the first plant tissue they were isolated from, carrot roots. Carotenes are also found in pumpkins and squash. Carotenes are also used for photosynthesis.
Now compare the colors in the growing spinach leaf to those of the autumn leaf. What colors are the same? What colors are missing in the leaves? What colors are in the leaves but not in the spinach?
Autumn leaves lose the green pigments first. The two types of chlorophyll actually break down as the leaves begin senescence. Does the plant make the the orange and yellow carotenes and xanthophylls in response to losing the chlorophyll? No, those pigments have been there all along in the growing leaf, but masked by the deeper green chlorophyll. When the green disappears, we can see the other pigments.
Another color, however, wasn’t present before. The intense red pigments found in some autumn leaves are called anthocyanins, and are known for making flowers, and vegetables like beets, red or purple. It turns out that certain trees make a lot of anthocyanins in their leaves in the fall. Leaves don't produce it earlier in the year.
Right now scientists don’t know for sure why certain types of trees produce the anthocyanins. One suggestion is that the anthocyanins act like sunscreen to help protect the leaves while the last bits of nutrients are being moved out and down to the roots for storage over the winter. Another suggestion is that red trees are less visible to insects. Harmful insects may lay their eggs on trees in the winter, and may choose trees that are yellow over those that are red.
- Scientific American has a similar Hidden Colors of Autumn Leaves activity
- See Share It! Science for Hidden Colors in a Leaf
Want to learn more? Feel free to leave questions in the comments.
To see our complete plant science lessons, either visit the plant science category (newest posts to oldest posts) or the plant science section of our experiment archive page (links to posts in order).
For more activities, try our Gardening/Plant Science for Kids Pinterest board.