Have you ever wondered why leaves turn color in the fall? They don’t really switch colors – some of those yellows and oranges are there all along!
Plants need food just like we do, only thing is, they make their own. They use a type of molecule, pigments, to capture sunlight. The plants can then use that energy to make sugar from carbon dioxide and water (photosynthesis).
Each kind of pigment works best at certain wavelengths (the different colors in a rainbow). The most common pigment group, chlorophyll, is really good at using most of the spectrum, except green. Then why are leaves green? That’s because chlorophylls soak up all the other colors. The green light bounces off, hits your eyes, and you see green.
Some pigments are better at absorbing other parts of the spectrum. The carotenoids give plants such as carrots or bananas bright yellows and oranges. The anthocyanins appear red to blue and are mostly present in flower petals and fruits such as cranberries or cherries. The uncommon betalains are found in only a few plants, but are responsible for red beets and colorful bougainvillea! Plants use a mixture of pigments in their leaves to capture as much of the energy in sunlight as possible.
In the fall, as
the days get shorter, plants slowly stop making the chlorophylls and eventually
their green fades away. Once the carotenoids are not swamped out anymore, their
colors are finally revealed! Some plants will make anthocyanins as the days get
shorter, and so will turn red.
In this
experiment, you will try to find out how many pigments are in different leaves.
You will separate the pigments using filter paper and rubbing alcohol, a
solvent.
Materials:
- Filter paper (coffee filter paper works though high-quality filter paper gives the crispest results), cut into long rectangular strips
- Rubbing alcohol Supervise young children! Rubbing alcohol is toxic if ingested!
- A pencil
- Tape
- A quarter
- A small, clear glass
- Leaves – choose some you know will turn color in the Fall
Methods:
1. Tape top of
paper strip to a pencil. Balance pencil on top of glass. Trim bottom of strip
so it is close to, but not touching the bottom of the glass.
2. Lay paper
strip on flat surface. Place leaf near bottom of strip and rub a dark line onto
the paper, parallel to the bottom of the strip (see photo). The neater and
darker your line, the better your results will be.
3. Replace pencil
on glass. Make sure the paper is hanging straight down. Carefully pour in the
rubbing alcohol until it just touches the bottom of the paper, but does not
cover your pigment line. Try not to splash the paper.
4. Observe over
the next hour as the alcohol reaches the top of the strip. Remove the strip and
lay flat to dry.
As the rubbing
alcohol moves up the paper (by capillary action), it carries the pigment
molecules. Some of the pigments are larger than others. The smallest ones
travel fastest so are nearer the top. The bigger ones are closer to the bottom.
This
chromatography strip is from a beet leaf. Notice the purple betalain pigment at
the top.
Questions:
1. How many
pigments do you see (there can be different shades of the same type of pigment
– for instance, “chlorophyll a” is almost teal-colored as compared to the
darker “chlorophyll b.”
2. Which of your
pigments are the largest?
3. Did you get
different results from different types of leaves?
4. If you
repeated this experiment every couple of weeks until the leaves drop, what
would you expect to see?
5. If plants did
not have pigments, and so could not make sugar, what would you eat? Think this
one through.
*Fun tip. Plant
pigments are important for us humans! For instance, the carotinoids are
powerful antioxidants and help your eyes stay healthy (eat your carrots and
tomatoes!).
Thank you so much, Heather! I can't wait to try this with my kids. And if you're looking for more fun science from Heather, check out this awesome Rainbow of Ants activity!
