Why Do Leaves Change Color in the Fall?

An essential question in the study of leaves is: Why do they change color in the Fall? This is a great way to start a lesson on photosynthesis and the role that chlorophyll plays in the process. Chlorophyll is the green pigment which captures the sun's energy through a complex series of electron transfers in proteins embedded in the thylakoids of the plant cells' chloroplasts. Chlorophyll absorbs the red/yellow and blue/violet wavelengths of visible light and reflects the green wavelengths, hence its green color to the human eye.

In the fall, the cooler temperatures and shorter amounts of sunlight signal the trees that winter's harsh conditions are approaching. The cells at the base of the leaves' stems begin to die. This prevents water which is necessary for photosynthesis from reaching the leaves. As a result, the chlorophyll in the leaves begins to break down and the yellow and orange carotenoid pigments which were present during the entire summer growing season now appear. The deep red color in leaves comes from anthocyanin pigments which are formed when there are cooler temperatures and photosynthesis slows.

You can see these pigments for yourself by using paper chromatography. Simply gather some fallen leaves, crush them in a glass and mix with either ethyl alcohol or acetone. Attach a strip of coffee filter, paper towel, or chromatography paper to a pencil so that the end of the paper touches the surface of the liquid. The paper/filter will absorb the liquid, and the pigments will travel up with smaller sized molecules traveling the farthest. This process may take up to an hour. Take the strips out and let them dry on clean paper towels. You can see here in this photo the pigments found in two Red Oak tree leaves. Note the green band of chlorophyll still present in the green leaf and none in the red leaf.