Originally posted September, 13 2010

Where do leaves get their colors?

September 13, 2010

After five years in southern California, I finally get to have a “real” autumn again. (I know, I’ve lived a very rough life. Pity me!) But, fall is most definitely on its way. Here in DC, the nights have been noticeably cooler, the days have been shorter, and some leaves have started to fall off of the trees. It is only a matter of time before those green leaves start turning red, orange or yellow.

Some lucky unknown person took this picture at a beautiful unknown location. Credit: ?

That nice green “leafy” color comes from the molecule chlorophyll. And, as molecules go, chlorophyll is pretty rad! It’s so rad that I’ve decided to not only post a picture of the molecule, but I’ve also posted a picture of its absorbance spectrum. (An absorby-whaty? Please don’t leave the site … it’s really not all that bad! It’s actually pretty simple.) Leaves are green because chlorophyll is green. Chlorophyll is green because it doesn’t absorb any green light. When sunlight passes through a leaf, the leaf “sucks up” all of the other colors. It lets the green light bounce off of it or pass through it. (It allows us to see green light and keeps us from seeing the other colors of light.) The absorbance spectrum just shows which colors (and how much of each color) chlorophyll does absorb.

Leaves are green because chlorophyll lets green light bounce off of them or pass through them. In the top left is the chemical structure of chlorophyll (white=hydrogens, black=carbons, red=oxygens, blue=nitrogens, teal=magnesium). The bottom left shows which colors of light chlorophyll absorbs. Chlorophyll is green because it does not absorb any green light.

The energy that plants use to turn carbon dioxide and water into sugars and oxygen all comes from the light that chlorophyll absorbs. As you can imagine, being the energy conduit for a plant can be very draining. We damage our skin by being in the sun too long. Chlorophyll gets damaged in the same way. After absorbing too much sunlight, it becomes useless. The plant cells have to get rid of it and make new molecules. So, in the bright summer sun, plants are continuously producing chlorophyll. It is in their best interest to do so. But, as temperatures cool and the days become shorter plants stop making chlorophyll, and the green color goes away. This means that other colors (coming from other molecules, which absorb light differently) start to dominate once all of the chlorophyll has been degraded. Luteinand xanthophyll are yellowish/reddish (yes, those are the exact scientific definitions) pigments that are always present in leaves, but we don’t notice them until the chlorophyll is gone. Anthocyanins are reddish-purple pigments that aren’t synthesized by leaves until they start to have reduced amounts of chlorophyll.

Lutein (top – yellowish red), xanthophyll (bottom – yellowish red) and anthocyanin (right – reddish purple) give fall leaves their color.

So, go away chlorophyll! Bring on the Lutein, xanthophyll and anthocyanin! It’s been five years, and I am ready!

-mrh