Chemistry behind leaves changing color

You may have seen change in color of some trees from green to red, yellow or orange. This transition looks beautiful and we enjoy this change. 

The color change is an indication that the trees are preparing for winter and it involves a complex chemistry. 

Color in tree leaves is due to some chemical compounds-the pigments. 

In general, trees are green in color. The green color comes from a compound known as chlorophyll present in the chloroplasts of leaf cells. From our high school science we know that this pigment is involved photosynthesis-plants cooking food. 

Chlorophyll is a dominant pigment during growing season. This green pigment is not very stable. Therefore plants have to produce chlorophyll constantly. However, the production slows down in low sunlight and lower temperature as fall/autumn season progress. 

At this stage, other pigment molecules come to front stage and play their dominant role. Some pigments are already present in the leaves that are just unmasked as chlorophyll gets away from the business. Whereas some pigments are produced in this condition unfavorable for chlorophyll.

In some trees as chlorophyll fades away, a yellow pigment called carotene get unmasked. Carotenoids and another pigment, xanthophylls, are responsible for the yellow and orange color of leaves.

The vivid red and purple colors in some other trees come from anthocyanins which are produced in the fall from sugars in the leaf. 

General structures of pigments found in leaves
As you see in these structures, one thing is common in all of them. The alternating double and single bonds: conjugation system. This conjugation system allows them to absorb light in the visible range. This leads to the appearance of color to us.

Enjoy the season. 

Here is a nice explanation of the chemistry of color change in a video produced by American Chemical Society.

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