Alaska's Colors
Alaska's summers abound with colors: the trees and grasses turn green, the flowers display the reds, yellows and other colors of the rainbow, and the vegetables of various hues dominate our gardens. Additional colors are apparent in the birds, fish, and insects which have been absent during the winter. What is the source of all these colors?
The profusion of green is caused by the familiar pigment chlorophyll, which always displays a green color. However, associated with chlorophyll are pigments called carotenoids which normally have red, yellow or orange colors. These pigments are always present in the green leaves of plants, but their presence is hidden during the summer months. Their existence may be noticed in the spring as the new leaves have a yellow-green color. As fall approaches the chlorophylls fade rapidly, leaving the bright yellows and reds which persist because of their more stable molecular structure.
The carotenoids are widely distributed in nature and are responsible for most of the colors we observe in plants. It is estimated that the world contains about 100 million tons of these pigments, of which some 500 different kinds have been identified and characterized. The colors of flowers and fruits are mostly due to a wide range of carotenoid compounds that dominate the plant kingdom, but most of the colors in Alaska result from only about a dozen of these compounds.
The basic chemical structure of the carotenoids is based on about 40 carbon atoms which are bonded to an appropriate number of hydrogen atoms. The carbon atoms are arranged in a chain with two 6-carbon rings at the chain ends. The carbon-atom chain is also associated with alternating double bonds, or polyunsaturation. The variation in carotenoids results from adding differing combinations of hydrogen, carbon, and oxygen to this basic structure. The addition of alcohol groups and the positions of the carbon double bonds account for a great number of the different carotenoids.
Probably the most familiar carotenoid is beta-carotene, which causes the orange color in carrots. It is one of the most widely distributed compounds in nature, and occurs, together with other carotenoids, in green leaves, flowers, and fruits of almost all plants. It is also a food coloring and is used to color margarine yellow.
Beta-carotene is considered a drug or dietary supplement and can be purchased in local stores. The human body is able to store high concentrations of carotenoids without apparent harmful effects, and when needed, a carotenoid molecule is modified to form vitamin A. Since vitamin A itself is toxic in large amounts, beta-carotene is the choice of many people as a source of vitamin A. Beta-carotene is also being studied as an anti-tumor agent, especially in relation to skin diseases caused by sunlight.
Many common plants contain other carotenoids. The red pigment in tomatoes is lycopene, while Zeaxanthin produces the yellow in corn. The dominant carotenoid in leaves is lutein and violaxanthin is found in yellow flowers. These pigments differ only slightly in chemical structure from beta-carotene.
Just as the artist uses a brush and paints to produce a work of art, nature is skilled at synthesizing the appropriate amounts and types of carotenoids to create spectacular arrangements of colors. Virtually all of the colored leaves, flowers, and berries in Alaska contain carotenoids, even the blue and purple ones. Plants are able to mix carotenoids in just the right proportions to produce the desired colors. They will chemically modify or complex them with other molecules to create additional colors.
These ubiquitous pigments are also responsible for many of the colors in animals, fishes, birds, and insects. Warblers, robins, and grosbeaks are colored with yellow, orange, and red carotenoids. They produce the red flesh and skin colors of salmon, the orange color of crabs, and the varied pigments in butterflies and other insects. They even occur in humans, especially in the skin.
Not only do these amazing pigments brighten our lives with colors, but they have many other vital functions. Plants use them as light collectors in photosynthesis (converting carbon dioxide into plant tissue and oxygen gas) and to protect the plant from intense sunlight. Without these pigments, the sun would rapidly destroy the plant.
The eye is able to produce visual images because molecules of a substance called retinaldehyde, which are located on the retina of the eye, absorb light and produce electrical signals. Carotenoids can be split in half to form retinaldehyde. There is some truth to the old wives' tale that eating carrots (also cantaloupes, mangoes, and almost any colored food) is good for your eyesight.
