Why Is Milk White? (4 page)

But sunlight is needed to make vitamin D in your skin. So when people started living in northern countries that don't get as much sun as tropical countries do, they evolved to produce less melanin, so what little sunlight there was could get into the skin and produce enough vitamin D.

When ultraviolet light from the sun damages the DNA in your skin, the damage is detected and the skin produces more melanin. The melanin produced absorbs ultraviolet light very well and protects the DNA in the skin from further damage. If there is too much damage to the DNA, skin cancer can result.

Melanin is found in other places in the body besides the skin. It is the pigment that colors hair, and it is present in the iris of the eye. Even blue-eyed people usually have small areas of the iris that contain melanin.

There are many types of melanin, and they have different colors. Brown hair, black hair, blond hair, and red hair are colored by different amounts of the different types of melanin.

One type of melanin,
pheomelanin
, is pinkish or reddish and is what colors red hair. Another form,
eumelanin
, can be black or brown. If most of the melanin in hair is missing, except for a little bit of black eumelanin, you get gray hair. If most melanin is missing except for a little bit of brown eumelanin, you get blond hair.

Other animals also use melanin for coloration and protection from ultraviolet light. Octopi and squid use it to make the black ink they produce when attacked. Some bacteria and fungi use it as a sunscreen and as an antioxidant, and it can protect some microorganisms from attack by a host animal's immune system.

All living things use chemistry, not just people and animals. A large part of the tree of life is the plant kingdom. Since plants can't run away or fight with tooth and claw, they have developed quite sophisticated chemical methods to get what they need or defend what they have.

Many of the chemical changes you see in plants have to do with their color. The leaves change color with the seasons, flowers come in bright colors to attract pollinators, and even the green of the leaves is due to the important chemistry involved with making sugar from water, air, and sunlight.

We use plants mostly for food. Any industrial uses for plants would compete for land that could be growing food, or for food itself. Despite that, there are many industrial chemicals that are made from plants.

An example is the alcohol that is added to gasoline. Normal economics would prevent corn from being used to power cars, since it is more valuable as food. But governments pay distillers to make alcohol from corn, so that corn prices will be higher and benefit the corporations that grow the corn.

But there are many non-food products that can be more cheaply grown than manufactured. Carnauba wax, candelilla wax, jojoba oil, gum arabic, gum tragacanth, and natural rubber are just a few.

In some cases, plants are most useful as sources of chemicals, such as in medicine and pest control. Because plants make many very complex molecules that are very hard to produce synthetically, many medicinal proteins and drugs come from plants. Roughly one-quarter of all prescription drugs are derived from plants.

Plant-derived insecticides and insect repellants are another class of molecule that is cheaper to get from plants than to try to make in a lab. These molecules also have the benefit of being easily biodegradable, so they don't linger in the environment.

Plant-derived dyes are another class of chemicals that are cheaper to grow than to make.
Carotenoids
—the red, yellow, and orange molecules in autumn leaves—are widely used in industry. Indigo blue, the browns of henna, and the yellows of saffron and turmeric are other examples.

In addition to food, medicine, and industrial chemicals, we mostly use plant materials for their structural qualities.

Plants use four types of molecules to keep their shape and give strength to their cell walls. This strength is what holds up trees such as giant redwoods against gravity. The four molecules are lignin, cellulose, hemicellulose, and pectin, in order of how strong the molecules make the cell walls.

Lignin is a huge molecule, and it links together many other molecules. It is what makes wood a good structural material.

Cellulose is also a huge molecule, a
polymer,
made up of long chains of thousands of molecules of the simple sugar glucose.

Hemicellulose is made up of a number of different simple sugars all chained together. It is a more random molecule than cellulose—it doesn't crystallize, and has less strength.

Pectin is another large molecule made up of many different simple sugars all linked together. Pectins are why unripe fruits are hard. As the fruit ripens, the pectins break down, and the soft fruit can be eaten by animals that distribute the seeds for the plant.

Cellulose and lignin are the two most abundant organic polymers on earth. We use them together in wood products to build our houses and furniture and to make paper.

The lignin in newsprint paper is what makes the paper yellow with age. More expensive papers are made by removing almost all of the lignin, leaving mostly cellulose, which makes a nice white paper.

Cotton and flax are plant fibers used for making cloth and paper. Linen (made from flax fibers) is made of lignin and cellulose. Cotton is 95 percent cellulose.

Plants have three main pigment molecules in their leaves. These are chlorophylls, carotenoids, and anthocyanins. Besides the leaves, plants may also have colorful bark.

Most leaves are various shades of green. This is due to the chlorophylls. The name chlorophyll comes from the Greek words
chloros
(green) and
phyllon
(leaf). There are six types of chlorophylls in plants. The two main chlorophylls are chlorophyll a and chlorophyll b.

Chlorophyll a absorbs purple and orange light the most. Chlorophyll babsorbs mostly blue and yellow. Neither one absorbs green, so the leaf looks green because that light is reflected to our eyes instead of being absorbed by the leaf.

Chlorophyll molecules have a ring shape at one end, with a magnesium atom in the center. If you boil a leaf in water, this magnesium atom gets replaced by a hydrogen atom, and the color changes from bright green to the dull color of overcooked broccoli.

Carotenes are the pigments that make the yellows and oranges of corn, squash, and carrots.

Look how long the beta carotene molecule is. It has lots of double bonds (where you see two lines close together) alternating with single bonds (where there is only one line). These bonds between the carbons actually smear together, so that the electrons slosh from one end of the molecule to the other, like water in a bathtub. The longer the molecule, the longer it takes to slosh to the other side. The sloshing electrons reflect light whose wavelength matches the sloshing—a long molecule reflects reds. Shorter groups of alternating double and single bonds, like the ring in chlorophyll, reflect shorter wavelengths of light, in this case green.

Anthocyanins are the third pigment plants use. They also have rings with alternating double and single bonds. They tend to be smaller, so they reflect blue and violet colors. Grape juice is purple because of anthocyanins.

A fun thing about anthocyanins is that they change their color if you change their acidity. If you add vinegar to grape juice, it turns red.

Sunlight makes plants produce chlorophyll. There is a lot more chlorophyll in plants than carotenoids and anthocyanins, so leaves look green. The chlorophyll is what absorbs sunlight to give the plant the energy it needs to make food out of water and the carbon dioxide in the air.

Chlorophyll is hard for a plant to make. A plant only makes it in places where it will do the most good (that is, collect the most sunlight). Most of the leaves are at the ends of branches and twigs and cast shadows on the rest of the plant, so the trunks of trees and the branches have little or no chlorophyll. The center leaves in cabbage, lettuce, and celery are also lighter in color because the plant does not waste precious chlorophyll on parts that get no sunlight.

If you look at a bean sprout, you will see that it is mostly white. That is partly because they are grown in the dark and don't get enough sunlight. The plant only produces chlorophyll when enough light hits it to make chlorophyll production worthwhile.

Two things. One is the loss of chlorophyll, which removes the green pigment and allows the yellow carotenoid pigments to show. The other is the production of anthocyanins, which can be the blues and purples of flowers and fruits or the dark reds of autumn leaves.

Plants lose their leaves on purpose. When a leaf is damaged by wind or too much sun or when water and light are harder to come by, such as in a cold autumn, the plant will drop the leaf and either produce a new leaf or go dormant for the winter.

When a tree is about to lose a leaf, it stops sending nutrients to it and starts reclaiming some of the useful molecules in the leaf, to be stored or used elsewhere in the plant. Chlorophyll fades away and is not replaced. You can see the effects of this as a banana ripens. The green banana becomes yellow as the chlorophyll is lost and the yellow carotenoid pigments show through.

Anthocyanins are produced in some leaves as they prepare to fall. These pigments prevent damage from oxygen as the leaf is starved of nutrients, allowing time for the plant to absorb more useful molecules from the dying leaf. They also may be useful when they fall, since anthocyanins can prevent other plants from
growing in the soil under the tree, leaving more resources for the tree in the spring.

Plants that grow in dry areas and plants that don't lose their leaves in the fall have to protect themselves from drying out in hot summers and in winters when the water is frozen.

There are several tricks the plants can use to protect their leaves from drying out. One trick is to make the leaves into long, thin needles, so there is less surface to dry out. Pine trees use this trick.