Biological pigment

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Overview

The Blue Morpho butterfly, native to Central America, derives its distinctive blue coloring from iridescence rather than from pigmentation.

In biology, a pigment or biochrome is any material resulting in color of plant or animal cells, which is the result of selective color absorption. Many biological structures, such as skin, eyes, fur and hair contain pigments (such as melanin) in specialized cells called chromatophores.

Pigment color differs from structural color in that it is the same for all viewing angles, whereas structural color is the result of selective reflection or iridescence, usually because of multilayer structures. For example, butterfly wings typically contain structural color, although many butterflies have cells that contain pigment as well.

Biological pigments

Pigments in plants

Space-filling model of the chlorophyll molecule.
Anthocyanin gives these pansies their dark purple pigmentation.

Among the most important molecules for plant function are the pigments. Plant pigments include a variety of different kinds of molecules, including porphyrins, carotenoids, and anthocyanins. All biological pigments selectively absorb certain wavelengths of light while reflecting others. The light that is absorbed may be used by the plant to power chemical reactions, while the reflected wavelengths of light determine the color the pigment will appear to the eye. Pigments also serve to attract pollinators.

Chlorophyll is the primary pigment in plants; it is a porphyrin that absorbs red and blue wavelengths of light while reflecting green. It is the presence and relative abundance of cholophyll that gives plants their green color. All land plants and green algae possess two forms of this pigment: cholorphyll a and cholorphyll b. Kelps, diatoms, and other photosynthetic heterokonts contain chlorophyll c instead of b, while red algae possess only chlorophyll a. All chlorophylls serve as the primary means plants use to intercept light in order to fuel photosynthesis.

Carotenoids are red, orange, or yellow tetraterpenoids. They function as accessory pigments in plants, helping to fuel photosynthesis by gathering wavelengths of light not readily absorbed by chlorophyll. The most familiar carotenoids are carotene (an orange pigment found in carrots), lutein (a yellow pigment found in fruits and vegetables), and lycopene (the red pigment responsible for the color of tomatoes). Carotenoids have been shown to act as antioxidants and to promote healthy eyesight in humans.

Anthocyanins (literally "flower blue") are water-soluble flavonoid pigments that appear red to blue, according to pH. They occur in all tissues of higher plants, providing color in leaves, stems, roots, flowers, and fruits, though not always in sufficient quantities to be noticeable. Anthocyanins are most visible in the petals of flowers, where they may make up as much as 30% of the dry weight of the tissue.[1] They are also responsible for the purple color seen on the underside of tropical shade plants such as Tradescantia zebrina; in these plants, the anthocyanin catches light that has passed through the leaf and reflects it back towards regions bearing chlorophyll, in order to maximize the use of available light.

Betalains are red or yellow pigments. Like anthocyanins they are water-soluble, but unlike anthocyanins they are indole-derived compounds synthesized from tyrosine. This class of pigments is found only in the Caryophyllales (including cactus and amaranth), and never co-occur in plants with anthocyanins. Betalains are responsible for the deep red color of beets, and are used commercially as food-coloring agents.

Pigments in animals

The monarch butterfly's distinctive pigmentation reminds potential predators that it is poisonous.

Pigments in animals may serve to protect tissues from ultraviolet radition, such as melanin in the skin. Pigments may also aid in sexual reproduction, identifying species and gender of animals to potential mates, or signalling readiness to breed.

Some cephalopods use pigmented chromatophores to communicate.

Pigmentation is used by many animals for protection, by means of camoflauge, mimicry, or warning coloration.

Diseases and conditions

A variety of diseases and abnormal conditions that involve pigmentation arise in humans and animals, either from absence of or loss of pigmentation or pigment cells, or from the excess production of pigment.

  • Albinism is an inherited disorder characterized by total or partial loss of melanin. Humans and animals that suffer from albinism are called "albinistic" (the term "albino" is also sometimes used, but may be considered offensive when applied to people).
  • Lamellar ichthyosis, also called "fish scale disease", is an inherited condition in which one symptom is excess production of melanin. The skin is darker than normal, and is characterized by darkened, scaly, dry patches.
  • Melasma is a condition in which dark brown patches of pigment appear on the face, influenced by hormonal changes. When it occurs during a pregnancy, this condition is called the mask of pregnancy.
  • Vitiligo is a condition in whch there is a loss of pigment-producing cells called melanocytes in patches of skin.

Commercial uses

Pigments may be extracted and used as dyes.

References

  1. Robinson, Trevor. 1963. The Organic Constituents of Higher Plants, page 183 (Minneapolis: Burgess Publishing).

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