Biological pigment


A biological pigment, also known simply as a pigment or biochrome, is a substance produced by living organisms that have a color resulting from selective color absorption. Biological pigments include plant pigments and flower pigments. Many biological structures, such as skin, eyes, feathers, fur and hair contain pigments such as melanin in specialized cells called chromatophores. In some species, pigments accrue over very long periods during an individual's lifespan.
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

See conjugated systems for electron bond chemistry that causes these molecules to have pigment.
The primary function of pigments in plants is photosynthesis, which uses the green pigment chlorophyll and several colorful pigments that absorb as much light energy as possible. Pigments are also known to play a role in pollination where pigment accumulation or loss can lead to floral color change, signaling to pollinators which flowers are rewarding and contain more pollen and nectar.
Plant pigments include many molecules, such as porphyrins, carotenoids, anthocyanins and betalains. All biological pigments selectively absorb certain wavelengths of light while reflecting others.
The principal pigments responsible are:
  • Chlorophyll is the primary pigment in plants; it is a chlorin that absorbs blue and red wavelengths of light while reflecting a majority of green. It is the presence and relative abundance of chlorophyll that gives plants their green color. All land plants and green algae possess two forms of this pigment: chlorophyll a and chlorophyll 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. During the process of photosynthesis, they have functions in light-harvesting, in photoprotection, and also serve as protein structural elements. In higher plants, they also serve as precursors to the plant hormone abscisic acid.
  • Betalains are red or yellow pigments. Like anthocyanins they are water-soluble, but unlike anthocyanins they are synthesized from tyrosine. This class of pigments is found only in the Caryophyllales, and never co-occur in plants with anthocyanins. Betalains are responsible for the deep red color of beets.
  • Anthocyanins 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, plant stem, roots, flowers, and fruits, though not always in sufficient quantities to be noticeable. Anthocyanins are most visible in the petals of flowers of many species.
Plants, in general, contain six ubiquitous carotenoids: neoxanthin, violaxanthin, antheraxanthin, zeaxanthin, lutein and β-carotene. Lutein is a yellow pigment found in fruits and vegetables and is the most abundant carotenoid in plants. Lycopene is the red pigment responsible for the color of tomatoes. Other less common carotenoids in plants include lutein epoxide, lactucaxanthin, and alpha carotene.
File:Bougainvillea-3colors.jpg|thumb|Bougainvillea bracts get their color from betalains.
A particularly noticeable manifestation of pigmentation in plants is seen with autumn leaf color, a phenomenon that affects the normally green leaves of many deciduous trees and shrubs whereby they take on, during a few weeks in the autumn season, various shades of red, yellow, purple, and brown.
Chlorophylls degrade into colorless tetrapyrroles known as nonfluorescent chlorophyll catabolites.
As the predominant chlorophylls degrade, the hidden pigments of yellow xanthophylls and orange beta-carotene are revealed. These pigments are present throughout the year, but the red pigments, the anthocyanins, are synthesized de novo once roughly half of chlorophyll has been degraded. The amino acids released from degradation of light harvesting complexes are stored all winter in the tree's roots, branches, stems, and trunk until next spring when they are recycled to re‑leaf the tree.

Pigments in algae

Algae are very diverse photosynthetic organisms, which differ from plants in that they are aquatic organisms, they do not present vascular tissue and do not generate an embryo. However, both types of organisms share the possession of photosynthetic pigments, which absorb and release energy that is later used by the cell. These pigments in addition to chlorophylls, are phycobiliproteins, fucoxanthins, xanthophylls and carotenes, which serve to trap the energy of light and lead it to the primary pigment, which is responsible for initiating oxygenic photosynthesis reactions.
Algal phototrophs such as dinoflagellates use peridinin as a light harvesting pigment. While carotenoids can be found complexed within chlorophyll-binding proteins such as the photosynthetic reaction centers and light-harvesting complexes, they also are found within dedicated carotenoid proteins such as the orange carotenoid protein of cyanobacteria.
GroupPigment
Green algae
Red algae
  • Chlorophyll a
  • Phycobiliproteins
  • Phycoerythrin red-purple pigment, it is the dominant one in the species that have a red-purple color.
    • Golden and Brown algae
  • Chlorophyll a and c
  • Xanthophyll
  • Fucoxanthin

    • Pigments in bacteria

    Bacteria produce pigments such as carotenoids, melanin, violacein, prodigiosin, pyocyanin, actinorhodin, and zeaxanthin. Cyanobacteria produce phycocyanin, phycoerythrin, scytonemin, chlorophyll a, chlorophyll d, and chlorophyll f. Purple sulfur bacteria produce bacteriochlorophyll a and bacteriochlorophyll b. In cyanobacteria, many other carotenoids exist such as canthaxanthin, myxoxanthophyll, synechoxanthin, and echinenone.
    GroupPigment
    Cyanobacteria
    Purple bacteria
  • Bacteriochlorophyll a and b
    • Green bacteria
  • Bacteriochlorophyll c and e
    • Chromobacterium
  • Violacein
    • Streptomyces
  • Melanin
    • Micromonospora
  • Anthraquinone

    • Pigments in animals

    Pigmentation is used by many animals for protection, by means of camouflage, mimicry, or warning coloration. Some animals including fish, amphibians and cephalopods use pigmented chromatophores to provide camouflage that varies to match the background.
    Pigmentation is used in signalling between animals, such as in courtship and reproductive behavior. For example, some cephalopods use their chromatophores to communicate.
    The photopigment rhodopsin intercepts light as the first step in the perception of light.
    Skin pigments such as melanin may protect tissues from sunburn by ultraviolet radiation.
    However, some biological pigments in animals, such as heme groups that help to carry oxygen in the blood, are colored as a result of happenstance. Their color does not have a protective or signalling function.
    Pea aphids, two-spotted spider mites, and gall midges are the only known animals capable of synthesizing carotenoids. The presence of genes for synthesizing carotenoids in these arthropods has been attributed to independent horizontal gene transfer events from fungi.

    Diseases and conditions

    A variety of diseases and abnormal conditions that involve pigmentation are 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".
    • 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.
    • ocular pigmentation is an accumulation of pigment in the eye, and may be caused by latanoprost medication.
    • Vitiligo is a condition in which there is a loss of pigment-producing cells called melanocytes in patches of skin.

      Pigments in marine animals

    Carotenoids and carotenoproteins

    s are the most common group of pigments found in nature. Over 600 different kinds of carotenoids are found in animals, plants, and microorganisms.
    Marine animals are incapable of making their own carotenoids and thus rely on plants for these pigments. Carotenoproteins are especially common among marine animals. These complexes are responsible for the various colors to these marine invertebrates for mating rituals and camouflage. There are two main types of carotenoproteins: Type A and Type B. Type A has carotenoids which are stoichiometrically associated with a simple protein. The second type, Type B, has carotenoids which are associated with a lipo protein and is usually less stable. While Type A is commonly found in the surface of marine invertebrates, Type B is usually in eggs, ovaries, and blood. The colors and characteristic absorption of these carotenoprotein complexes are based upon the chemical binding of the chromogen and the protein subunits.
    For example, the blue carotenoprotein, linckiacyanin has about 100–200 carotenoid molecules per every complex. In addition, the functions of these pigment-protein complexes also change their chemical structure as well. Carotenoproteins that are within the photosynthetic structure are more common, but complicated. Pigment-protein complexes that are outside of the photosynthetic system are less common, but have a simpler structure. For example, there are only two of these blue astaxanthin-proteins in the jellyfish, Velella velella, contains only about 100 carotenoids per complex.
    A common carotenoid in animals is astaxanthin, which gives off a purple-blue and green pigment. Astaxanthin's color is formed by creating complexes with proteins in a certain order. For example, the crustochrin has approximately 20 astaxanthin molecules bonded with protein. When the complexes interact by exciton-exciton interaction, it lowers the absorbance maximum, changing the different color pigments.
    In lobsters, there are various types of astaxanthin-protein complexes present. The first one is crustacyanin, a slate-blue pigment found in the lobster's carapace. The second one is crustochrin, a yellow pigment which is found on the outer layer of the carapace. Lastly, the lipoglycoprotein and ovoverdin forms a bright green pigment that is usually present in the outer layers of the carapace and the lobster eggs.