Science of Light

Lesson Eight

Biological Coloration

    “You couldn’t live on this earth unless you had some of every one of its elements in you.

    “Matter is always becoming.  Matter never is anything, but it is always becoming something else.”

Father Paul

    The vast diversity of color in nature, and in living forms, is a subject of universal wonderment, a delight to the eye.  But aside from the aesthetic appeal of color, pigmentation also holds a special interest for the professional in many fields of activity.  Chromatology, or the study of colors, finds useful application in medicine, agriculture, and many other arts and industries.

    In the field of biochemistry the comparative metabolism of colored molecules is one of the important areas of investigation.  In the diagnosis and treatment of disease, the physician is often aided by visible signs and symptoms concerning the state of health as reflected by the colors of tissues and body fluids; and in agriculture, the farmer quickly recognizes the stages of growth and ripening of his crops by observing the changing color pigmentation as they mature.

    The reason why snow appears white is the same as that which imparts whiteness to animal structures.  It is the total reflection of light which often results from the separation of finely-divided structural materials by air spaces.  The appearance of whiteness may also come from secretions or deposits in animal tissues which contribute to the totally scattered reflection.

    The color of a chemical compound depends upon the selective absorption of light within definite wavelengths, the unabsorbed rays being reflected or transmitted to the eye.  This capacity to absorb visible light is due to varying kinds and degrees of chemical unsaturation in chromospheres (color-carrying groups) within the molecule.  In fact, the basis of color-manifestation in a compound is related to modification in the speed or frequency of motion of one or more pairs of the compound’s many rapidly-vibrating electrons.

Animal pigments:

    Carotenoids are a group of red, orange or yellow pigments.  These are present in many plants and creatures, concentrated particularly in the yolks of eggs, sexual organs, hair, skin, eyes and milk.  Marine animals derive carotenoids from rich supplies of seaweed or microscopic underwater plants.  In man, the skin may turn slightly yellow from an excessive intake of such carotene-rich foods as carrots or oranges.  This otherwise harmless condition is call artificial jaundice, and clears up when intake is reduced.

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    The bright color of the flamingo, as well as many fishes and other creatures, is due to some derivative of carotenoid present in their systems.

    Naphthoquinones and Anthraquinones are pigments less well known, used particularly for dyes.  The former produces yellow, orange, red and purple pigments, especially from sea animals.  The latter produces various red pigments obtained from certain insects.

    Flavones and Tetrapyrroles:  The flavones impart yellow color to certain flowers and are found in some insects.  The tetrapyroles are nitrogenous, water-soluble pigments called porphyrins.  They are found in plant chlorophyll, and in animal hemoglobins, present in the red blood cells of most creatures.  Hemoglobins are responsible for the pink to red color of the combs and wattles of birds, and the skin of man.  Certain underwater creatures fade in aerated water, but increase in redness when placed in water with a poor supply of oxygen – apparently a physiological adaptation toward survival. 

     Hemoglobin is also present in the bacteria-harboring root of peas, beans and other leguminous plants.  It is believed to serve as a catalyst for the chemical fixation of atmospheric nitrogen in the soil, a well-known property of the root nodules of legumes.  There are many related pigments, some in the blue and green range.

Melanins:

    If an apple is cut so that its flesh is exposed to the air, the surface of the cut begins to turn brown.  The brown color is caused by a pigment called melanin that is formed by the action of the air on one of the chemicals that is in the apple.  Melanin is also found in the skin and hair of human beings.

     Because of the high frequency, each photon of ultra-violet light has a high amount of energy.  It can damage the cells in a living body.  So the body needs protection against ultraviolet rays.  The melanin in our skins gives us this protection by absorbing the ultra-violet rays before they can do any harm.

    Dark colors evidence the presence of the melanin pigment, dark feathers, hair or eyes.  Melanin is an end-product of metabolism, formed as a result of oxidation and polymerization of phenolic compounds.  Certain albino animals fail to develop melanin in their tissues.

    Urochrome, the principle yellow pigment of urine, is considered to be a modified melanin.  In certain diseases melanin precursors cause urine to darken as oxidation occurs on standing.

    Melanin can be bleached by such oxidants as hydrogen peroxide, chlorine, chromate or permanganate.  Peroxide is, of course, sometimes used to bleach hair to create a blond effect.

    The dark hairs of mammals contain a higher trace of copper than do pale hairs.  If the intake of copper falls well below the minimal requirement of a fraction of a milligram per day, the new hairs which emerge are less dark.  Ellipsoidal or spherical microgranules of

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melanin are randomly distributed with the dried cortical cells of all colors of hair, imparting varying degrees of hue from light to dark, depending on the microgranules of melanin which are present.  Human red hair, unlike any other hair from humans or animal, is unique in its iron-rich pigment.  Red poultry feathers yield a similar substance.

    All human skin, except in albinos, contains greater or lesser amounts of melanin.  In fair-skinned races the corium, or deeply-lying skin layer, contains but little of the pigment.  But darker races carry heavier dermal deposits, fortified by numbers of smaller melanocytes in the upper skin layer, or epidermis.  Exposure to sunlight causes tanning of man’s skin, with a gradual increase of melanin pigment, which in turn helps protect underlying tissues from injurious sunrays.

   Certain fishes placed in black-lined containers have been found to increase the melanophores of the skin, while after transfer to pale containers, they gradually lose it again.  Another interesting phenomena has been observed among the fishes, of rapid darkening of the skin through melanization.

The Tasmanian whitebait as it approaches sexual ripeness develops an increasing number of melanosphores, then after spawning shows extensive darkened areas of skin.

Indigoid derivatives:

    Like melanins, the indigo compounds are excretory products of certain animals, but their distribution as pigmentary compounds is limited.  Unlike the more somber melanins, many indigoids are red, green, blue or purple.     

    Indigo occurs in many plants, and has long been useful as a blue dye.  It does not occur in the tissue of healthy animals but certain chemical derivatives of it are found in secretory and excretory products.

    Tyrian Purple is called a “dibromindigo”, and is a purple known to the ancients, the red-violet dye employed commercially in some countries.  It is a product secreted by several species of snail, of the genera murex and purpura.

Purines and Pterins:

    The purine compounds are hardly true pigments, since they are usually white crystals, but they often contribute to the color scheme of some lower animals.  Solid white uric acid is found in the excrements of birds and other creatures.  Small amounts of uric acid are found in man and apes.  (Gout is partially caused by the deposition of sodium urate in the joints).

    The brilliant whiteness of some anemones results partly from microcrystalline deposits of uric acid in the tissues.  Other purines occur in the wings of butterflies.

    Purine compounds constitute part of the complex nucleic acids which abound in the nuclear material of all cells, and therefore play an important part in cell metabolism.

    There are several other related compounds in the white and yellow range.

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Lyochromes (Flavins):   

    This is a class of yellow, greenly-fluorescent water-soluble pigments widely distributed in plant and animal tissues, but in such small amounts as to make no appreciable color change.  Lyochromes are synthesized by bacteria, yeasts and green plants.  A very important one of these, called riboflavin, is identical with vitamin B₂.   This is not manufactured by animals, but must be derived from plant sources.  It is part of an enzyme capable of combining with molecular oxygen, thus developing a yellow color.  The release of oxygen in intracellular oxidation processes brings about simultaneous loss of color, which is restored by fresh supplies of oxygen.

    A nutritional lack of riboflavin in the diets of test animals retarded growth, caused development of cataracts, and impairment of cellular respiration.  The compound is not stored in quantity.  Milk, eggs, liver, kidney, blood and muscles contain riboflavin.

    There are many other animal coloring matters, which are of lesser importance, or not as yet under-stood, which we may by-pass at this stage. 

    Most pigments have roles related to their light-absorbing or light-reflecting qualities.  In the eyes of some creatures, certain pigments in the violet range regular the admission of light.  Reflecting pigments cause the night eye-shine of others.

    It is possible that light-absorbing and –reflecting pigments in the skin may be involved in a primitive mechanism for temperature regulation in certain cold-blooded species, for example the desert horned toad.  In the cool of morning its skin is dark, and absorbs heat rays; as the temperature rises during the day its skin blanches, thereby reflecting heat rays away from the body.

    White in the animal kingdom is sometimes due to special white substances deposited in the tissues; in other cases it is due to the lack of colored substances – their place being taken by air – in the hair of white mammals and the plumage of white birds, this may be of value in retarding heat radiation.  It appears that these are given them for protective coloration, as certain smaller arctic animals who change their white winter coat for darker fur during the summer.

    There are many animals who can change their shade, or even their actual color, slowly or almost instantaneously, to conform to their background, and camouflage their whereabouts.

    Male birds are usually more brilliant of plumage than the female.  This again one can suppose is for the protection of the female during the nesting period when she must melt into the background, unobserved, sitting on the nest until the babies have hatched and are safely launched.

    Plant colors are predominantly green due to the prevalence of green chlorophyll in the leaves and stems of most plants, grasses, and trees.  Chlorophyll, one of the most important pigments in nature, is capable of channeling the radiant energy of sunlight into chemical energy usable in the reactions of the cell through the process called photosynthesis.

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Chemically it is related to hemoglobin, the “heme” in the red blood pigment, as well as to the respiratory enzymes called cytochromes. 

    Chlorophyll absorbs nearly all of the red light that falls on it.  Plants use the energy of the absorbed light to build sugar molecules out of water and carbon dioxide.  The light reflected by the chlorophyll in leaves is that which is left over after the red light has been absorbed.  This kind of light mixture imparts to growing plants their green appearance.

Carotene:

    In the plant world, the carotenoids are almost universally present in the yellow to orange-red colorants of nature, such as in carrots, or marigolds.  Carotene is the raw material from which vitamin A is made.  It is changed into vitamin A by the action of ultra-violet rays.

    When the leaves change color from green to different shades of yellow and red, this is the result of carotene in the leaves.

Flavonoids:

    The autumn coloring of leaves is due to the disappearance of chlorophyll, as it decomposes at the approach of winter, and the formation of anthocyanins.  Anthocyanin gives both purple-red color to autumn leaves, and the red-purple appearance to young new growth.  Certain mineral deficiency of plants can be detected by the formation of red anthocyanin coloration.

    The flavonoids include anthocyanin, responsible for red, blue, mauve, purple and violet colors; and the anthoxanthins, ranging from colorless to yellow.  The latter is responsible for white flowers, cream or ivory.

    This is just the threshold of the subject of pigmentation in nature, but it gives some idea of the vast possibilities of study and research in this field.