In plants the major pigments are the carotenes (reddish orange to yellow), the anthocyanins (red, blue, and violet), and the chlorophylls (green). The red and yellow colors of autumn foliage are due to the exposure of the anthocyanins after the green chlorophyll pigments, which usually mask them, have decomposed and faded. The major animal pigments are the hemes (red) of blood hemoglobin, the carotenes, the melanins (black and brown), and guanine (white and iridescent). The latter three produce the surface coloration of most animals.
Pigments not only provide external coloration but also function in some important physiological processes. In the retina of the eye the pigment cells (rods and cones) adjust or regulate the entering light (see vision). Among its other functions, carotene operates in the synthesis of vitamins and of chlorophyll. Chlorophyll is essential for plant photosynthesis. Hemoglobin in the blood carries oxygen for respiration. Chlorophyll and hemoglobin are structurally quite similar, both belonging to the pyrrole group of pigments.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.