Biological pigment

Biological pigments, also known as biochromes, are substances produced by living organisms. These molecules create color through a process called selective color absorption. When light hits a pigment, the substance absorbs specific wavelengths of light. The wavelengths that are not absorbed are reflected back to our eyes. This allows us to see different colors in the natural world.

It is important to distinguish pigments from structural color. Pigments produce the same color regardless of the viewing angle. In contrast, structural color results from selective reflection or iridescence. This often happens because of complex multilayer structures, such as those found in butterfly wings.

In the plant kingdom, pigments serve vital biological functions. The primary role of most pigments is to assist in photosynthesis. This is the process where organisms convert light energy into chemical energy. Chlorophyll is the most important pigment for this task. It is a chlorin that absorbs blue and red wavelengths of light. Because it reflects most green light, plants appear green to us.

Different organisms use different types of chlorophyll. For example, all land plants and green algae use chlorophyll a and b. However, red algae possess only chlorophyll a. Kelps and diatoms use chlorophyll c instead of chlorophyll b.

Plants also utilize accessory pigments to maximize their energy intake. Carotenoids are a group of red, orange, or yellow tetraterpenoids. These molecules help with light-harvesting and provide photoprotection. Photoprotection allows the plant to dissipate energy and prevent damage from light. Carotenoids also serve as structural elements for proteins. Within this group, there are several specific types. Lutein is the most abundant carotenoid in plants. Lycopene is the red pigment found in tomatoes. Other examples include alpha carotene in carrots and lactucaxanthin in lettuce.

Other plant pigments include anthocyanins and betalains. Anthocyanins are water-soluble flavonoid pigments. They can appear red or blue depending on the pH level of the plant tissue. These are often highly visible in flower petals. Betalains are also water-soluble but are synthesized from tyrosine. These pigments are found only in the order Caryophyllales, which includes cacti. They are responsible for the deep red color in beets.

Interestingly, betalains and anthocyanins never co-occur in the same plant.

Seasonal changes in deciduous trees provide a clear look at pigment shifts. During autumn, the green chlorophyll begins to degrade. As chlorophyll breaks down, it turns into colorless substances called nonfluorescent chlorophyll catabolites. This degradation reveals pigments that were already present, such as yellow xanthophylls and orange beta-carotene. Additionally, some plants synthesize new red anthocyanins once half of the chlorophyll has degraded. The amino acids released during this process are stored in the tree's roots and branches. They are recycled the following spring to grow new leaves.

Animals use pigmentation for survival and communication. Many species use pigments for camouflage to hide from predators. Others use mimicry or warning coloration to signal to other animals. Some animals, such as cephalopods, use specialized cells called chromatophores to communicate.

In humans and other animals, melanin is a common pigment in skin and hair. Melanin helps protect tissues from damage caused by ultraviolet radiation from the sun. Some pigments, like the heme groups that carry oxygen in blood, are colored by happenstance. They do not serve a specific signaling or protective function.

Marine life also relies heavily on pigment chemistry. Most marine animals cannot synthesize their own carotenoids. Instead, they must consume plants to obtain these pigments. Many marine invertebrates use carotenoproteins for mating rituals and camouflage. There are two main types: Type A and Type B. Type A involves carotenoids associated with a simple glycoprotein. Type B involves carotenoids associated with a lipoprotein and is usually less stable. These complexes can create a wide range of colors, including blue, red, and green.

Sometimes, biological systems experience issues with pigment production. Albinism is an inherited disorder involving the total or partial loss of melanin. In humans and animals, this results in a lack of pigment. Other conditions involve the excess production of pigment. Lamellar ichthyosis, or "fish scale disease," causes the skin to be darker than normal with scaly patches. Melasma causes dark brown patches to appear on the face, often influenced by hormones. Vitiligo occurs when pigment-producing cells, called melanocytes, are lost in patches of skin.