Carotenoids are yellow, orange, and red organic pigments that are produced by plants and algae, as well as several bacteria, and fungi.

Carotenoids give the characteristic color to pumpkins, carrots, parsnips, corn, tomatoes, canaries, flamingos, salmon, lobster, shrimp, and daffodils.

Carotenoids can be produced from fats and other basic organic metabolic building blocks by all these entities. 

Carotenoids can be produced by endosymbiotic bacteria in whiteflies.

Carotenoids from the diet are stored in the fatty tissues of animals.

Exclusively carnivorous animals obtain the compounds from animal fat. 

In the human diet, absorption of carotenoids is improved when consumed with fat in a meal.

Cooking carotenoid-containing vegetables in oil and shredding the vegetable both increase carotenoid bioavailability.

There are over 1,100 known carotenoids.

Carotenoids can be categorized into two classes, xanthophylls (which contain oxygen) and carotenes (which are purely hydrocarbons and contain no oxygen).

In general, carotenoids absorb wavelengths ranging from 400 to 550 nanometers (violet to green light), causing  the compounds to be deeply colored yellow, orange, or red. 

Carotenoids are the dominant pigment in autumn leaf coloration of about 15-30% of tree species, but many plant colors, especially reds and purples, are due to polyphenols.

Carotenoids serve two key roles in plants and algae: they absorb light energy for use in photosynthesis, and they provide photoprotection.

Carotenoids that contain unsubstituted beta-ionone rings and have vitamin A activity.

They  can be converted to retinol.

In the eye, lutein, meso-zeaxanthin, and zeaxanthin are present as macular pigments which are important in visual function.

The structure of carotenoids allows for biological abilities, including photosynthesis, photoprotection, plant coloration, and cell signaling.  

The general structure of the carotenoid is a polyene chain consisting of 9-11 double bonds and possibly terminating in rings. 

This structure of conjugated double bonds leads to a high reducing potential, or the ability to transfer electrons throughout the molecule.

They are a type of antioxidant, which means that they protect the body from damage caused by harmful molecules called free radicals. 

Some common carotenoids include beta-carotene, lycopene, and lutein. 

These compounds are found naturally in many different types of foods, including carrots, sweet potatoes, tomatoes, and leafy greens. 

Eating a diet rich in carotenoids has been linked to a lower risk of certain chronic diseases, such as cancer and heart disease.

Carotenoids can transfer excitation energy in one of two ways: 1) singlet-singlet energy transfer from carotenoid to chlorophyll, and 2) triplet-triplet energy transfer from chlorophyll to carotenoid. 

Carotenoids also participate in different types of cell signaling, and are able to regulate plant growth, seed dormancy, embryo maturation and germination, cell division and elongation, floral growth, and stress responses.

Carotenoids with molecules containing oxygen, such as lutein and zeaxanthin, are known as xanthophylls.

The unoxygenated carotenoids such as α-carotene, β-carotene, and lycopene, are known as carotenes. 

Carotenes typically contain only carbon and hydrogen and are in the subclass of unsaturated hydrocarbons.

Their color, ranges from pale yellow through bright orange to deep red, and is directly linked to their structure. 

Xanthophylls are often yellow.

The double carbon-carbon bonds interact with each other in a process called conjugation, which allows electrons in the molecule to move freely across these areas of the molecule. 

This causes the range of energies of light absorbed by the molecule to decrease. 

As more wavelengths of light are absorbed from the longer end of the visible spectrum, the compounds acquire an increasingly red appearance.

Carotenoids are usually lipophilic due to the presence of long unsaturated aliphatic chains.

 The physiological absorption of these fat-soluble vitamins depends directly on the presence of fats and bile salts.

Beta-carotene, found in pumpkins, sweet potato, carrots and winter squash, is responsible for their orange-yellow colors.

Dried carrots have the highest amount of carotene of any food per 100-gram serving.

Green, kale, spinach, collard greens, and turnip greens contain substantial amounts of beta-carotene.

The diet of flamingos is rich in carotenoids, imparting the orange-colored feathers of these birds.

Carotenoids are located primarily outside the cell nucleus in different cytoplasm organelles, lipid droplets, cytosomes and granules. 

Carotenoids play an important role in biological oxygenation. 

In plant cells they are involved in the control of trans-membrane transport of molecular oxygen released in photosynthesis, and in animals carotenoids play an important role to support oxygen in its transport, storage and metabolism.

Carotenoids are hydrophobic and are typically present in plasma lipoproteins and cellular lipid structures.

Lipids provide a more favorable environment for O2 solubility than in aqueous mediums.

By protecting lipids from free-radical damage, carotenoids support crystalline architecture and hydrophobicity of lipoproteins and cellular lipid structures.

Carotenoids can be involved in the intracellular deposition of oxygen.

Carotenoids can also stimulate the formation of intracellular lipid droplets, which can store additional molecular oxygen.

These properties of carotenoids help animals to adapt to environmental stresses, high altitude, intracellular infections and other hypoxic conditions.

Carotenoids increases oxygen diffusion and the oxygen carrying capacity of plasma lipoproteins, and can stimulate oxygen delivery into body tissues. 

This improves tissue and cellular oxygenation and stimulates the growth and respiration of mitochondria.

Oxygen is required in many intracellular reactions including hydroxylation, which is important for metabolic activation of prodrugs and prohormones, such as vitamin D3. 

Carotenoids provide support for intracellular oxygenation and can also improve efficacy of these molecules.

Carotenoids can form physical complexes with different molecules. With hydrophobic molecules this could be self-assembly. 

Foods high in carotenoids appear to be protective against head and neck cancers.

Several studies found correlations between diets rich in carotenoids appear to have a protective effect on prostate cancers.

Carotenoids are important components of the dark brown pigment melanin, which is found in hair, skin, and eyes. 

Several studies have observed positive effects of high-carotenoid diets on the texture, clarity, color, strength, and elasticity of skin.

High carotenoid diets help reduce symptoms of eyestrain and improve night vision.

Humans are mostly incapable of synthesizing carotenoids, and must obtain them through their diet. 

Carotenoids are a common and often ornamental feature in animals:

pink color of salmon, and the red coloring of cooked lobsters and scales of the yellow morph of common wall lizards are due to carotenoids.

Apricots, rich in carotenoids.

The most common carotenoids include lycopene and the vitamin A precursor β-carotene. 

In plants, the xanthophyll lutein is the most abundant carotenoid and it may have a role in preventing age-related eye disease.

Lutein and the other carotenoid pigments found in leaves are often not obvious because of the masking presence of chlorophyll. 

When chlorophyll is not present, as in autumn foliage, the yellows and oranges of the carotenoids are predominant. 

For the same reason, carotenoid colors often predominate in ripe fruit after being unmasked by the disappearance of chlorophyll.

Carotenoids are responsible for the brilliant yellows and oranges that tint deciduous foliage: dying autumn leaves) of certain hardwood species as hickories, ash, maple, yellow poplar, aspen, birch, black cherry, sycamore, cottonwood, sassafras, and alder. 

Carotenoids are the dominant pigment in autumn leaf coloration of about 15-30% of tree species.

The  reds, the purples, and their blended combinations that decorate autumn foliage usually come from pigments in the cells called anthocyanins. 

Unlike the carotenoids, these pigments are not present in the leaf throughout the growing season, but are actively produced towards the end of summer.

Dietary carotenoids and their metabolic derivatives are responsible for bright yellow to red coloration in birds.

Carotenoid coloration exhibits high levels of sexual dimorphism: adult male birds generally displaying more vibrant color  than females of the same species.

The  sweet floral smells present in black tea, aged tobacco, grape, and many fruits are due to the aromatic compounds resulting from carotenoid breakdown.

Carotenoids may be produced by bacteria to protect themselves from oxidative immune attack. 

This carotenoid antioxidant action that helps the microbe evade death by reactive oxygen species used by the host immune system.

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