Carotenoids Definition and Function

Carotenoids are a type of accessory pigment, created by plants to help them absorb light energy and convert it to chemical energy. There are two types of carotenoids, xanthophylls and carotenes, which differ only in their oxygen content. Carotenoids have a similar base structure consisting of 8 isoprene molecules. Isoprene molecules have 5 carbons, and 8 of them together has 40 carbons. All carotenoids share this structure, and as such are also called tetraterpenoids.

Accessory pigments like the carotenoids are used for a variety of reasons in plants. Some are used to collect different wavelengths of light than the primary pigment, chlorophyll. Other carotenoids are used to take energy from chlorophyll as it becomes excited by light, and pass the energy down the chain. While leaves typically appear green because of the abundance of green chlorophyll, they will turn red and brown in the fall. This is because the chlorophyll degrades as there is less light available. As the leaves die, the carotenoids remain, producing a wide variety of orange and red colors.

Animals do not produce carotenoids naturally, and must obtain carotenoids in their diet. Strictly carnivorous animals, which eat no plants, get their carotenoids from the excess stored in the fat reserves of their prey. Some animals have also developed pathways to concentrate and display these carotenoids. Animals like flamingos, salmon, and lobsters have all developed a similar coloration due to their storage of carotenoids.

Carotenoids, like other antioxidants, have some protective effects against certain cancers. Further, certain carotenoids are needed in the diet. It is from these carotenoids that the body creates necessary vitamins. Carotenoids also serve as a basis for animal-molecules such as cholesterol and certain hormones necessary for regulating the metabolism.

Types of Carotenoids

Xanthophylls

There are two main types of carotenoids, the xanthophylls and the carotenes. Xanthophylls are easily recognized by their yellow coloration, and are present in high quantities in leaves. In the fall, these carotenoids are responsible for yellow leaves. Xanthophylls also give color to fruits and vegetables like papaya, squash, and peaches. The macula lutea in the human retina gets its coloration from these carotenoids, which play a significant role in vision. They help protect the retina from blue and ultraviolet light, which tends to cause radical ions in the tissue.

Carotenes

Carotenes, unlike xanthophylls, are carotenoids with no oxygen atoms. They reflect mostly red and orange light. Carotenes are responsible for the color of everything from carrots to sweet potatoes to cantaloupe. Carotenes, as an accessory pigment, work by transferring the energy they gather from light into chlorophyll, which can then be used to store energy in the form of glucose. Carotenes are present in nearly every vegetable and fruit in some quantity. While animals cannot produce these carotenoids, they have important biochemical functions, and serve as precursors to many molecules.

Examples of Carotenoids

Beta-Carotene

Beta-carotene is a specific carotenoid found plants and fruits. It has a red-orange coloration when isolated. Beta-carotene is the most common carotene found in plants. In humans and other animals, beta-carotene becomes a precursor for vitamin A, and must be consumed in the diet for survival. Beta-carotene is found in carrots, pumpkins, sweet potatoes, and even leafy greens like spinach and kale. Over consumption of beta-carotene, while not expressly harmful, will leave the skin with an orange coloration, as the carotenoid is stored in the fat layer just under the skin.

Lutein

Lutein is a xanthophyll, found in leafy green plants. Lutein is a yellow colored pigment. It bestows yellow color to egg yolks, and yellow carrots. Like all carotenoids, it is synthesized in plants. Animals can store the pigment in fat, and recent studies have shown that it may have some function in the human eye. Diseases like macular degeneration may be caused by the body’s inability to incorporate and use carotenoids like lutein.

Quiz

1. The puffin is a sea-faring bird, which survives mainly on a diet of small bait fish. The bait fish survive mainly on krill, which eat mostly algae. Puffins have bright orange patches on their beaks, caused in part by the build-up of carotenoids. Which link in the food chain provides these carotenoids?
A. The krill
B. The algae
C. The baitfish

Answer to Question #1
B is correct. The algae is the only organism capable of producing carotenoids. The rest of the organisms just accumulate it in their bodies. Some, like the krill and puffin, may use it to provide coloration. The bait fish may just accumulate it in the fat. Eventually, it makes its way to the puffin, providing the rich coloration puffins seek in their mates.

2. Flamingos are pink because of the extra carotenoids they store in their feathers. They eat tiny brine shrimp, which in turn accumulate carotenoids from the algae they eat. What would happen if flamingos were fed a diet with no carotenoids?
A. They would become red
B. They become white
C. They would die immediately

Answer to Question #2
B is correct. Flamingos without carotenoids in their diet will become white as they use up the carotenoids in their system and replace their pink feathers. While they will likely receive enough to live, they do need some level of carotenoids to produce essential vitamins. Flamingos in the wild are vividly pink because they amass large amounts of carotenoids. In a way, this is a direct signal to potential mates about how competitive the flamingo is. The pinker the bird, the more they have eaten, proving success to the mate.

3. What would happen to a plant without carotenoids?
A. It would be able to absorb a wider range of specific frequencies
B. Photosynthesis would break down
C. Carotenoids essentially do nothing for the plant

Answer to Question #3
B is correct. Carotenoids provide a variety of functions for photosynthesis. Not only do they expand the range of wavelengths from which energy can be extracted, but they also serve as crucial buffers to absorb free radicals created during the process of photosynthesis. Without these buffers, the chlorophyll pigments would be destroyed and the whole chain of events would be disrupted.

References

  • Bruice, P. Y. (2011). Organic Chemistry (6th ed.). Boston: Prentice Hall.
  • McMahon, M. J., Kofranek, A. M., & Rubatzky, V. E. (2011). Plant Science: Growth, Development, and Utilization of Cultivated Plants (5th ed.). Boston: Prentince Hall.
  • Nelson, D. L., & Cox, M. M. (2008). Principles of Biochemistry. New York: W.H. Freeman and Company.