Both chlorophyll and carotenoids are pigments, or chromophores, involved in photosynthesis. Chlorophyll and carotenoids are responsible for collecting light, absorbing photons, and transferring excitation energy to the photosynthetic reaction center. However, only chlorophyll functions within the reaction center to effect charge separation across the cell membrane. It is chlorophyll that triggers a series of electron transfer reactions that ultimately reduce carbon dioxide (CO2) to carbohydrates.
With a name meaning “green leaf” in Greek, chlorophyll was first identified in 1818 by Pierre Joseph Pelletier and Joseph Bienaime Caventou. Chlorophyll is known for its green appearance and for being the most abundant photosynthetic pigment on Earth. Since its original discovery, dozens of types of chlorophyll molecules have been discovered. Molecularly, they are all cyclic tetrapyrroles and generally contain a central magnesium ion. The chemical structure of chlorophyll has the potential to gain or lose electrons easily, allowing it to absorb photons and transfer excitation energy to and within the photosynthetic reaction center.
Chlorophyll and carotenoids are light-capturing pigments, but chlorophyll is the most abundant and most critical for photosynthesis. The different types of chlorophylls, working in combination, are able to absorb light in much of the photosynthetic spectrum, from 330 to 1,050,500 nanometers. An exception is the so-called “green hole”, about XNUMX nanometers. Accessory pigments are needed to fill this absorption void.
A second limitation of chlorophyll comes from the same characteristic that makes them such potent pigments in the photosynthetic system: their ability to maintain long-lasting excited states. This ability, however, also leads to a tendency to generate toxic reactive oxygen species. Again, accessory pigments, especially carotenoids, can help solve this problem.
Carotenoids are chromophores that are usually red, orange, or yellow. Perhaps the best known carotenoid is carotene, which gives carrots their orange color. Carotenoids have two main functions: to collect light energy for photosynthesis and to protect chlorophyll from light damage.
For their main function, carotenoids absorb the light energy of photons. Together with biliproteins, they help absorb energy in the “green hole” about 500 nanometers. They are not able to transfer this energy directly to the photosynthetic pathway of the reaction center. Rather, they transfer excitation energy directly to the chlorophyll molecules, which then transfer energy to the reaction centers and the photosynthetic pathway. Therefore, carotenoids are known as accessory pigments, and chlorophyll and carotenoids together form the light-gathering antenna within cells.
Perhaps the most important function of carotenoids is to protect chlorophyll and the surrounding cell from light damage. Chlorophylls often generate toxic reactive oxygen species, which cause various cellular damage, and are especially prone to generate these free radicals under bright light conditions. Carotenoids are able to absorb excess light, diverting it from chlorophyll. Unlike chlorophyll, carotenoids can safely convert excess excitation energy into heat.