Chlorophyll and carotenoids are both pigments, or chromophores, that are involved in photosynthesis. Both chlorophyll and carotenoids are responsible for harvesting light, absorbing photons and transferring the excitation energy to the photosynthetic reaction center. Only chlorophyll, however, functions within the reaction center to perform charge separation across the cell membrane. It is chlorophyll that sets off a series of electron transfer reactions that eventually reduces 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 well-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 usually contain a central magnesium ion. Chlorophyll’s chemical structure has the potential to gain or lose electrons easily, which is what allows it to absorb photons and transfer the excitation energy to and within the photosynthetic reaction center.
Chlorophyll and carotenoids are both light-harvesting pigments, but chlorophyll is the most abundant and the most critical for photosynthesis. The different types of chlorophylls, working in combination, are able to absorb light over much of the photosynthetic spectrum, from 330-1,050 nanometers. One exception is what is called the “green gap,” around 500 nanometers. Accessory pigments are required to fill this absorption gap.
A second limitation of chlorophylls arises out of the very characteristic that makes them such powerful pigments in the photosynthetic system: their ability to maintain long-lived excited states. That ability, however, also leads to a tendency to generate toxic reactive oxygen species. Again, accessory pigments, carotenoids in particular, are able to help solve this problem.
Carotenoids are chromophores that are usually red, orange or yellow in color. The most well-known carotenoid is probably carotene, which gives carrots their orange color. Carotenoids have two main functions: harvesting light energy for photosynthesis and protecting chlorophyll from light damage.
For their primary function, carotenoids absorb light energy from photons. Along with biliproteins, they help absorb energy in the “green gap” near 500 nanometers. They are not able to transfer this energy directly into the photosynthetic pathway in the reaction center. Rather, they transfer the excitation energy directly to chlorophyll molecules, which then transfer the energy to reaction centers and into the photosynthetic pathway. Carotenoids are thus known as accessory pigments, and chlorophyll and carotenoids together make up the light-harvesting antenna within cells.
Perhaps the most important function of carotenoids is protecting chlorophyll and the surrounding cell from light damage. Chlorophylls often generate toxic reactive oxygen species, which cause diverse cellular damage, and they are particularly prone to generating such free radicals under high light conditions. Carotenoids are able to absorb excess light, diverting it from chlorophyll. Unlike chlorophyll, carotenoids can harmlessly convert excess excitation energy to heat.