The compounds present in plants that, in a way, assist in the absorption of light during the process of photosynthesis, are accessory pigments. As their name suggests, they act as helpers or assist chlorophyll in absorption of light. These may be chlorophyll variants or other classes. There are different kinds present in plants that have been discovered recently.
What are Accessory Pigments?
Photosynthetic pigments are broadly categorized into three classes. They are
Carotenoids are also accessory pigments, which are red, orange, and yellow in color. They are abundantly found in plants and are present in chloroplasts. There are around 600 types of carotenoids. These are made up of two, six carbon rings, that are connected by carbon atoms. Carotenoids are incapable of transferring the absorbed light to the photosynthetic pathway. Thus, the energy that is absorbed by them is transferred to chlorophyll a, through various reactions, which then transfers energy into the photosynthetic pathway. They are present in the chloroplasts and chromoplasts of plants. They are further classified as xanthophylls, which contain oxygen and carotenes, which are basically just hydrocarbons with no oxygen. They are found in various plants and also types of algae, fungi, and bacteria.
Phycobilins are water soluble and they are present in the cytoplasm or the stroma of the chloroplasts. They are only found in cyanobacteria and rhodophyta. The four types of phycobilins are phycoerythrobilin, phycourobilin, phycoviolobilin, and phycocyanobilin.
What is the Function of Accessory Pigments?
To understand their function, it is necessary to know the chloroplast structure and function, that plays an important role in the process of photosynthesis. Chlorophyll is the primary pigment that intercepts sunlight to utilize it for the process of photosynthesis. All of them have different absorption spectra, that is, they absorb light at different wavelengths. The porphyrin ring of chlorophyll absorbs yellow as well as blue wavelengths, and reflects green wavelengths. They absorb light from different wavelengths (carotenoids absorb the green wavelength along with chlorophyll b), which are not readily absorbed by chlorophyll.
Hence, their basic function is transferring energy to chlorophyll a. Chlorophyll a then triggers chemical reactions in the chloroplasts, known as the light dependent process of photosynthesis or just Light Reactions. Products of these reactions are then utilized in the Dark Reactions of photosynthesis. Carotenoids are the most important ones in the process of photosynthesis. The carbon bonds in the two rings of carotenoids interact with each other, this is the conjugated system. This system makes the movement of electrons relatively free, and it therefore becomes energy conserving. As a result, the range of energies of light that is absorbed, decrease. More light is absorbed from the short end of the spectrum, hence giving the pigment a red appearance.
In aquatic ecosystem, the absorption spectrum of pigments differs due to the presence of water and other organic and dissolved matter. It is the phycocyanin that absorbs wavelengths of red, and therefore cyanobacteria can survive in dark inland water.
Thus, it is the presence of these accessory pigments in plants that makes it possible for the plants to absorb a wide range of wavelengths from the visible spectrum. Thus, they play a vital role in the survival of photosynthetic plants and contribute a lot more to the ecosystem as well.