Accessory Pigments

Accessory pigments are compounds in plants that in a way assist in the absorption of light in the process of photosynthesis. These pigments as the name suggests act as helpers or assist chlorophyll a in absorption of light. These pigments may be chlorophyll variants or other classes. There are different kinds of these pigments in plants that have been discovered recently. This article will tell you more about these pigments, it's types and functions.

What are Accessory Pigments?

Photosynthetic pigments are broadly categorized into three classes. They are

• Chlorophylls
• Caretenoids
• Phycobilins

These classes are further divided into various sub-classes. Amongst these chlorophyll has various types such as chlorophyll a, chlorophyll b (found in green algae), and chlorophyll c (found in members of Chromista and dinoflagellates). Chlorophyll a is the most important pigment and the rest are therefore termed as accessory pigments. Chlorophyll pigments have a porphyrin ring within which electrons can move freely. Due to this property of these rings, they can lose or gain electrons and therefore are capable of capturing light.

Carotenoids are also accessory pigments which are red, orange and yellow pigments. They are abundantly found in plants and are present in chloroplasts. There are around 600 types of carotenoids. These pigments are made up of two six carbon rings that is connected by carbon atoms. Carotenoids are incapable of transferring the absorbed light to the photosynthetic pathway. Thus, the energy that is absorbed by these pigments 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.. These pigments are found in various plants and also types of algae, fungi and bacteria

Phycobilins are water soluble and 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 the function of these pigments, 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 the pigments have different absorption spectra, that is they absorb light at different wavelengths. The porphyrin ring of chlorophyll absorbs yellow and blue wavelengths and reflects green wavelengths. The pigments absorb light from different wavelengths (carotenoids absorb the green wavelength along with chlorphyll b) which are not readily absorbed by chlorophyll.

Hence the basic function of these pigments 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. Product of these reactions are then utilized in the Dark Reactions of photosynthesis. Carotenoids are the most important pigments 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 therefore becomes energy conserving. As a result the range of energies of light absorbed decrease. More light is absorbed from the short end of the spectrum therefore 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 (accessory pigment - phycobilin) 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, these pigments play a vital role in the survival of photosynthetic plants and contribute a lot more to the ecosystem as well.