Photosynthetic Pigments

Introduction

Plants contain a variety of pigments which give rise to varied colors which absorb the light. Flowers and fruits are definitely rich in organic compounds that absorb light. Pigments can also be found in the leaves, stems and roots. Anthocyanins, flavonoids, flavines, quinones and cytochromes are  few examples of pigment compounds. But none of these should be termed photosynthetic pigments. So, photosynthetic pigments are the only pigments that have the ability to collect sunlight energy and transfer it to photosynthetic equipment. There are two types of photosynthetic pigments in plants: chlorophylls and carotenoids.

List of photosynthetic pigments :

Carotene: an orange pigment

Xanthophyll: a yellow pigment

Chlorophyll a: a blue-green pigment

Chlorophyll b: a yellow-green pigment

The most prevalent  is chlorophyll a, which is found in every plant that performs photosynthesis. In a distinct portion of the electromagnetic spectrum, each pigment absorbs light more efficiently. Chlorophyll a absorbs well at 400–450 nm and 650–700 nm, while chlorophyll b absorbs well at 450–500 nm and 600–650 nm. At 400–530 nm, xanthophyll absorbs efficiently. However, none of the pigments absorb well in the green-yellow area, which accounts for the abundance of green in nature.

Chlorophylls

Most land plants have two types of chlorophyll (Chl): Chl a and Chl b. They differ in that Chl a has a methyl ( -CH3) group on the perimeter of its big ring (called a tetrapyrrole), which in Chl b is oxidized to generate a formal (-CH=O) group. Because of this distinction, these two pigment molecules can absorb light at slightly different wavelengths. 

The lowest excited singlet state( singlet state is molecular electronic state in which all electron spins are paired)  necessitates the absorption of energy in the form of a red photon (640 to 700 nm). The second excited singlet state has a greater energy level than the ground state and necessitates energy absorption in a blue photon (430 to 475 nm) since photons with shorter wavelengths have more energy. Because chlorophyll has two excited singlet states, it absorbs photons from both the red and blue parts of the visible spectrum. We perceive leaves to be green when we see light reflected by or transmitted through them. This is because chlorophylls are the primary leaf pigments and do not absorb green photons (about 500 to 600nm).

In addition, the chlorophyll structure includes a long hydrophobic chain that is ester-bonded to the tetrapyrrole ring. This phytyl group in the ‘tail’ renders the entire chlorophyll molecule insoluble in water. This insolubility maintains the chlorophyll pigments confined to the photosynthetic membranes of chloroplasts (intracellular organelles)..

Carotenoids

Carotenoids are another class of photosynthetic pigments. Most plants contain different kinds of carotenoids including beta carotene, lutein, neoxanthin and violaxanthin. Its core structure is made up of a five-carbon unit that repeats and branches. Isoprenoids are molecules produced from these five-carbon units. The red color in tomatoes is ultimately derived from the yellow carotenoid (lutein). Typically, carotenoids absorb photons best in the blue area of the light spectrum (400 to 500 nm) and appear yellow. But why are lycopene-containing fruits red and not yellow? The answer involves structures found around the lycopene molecule known as chromophores. In tomato fruit it is these chromophores called xanthophylls and carotenoids, that absorb blue-green light, returning that photon energy as longer wavelengths of light (orange and red) that our eyes perceive as red.

The function of Photosynthesis pigments

Pigment-protein complexes are significant because they contain specialised electron transfer components that aid in the capture of energy from the photosynthesis process. The photosynthetic pigments of the thylakoid membrane are organised into two distinct photosystems PSII and PSI  which catalyse the light-driven production of ATP. Each antenna pigment consists of an array of chlorophylls (except chlorophyll a) and carotenoids. They are only responsible for harvesting light energy and transferring it to a small number of pigment-protein complexes known as reaction centers( chlorophyll a680 and chlorophyll a700 ). The photon’s energy is utilised at the active center to stimulate an electron to a higher energy level (and thus a lower redox potential) so that it can react. Almost all life on earth depends on photosynthesis for its survival. In this approach, photon energy is used to ‘push’ electrons to higher energy levels. When the reaction center loses an electron, it oxidises and becomes capable of accepting electrons from an external source. Meanwhile, electron transport drives protons across the photosynthetic membrane, giving the energy to manufacture ATP. The chloroplast can finish the photosynthesis process by making sugars from CO2 if it has enough NADPH and ATP. Oxygen is released in photosynthesis which is require for living organisms.

Conclusion 

Photosynthetic pigments are such a complex family of molecules that it is absolutely imperative. Although photosynthetic pigments have only been discovered for about a century, these molecules are vital for sustaining life on the planet Earth and will continue to help humans for years to come. Chlorophyll and other light-sensitive pigments in photosynthetic cells capture solar energy. Such cells can transform solar energy into energy-rich organic molecules like glucose by carbon dioxide. These cells not only drive the global carbon cycle but also produce a large portion of the oxygen in the Earth’s atmosphere. Non- photosynthetic cells, in essence, employ photosynthesis products to conduct the reverse of photosynthesis: break down glucose and release carbon dioxide.