Photosynthetic Pigments: Check Types, Process and Examples

Photosynthetic Pigments: Pigments are brightly coloured, chemicals. These are chemical substances that exclusively reflect specific wavelengths of visible light. This gives them a “colourful” appearance. They give flowers, corals, and even animal skin its colours. Pigments’ ability to absorb particular wavelengths is more essential than their ability to reflect light.

Pigments are useful to plants and other autotrophs (organisms that create their own food through photosynthesis) because they interact with light to absorb only specific wavelengths. Pigments also allow energy to be absorbed from sunlight for the photosynthesis process in plants, algae, and cyanobacteria. However, because each pigment reacts with just a restricted range of the spectrum, it is frequently necessary to manufacture many types of pigments, each of a distinct colour, to achieve the desired effect. To know more about the process of Photosynthesis in plants and diagram of photosynthesis, scroll down.

What is the Photosynthesis Process?

The photosynthesis process is carried out by green plants and photosynthetic bacteria, where electromagnetic radiation is converted into chemical energy and uses light energy to convert carbon dioxide and water into glucose and oxygen. Below is the diagram of photosynthesis:

Chlorophyll Pigments: What are they?

Photosynthetic organisms contain light-absorbing molecules known as pigments. These photosynthetic pigments absorb only specific wavelengths of visible light while reflecting others, and the set of wavelengths absorbed by a pigment is its absorption spectrum. These are known as chlorophyll pigments.

Fig: Photosynthetic Pigments

The molecules of photosynthetic pigments are a little ubiquitous and are composed of pigments, i.e., chlorophyll, carotenoids, and phycobilin.

The photosynthetic systems contain another specific pigment called pheophytin (bacteriopheophytin in bacteria), which plays a very important role in the transfer of electrons.

In plants, another pigment can be found in particular photosynthetic systems, such as xanthophylls, carotenoids, etc.

Reaction Centre and Accessory Pigments

The reaction centre contains pigment chlorophyll-a, which results in the release of electrons by absorbing light. The energy created is used to reduce an electron acceptor (pheophytin or Ferredoxin reducing substance) and is critical for producing chemical energy during photosynthesis. Accessory pigments receive radiant energy and transfer it among themselves, and this transfer of energy is called resonance transfer. These are light-absorbing compounds found in photosynthetic organisms. The accessory pigments include chlorophyll-b and c carotenoids, xanthophylls and phycobilin.

Chlorophyll

i. Chlorophyll is the major pigment molecule, which is the principal photoreceptor (primary light-absorbing) in the chloroplasts of most green plants.
ii. Chlorophylls are very effective photoreceptors because they contain networks of alternating single and double bonds.
iii. Chlorophyll is a water-insoluble pigment and consists of a porphyrin ring that is bound to an ion \({\rm{M}}{{\rm{g}}^{2 + }}\) attached to a phytol chain.
iv. This chlorophyll pigment can absorb light at a wavelength between \({\rm{430 – 480}}\;{\rm{nm}}\) and between \({\rm{650 – 700}}{\mkern 1mu} \,{\rm{nm}}\)
v. As sunlight, i.e., white in colour, falls on a chlorophyll layer, the green light with a wavelength between \(480\) and \({\rm{550}}\;{\rm{nm}}\) is not absorbed but is reflected; that is why plant chlorophylls and whole leaves are green.
vi. All chlorophyll molecules in the thylakoid membrane are associated with specific chlorophyll-binding proteins.
vii. Chlorophyll is a green pigment with polycyclic and planar structures.
viii. Chlorophyll is formed from protochlorophyll in the presence of light. The addition of two \({\rm{H}}\)-atoms to protochlorophyll gives chlorophyll.
ix. Chlorophylls are found either in the cytoplasmic membranes of photosynthetic bacteria or in thylakoid membranes inside plant chloroplasts.
x. There are two different classes of chlorophylls. They are chlorophyll-a \(\left( {{\rm{chl – a}}} \right)\) and chlorophyll-b \(\left( {{\rm{chl – b}}} \right){\rm{.}}\)

Fig: Chlorophyll Structure

a. Chlorophyll-a

i. This is a bluish-green coloured pigment and has the molecular formula \({{\rm{C}}_{{\rm{55}}}}{\mkern 1mu} {{\rm{H}}_{{\rm{72}}}}{\mkern 1mu} {{\rm{O}}_{\rm{5}}}{\mkern 1mu} {{\rm{N}}_{\rm{4}}}{\mkern 1mu} {\rm{Mg}}\) group at \(3^{{\rm{rd}}}{\rm{C}}\) of \({{\rm{2}}^{{\rm{nd}}}}\) pyrrole ring.
ii. Chlorophyll-a contains a porphyrin ring.
iii. This chlorophyll-a in reflected light shows blood-red colour, while in transmitted light, it shows blue-green light.
iv. It is a universal pigment in all photosynthetic organisms except bacteria.
v. The energy absorbed from light results in the release of electrons, which is accepted by the primary electron acceptor.
vi. It has a long hydrophobic tail that anchors the molecule to other hydrophobic proteins in the thylakoid membrane of the chloroplast.

b. Chlorophyll-b

i. Chlorophyll-b is a yellowish-green colour pigment and has the molecular formula \({{\rm{C}}_{{\rm{55}}}}{{\rm{H}}_{{\rm{70}}}}{{\rm{O}}_{\rm{6}}}{{\rm{N}}_{\rm{4}}}{\rm{Mg}}\) having \({\rm{CHO}}\) group at \({{\rm{3}}^{{\rm{rd}}}}{\rm{ C}}\) of \({{\rm{2}}^{{\rm{nd}}}}\) pyrrole ring.
ii. This pigment looks dull brown in reflected light and yellowish-green colour in transmitted light.
iii. It is an accessory photosynthetic pigment with a molecular structure consisting of a chlorin ring with an \({\rm{Mg}}\) centre.
iv. Absorb energy that chlorophyll-a does not absorb and functions as a light-harvesting antenna in Photosystem-I.
v. This pigment is soluble in methyl alcohol.
vi. Chlorophyll-b is absent in BGA, red algae, brown algae, diatoms, etc.

Carotenoids

i. In addition to chlorophylls, thylakoid membranes contain secondary light-absorbing pigments or accessory pigments known as carotenoids.
ii. These carotenoid pigments may be yellow, red, or also purple.
iii. The brightly coloured carotenoids found in fruits such as the red of the tomato is lycopene, the yellow of corn seeds is zeaxanthin, and the orange of an orange peel is \({\rm{\beta – }}\) carotene.
iv. Lycopene is the common precursor structure for the synthesis of cyclic and bicyclic carotenoids.
v. These carotenoid pigments absorb light at wavelengths that are not absorbed by the chlorophylls, so these are called or referred to as supplementary light receptors.
vi. Carotenoids are of two types, i.e., carotenes and xanthophylls.
vii. Lutein is a type of vitamin called a carotenoid, and it is related to beta-carotene and vitamin A.
viii. Carotenoids function as a type of antioxidant for human beings.
ix. Lutein and zeaxanthin are yellow carotenoid antioxidants and are considered macular pigments.
x. Carotene is an orange-coloured pigment, whereas xanthophylls are yellow in colour.
xi. Fucoxanthin is a xanthophyll, and it is found as an accessory pigment in the chloroplasts of brown algae and most other heterokonts, giving them a brown or olive-green colour pigment.
xii. These accessory pigments protect the chlorophyll against photo-oxidation and then transfer light energy to chlorophylls.
xiii. Some carotenoids can form complexes with proteins (carotenoproteins) that are water-soluble and appear to stabilize carotenoids, as occurs with some crustaceans.
xiv. Carotenoids are naturally found in both forms and also esterified with fatty acids in many fruits, flowers, animals, microorganisms, and algae.
xv. Carotenoids also help in preventing photodynamic damage.
xvi. Carotenoids can also express themselves in mature leaves even if chlorophyll begins to disintegrate.

Fig: Lycopene

Fig: β-carotene

Phycobilin

i. Cyanobacteria or blue-green algae and red algae possess phycobilin such as phycoerythrobilin and phycocyanobilin as their light-harvesting pigments.
ii. These pigments are open-chain tetrapyrroles (\(4\) pyrrole rings) that lack \({\rm{M}}{{\rm{g}}^{{\rm{ +  + }}}}\) and the phytol tail.
iii. Phycobilins are covalently linked to specific binding proteins, forming phycobiliproteins, that associate to form highly ordered complexes called phycobilisomes that constitute the primary light-harvesting structures in microorganisms such as red algae or blue-green algae.
iv. Phycobilin is a water-soluble pigment.
v. It is responsible for maximum absorption in the green parts of the spectrum.
vi. It is distributed in the case of red and blue algae.
vii. Phycoerythrin is distributed in red algae, and phycocyanin is distributed in red as well as blue algae.

Bacteriorhodopsin

i. Bacteriorhodopsin is another class of photosynthetic pigment that exists only in halobacteria.
ii. Bacteriorhodopsin is composed of a protein that is attached to a retinal prosthetic group.
iii. This pigment is responsible for the absorption of light photons, leading to a conformational change in the protein, which results in the expulsion of the protons from the cell.

Main Photosynthetic Pigments

The different types of Photosynthetic Pigments are shown through the table below:

Photosynthetic Pigments Summary

All green plants are photosynthetic because they are green in colour. We understood that plants with chlorophyll pigment perform photosynthesis and prepare their food through this article. We also understood some microorganisms such as Cyanobacteria could also perform photosynthesis and prepare their food like green plants. Different photosynthetic organisms have a variety of different pigments. Hence, they can absorb energy from a wide range of wavelengths. Thus, photosynthetic pigments play a major role in organisms and the colour of fruits and vegetables. Students can write a short note on chloroplast from the section elaborated above.

FAQs on Photosynthetic Pigments

Q.1. What are the three types of photosynthetic pigments?
Ans: The three types of photosynthetic pigments are chlorophyll, carotenoids and phycobilin.

Q.2. What is primary photosynthetic pigment?
Ans: Chlorophyll-a is called the primary photosynthetic pigment.

Q.3. Is anthocyanin a photosynthetic pigment?
Ans: Anthocyanin is another important pigment that is not directly involved in photosynthesis, but it gives red stems, leaves, flowers, or even fruits their colour.

Q.4. Which pigments are present in the antenna? 
Ans: Accessory pigments or antenna are light-absorbing compounds, found in photosynthetic organisms and the accessory pigments include chlorophyll-b and c, carotenoids, xanthophylls and phycobilin.

Q.5. Which pigment is present in a leaf of higher plants?
Ans: Chlorophyll-a chlorophyll-b carotenoids, and xanthophylls are the pigment that is present in the leaf.

Q.6. What is the colour of various pigments on a chromatogram?
Ans: In paper chromatography/chromatogram, chlorophyll-a appears bright or blue-green, chlorophyll-b as yellow green/grass green, xanthophylls as yellow and carotenoids as yellow to yellowish orange.

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