Photosynthetic Pigments

 

PhotosyntheticPhotosynthetic Pigments

Photosynthetic pigments are the most important coloured components of the chloroplast lamellae. These pigments are molecules that strongly absorb visible light. They interact with sunlight to alter the wavelengths that are either reflected or transmitted by the plant tissue. These pigments are also found in cyanobacteria and algae.The chloroplasts contain pigments like chlorophylls,carotenoids and phycobillins.

The major groups of photosynthetic pigments are:

1. Chlorophyll
2. Carotenoids
3. Phycobilins
4. Flavonoids

Chlorophyll

The most widespread pigment in photosynthetic plants is chlorophyll. They are cyclic tetrapyrrole pigments chelated with magnesium. They share structural features with the haem and bile pigments of animals. These chlorophyll pigments can be found in fruits, flowers as well as leaves.

Chlorophyll a and chlorophyll b are the major types of chlorophylls found in plants. The former is a blue-green pigment and the latter is a yellow-green pigment. They give their characteristic green colour due to the strong absorbance of red and blue light.

Type of Chlorophyll

Chlorophyll a

This pigment is found in all higher plants. It is the most vital pigment used in photosynthesis. Some algae, cyanobacteria  and anaerobic phototrophs also show the presence of chlorophyll a. It has a strong rate of absorption. It absorbs violet-blue as well as orange-red light and reflects blue-green light.

Chlorophyll a is present universally in all plant groups except green and purple bacteria.it is a tetrapyrrole having a porphyrin head with Mg²⁺ in the centre forming a complex. The head part is hydrophilic. Attached to the head is a long hydrophobic chain of terpene alcohol called phytol which forms the tail part of chlorophyll

Chlorophyll – Chemical Formula – C₅₅ H₇₂ O₅ N₄ Mg

 

Chlorophyll b

This type of chlorophyll is seen in green algae and plants. It is an accessory pigment that aids chlorophyll a. This pigment usually absorbs the orange-red light and reflects a yellow-green colour. The chlorin ring of this chlorophyll has a CHO. Whereas, the chlorin ring of Chlorophyll-a contains CH3

                           Chlorophyll a                                                  Chlorophyll b

Chlorophyll c

This pigment is primarily seen in marine algae. Brown algae, Diatoms and Dinoflagellates show the presence of chlorophyll c. It is an unusual chlorophyll pigment that has a porphyrin ring. This can be further classified into chlorophyll c_1, c₂ and c_3. The chemical composition and the absorption rate differ in each sub-type.

Chlorophyll d

Chlorophyll d is present only in red algae and cyanobacteria. These organisms live in deep water, and thus use red light for photosynthesis.

Chlorophyll e

This is a rare pigment that is found in some golden algae. Chlorophyll e has been identified from the Xanthophytes (yellow-green algae).

Carotenoids

Carotenoids are also associated with the process of photosynthesis. Additionally, they give a bright colour to the fruits and flowers. They are terpenoid pigments present in all photosynthetic plants and they also occur in photosynthetic bacteria such as Rhodobacter and Erwinia. Carotenoids are orange, red and yellow pigments that usually occur in the roots, tubers, leaves, fruits, seeds and flowers.

Carotenoids are found essentially in all green plants.while chlorophylls absorb srongly in the red and the blue regions,carotenoids absorb mostly in the blue as well as in the green regions of the spectrum.that’s why leaves reflect green and yellow colour. Carotenoid function as accessory pigments for photosynthesis.They are also reuired for protecting photosynthetic pigments from the destructive photo oxidation that occurs in the presence of light and oxygen. The carotenes contain carbons and hydrogen while xanthophyll contains oxygen besides Carbon and Hydrogen.

Phycobilins

It is a light-capturing bile pigment found in the Chloroplast  of red algae and cyanobacteria. They have chromatophores that are primarily responsible for their colour. These are unique pigments as they are bonded to water-soluble phycobiliproteins which pass sunlight to chlorophyll and thus help in the process of photosynthesis.