Photosynthetic Pigments

Photosynthetic pigments

A pigment is a molecule that can absorb light in visible range.  Photosynthetic pigments are fat soluble.  All types of photosynthetic pigments are located in plastids such as chloroplast.  Embedded in the matrix of the chloroplast is a membrane system.  The membrane system is the site of the light reactions in photosynthesis.

The membrane system consists of many flattened fluid-filled lamellae termed as thylakoids.  Each thylakoid is made of lipid and aqueous protein layer.  The thylakoids form stacks termed as grana and each granum resembles a pile of coins.

Chloroplast of algae lack grana, but they have extensive chloroplast lamellae.  Cyanobacteria and photosynthetic bacteria have no chloroplast.  They have pigment bearing membrane (lamellae) lying naked in cytoplasm.  Their chief photosynthetic pigment is chlorophyll.  The other photosynthetic pigments are carotenes, xanthophylls and phycobilins which are termed as accessory pigments.

Chlorophyll

Chlorophylls are the green photosynthetic pigments which plants use in photosynthesis.  There are 10 types of chlorophyll- Chlorophyll a, chlorophyll b, chlorophyll c, chlorophyll d, chlorophyll e, bacteriochlorophyll a, b, c, d and e and bacterioviridin.  Of all, only two types i.e., chlorophyll a and chlorophyll b are widely distributed in green algae and higher plants.

Chlorophyll a is found in all photosynthetic plants except bacteria.  Hence, it is termed as universal photosynthetic pigment.  It is also called primary photosynthetic pigment because it performs primary reactions of photosynthesis or conversion of light into chemical or electrical energy.

Other photosynthetic pigments are therefore termed as accessory pigments.  The accessory pigments hand over the energy to chlorophyll a through electron spin resonance.  Some of the chlorophyll a molecules also function as reaction centres (P₇₀₀, P₆₈₀).  They bring about electrical charge separation and hence convert light energy into electrical energy.

Chlorophyll a is bluish green in pure state but chlorophyll b is olive green in pure state.  Both the chlorophylls are soluble in a number of organic solvents but chlorophyll a is more soluble in petroleum ether while chlorophyll b is more soluble in methyl alcohol.

Chlorophyll Structure

Chlorophyll structure was first studied by Wilstatter, Stoll and Fischer in 1912.  It has a head termed as porphyrin and a tail made up of long chain alcohol termed as phytol.  The head is hydrophilic (water loving), therefore in a thylakoid it is embedded in the aqueous protein layer.  But the tail is hydrophobic and is directed towards the lipid layer.

Chlorophyll molecule consists of 4 pyrrole rings.  These are arranged into a single tetrapyrrole ring called porphyrin with a magnesium atom in the centre.  It is conjugated (alternating) system of single and double bond.  Such tetrapyrrole ring structure is also present in cytochrome and haemoglobin with iron in place of magnesium.

Porphyrin ring have many side group that determine the properties of the pigment.  Chlorophyll a has a methyl group in second pyrrole ring while chlorophyll b has an aldehyde group there.  Both the chlorophyll have a long hydrocarbon side chain called phytol attached to the fourth pyrrole or porphyrin ring.

Phytol is a fatty alcohol and is insoluble in water.  The empirical formula of Chlorophyll a is C55 H72 O5 N4 Mg and Chlorophyll b is C55 H72 O5 N4 Mg.  Chlorophyll absorb light in blue, violet and red region of the spectrum.

Following figure shows the structure of chlorophyll molecule.  In case of chlorophyll a carbon-3 contains methyl group but in case of chlorophyll b carbon-3 contains aldehyde group.

 

Difference between Chlorophyll a and Chlorophyll b

Following are the differences between Chlorophyll a and Chlorophyll b :

Chlorophyll a	Chlorophyll b
1.  In pure state, chlorophyll a is bluish green.	1.    In pure state, chlorophyll b is olive green.
2.  It is a primary photosynthetic pigment.	2.  It is an accessory pigment.
3.  Carbon-3 contains methyl group.	3.  Carbon-3 contains aldehyde group.
4.  It is soluble in petroleum ether.	4.  It is soluble in 92% methyl alcohol.
5.  It absorbs more red wavelength than violet blue wavelength of light.	5.  It absorbs more violet blue wavelength of light than red wavelength of light.

Carotenoids

Carotenoids are lipid compounds which occur in almost all higher plants.  They are also present in several microorganisms.  Alongwith chlorophyll b the carotenoids are also called accessory pigments because they hand over the energy absorbed by them to chlorophyll a.

It is an accessory pigment in close association with chlorophyll in all photosynthesizing cells.  Most of the carotenoids are yellow or orange in colour present as chromoprotein in thyllakoid.  These pigments are made of two six membered rings with a highly unsaturated straight chain of hydrocarbons stretched between them.

The first carotenoid, termed as carotene was reported in carrot by Wackenroder (1831).  In general there are two major group of carotenoids- the carotene and carotenols (xanthophylls).

Carotenes

The carotenes are unsaturated hydrocarbons which absorb blue and green light and transmit yellow and red light.  These pigments consist of a long chain of hydrocarbons with a ring structure at each end.  β-carotene is an important carotene in association with chlorophyll in all photosynthetic organisms.  It is orange-yellow in colour.  The orange pigment of carrots is also β-carotene.

Xanthophylls

Xanthophylls are oxygen containing derivatives of carotenes.  These pigments, besides carbon and hydrogen also contain oxygen; e.g., lutein, violaxanthin, etc.

Both carotenes and xanthophylls are soluble in organic solvents like chloroform, ethyl ether etc.  Carotenes are more soluble in carbondisulphide as compared to xanthophylls.

Functions of carotenoids

Following are some of the functions of carotenoids:

1. Carotenoids functions as accessory pigments. They absorb radient energy in the mid region of visible spectrum and hand over the same to chlorophyll.
2. They also protect plants from excessive heat by preventing photo-oxidation (oxidative destruction by light) of chlorophyll.
3. β-carotene produces vitamin A in animals.
4. By their colour, the carotenoids make the flowers and fruits conspicuous to animals for pollination and dispersal.  Hence, they are helpful in pollination.

Phycobilins

Phycobilins are another group of accessory pigments present in red and blue-green algae.  They are open tetrapyrroles which neither contain magnesium nor phytol.  They are attached to the proteins and are water soluble.  The unit consisting of protein and phycobilins is termed as biliprotein or phycobilisome.

The pigments are of two types- blue (phycocyanin, allo-phycocyanin) and red ( phycoerythrin).  Usually both these types occur together, but their proportion may vary according to species and environment.  The pigments are also useful in chromatic adaptations. Hence, they are important accessory pigments of blue-green algae, cryptomonads and red algae.

In blue-green algae and red algae, the phycobilins are present inside submicroscopic structures termed as phycobilisomes.   Like carotenoids they also have an indirect role in photosynthesis; they absorb light energy and transfer it to chlorophyll a.  Hence, they are important for photosynthesis.