PHOTOSYNTHETIC PIGMENTS AND THEIR ROLE

Photosynthesis is a photo-biochemical reaction which is mediated through the absorption of light energy The light energy is absorbed by photosynthetic pigments in plants. These pigments have capacity to absorb light of certain wavelengths and reflect light of other wavelengths, imparting different col ours to different pigments. The co lour of the specific pigment indicates the wavelength reflected by the pigment.

Types of photosynthetic pigments:

In higher plants, there are chlorophyll-a, chlorophyll-b, carotene and xanthophyll, as main photosynthetic pigments. (Anthocyanin, purple colored pigment present in flower is not photosynthetic.)

There are three main types of photosynthetic pigments

Chlorophylls,
Carotenoids
Phycobilins

Chlorophylls

The chlorophylls are the most important and abundant active pigments of photosynthesis. They are green in co lour, are insoluble in water but soluble in organic solvents.

There are seven different types of chlorophylls.
These are chlorophyll a, b, c, d, e, bacteriochlorophyll and bacterioviridin.
Chlorophyll-a is present in all photosynthetic organisms (except photosynthetic bacteria). Chlorophyll-a (Blue green) - C₅₅H₇₂ O₅ N₄ Mg
Chlorophyll- b is also predominant and found in green algae, bryophytes and all vascular plants. Chlorophyll-b (Yellow green) - C₅₅H₇₀ O₆ N₄ Mg
Chlorophyll a possesses — CH3 (methyl group), which is replaced by — CHO (an aldehyde) group in chlorophyll b.
Chlorophyll molecule is made up of a squarish tetrapyrrolic ring known as head and a phytol alcohol called tail.
The magnesium atom is present in the central position of tetrapyrrolic ring.
Chlorophyll c, d, and e are found in brown and red algae along with chlorophyll a.
Chlorophyll a and b shows maximum absorption in blue-violet and red regions of visible light.

Carotenoids:

Carotenoids are widely distributed in chloroplasts and chromoplasts.
They show wide range in co lour, from yellow, orange to red.
They are insoluble in water but soluble in organic solvents.
They mainly absorb blue-violet region of visible light.
The carotenoids are unsaturated polyhydrocarbons being composed of eight isoprene (C₅H₈) units.
They are made up of two six-membered rings having a hydrocarbon chain in between.
They are sometimes called lipochromes due to their fat soluble nature.
Carotenoids absorb light energy and transfer it to Chl. a and thus act as accessory pigments.
They protect the chlorophyll molecules from photo-oxidation by picking up nascent oxygen and converting it into harmless molecular stage.
There are two main types of carotenoids viz. carotenes and xanthophylls.

Carotenes

They are orange red in co lour and have general formula C₄₀H₅₆.

They are isolated from carrot. They are found in all groups of plants i.e., from algae to angiosperms.

Some of the common carotenes are α, β, γ and δ carotene; phytotene, lycopene, neurosporene etc.

The lycopene is a red pigment found in ripe tomato and red pepper fruits.

The β-carotene on hydrolysis gives vitamin A, hence the carotenes are also called provitamin A.

β-carotene is black yellow pigment of carrot roots.

Xanthophylls

They are yellow colored carotenoid also called xanthols or carotenols.

They contains oxygen also along with carbon and hydrogen and have general formula C₄₀H₅₆O₂.

Lutene a widely distributed xanthophyll which is responsible for yellow co lour in autumn foliage.

Fucoxanthin is another important xanthophyll present in Phaeophyceae (Brown algae).

Phycobilins

These pigments are mainly found in blue-green algae (Cyanobacteria) and red algae.

These pigments have open tetrapyrrolic in structure and do not bear magnesium and phytol chain.

Blue-green algae have more quantity of phycocyanin and red algae have more phycoerythrin.

Phycocyanin and phycoerythrin together form phycobilins. These water soluble pigments are thought to be associated with small granules attached with lamellae.

Like carotenoids, phycobilins are accessory pigments i.e. they absorb light and transfer it to chlorophyll a.