Photosynthetic Pigments

There are mainly three types of pigments: chlorophylls, carotenoids and phycobilins (in BGA and red algae).

(1) Chlorophylls (green pigments)

The chlorophylls are main pigments which plants use in Photosynthesis.

They are specialized lipid molecules embedded in thylakoid membrane.

They are the main pigment concerned with harvesting or trapping of solar energy. Arnof and Allen (1966) recognised nine types of chlorophyll – (i) chlorophyll a; (ii) chlorqphyll b, (iii) chlorophyll c, (iv) chlorophyll d; (v) chlorophyll e; (vi) bacteriochlorophyll a, (vii) bacterio -chlorophyll b, (viii) chlorobium chlorophyll 650 and (ix) chlorobium chlorophyll 666.

Out of these, the two important types of chlorophyll found in green plants are chlorophyll a and

Of these, chlorophyll a is the primary photosynthetic pigment.

It is found in all photosynthetic eukaryotes and blue green algae (cyanobacteria).Light energy absorbed by all other types of chlorophylls and accessory pigments is ultimately transferred to chlorophyll a.

Tadpole like structure of chlorophyll-a

(a) Chlorophyll a : The chlorophyll a has a empirical formula as C55H72O5N4 Mg. The molecule is distinguishable into a 'head' of size 15 × 15 Å and a 'tail' of 20 Å length. The 'head' is made up at porphyrin, (a tetrapyrrole closed ring derivative) and the 'tail' of phytol (C20H39OH).

There is a 5th isocyclic ring of cyclopentanone. A non-ionic magnesium atom is held within tetrapyrrole ring by two covalent and two co-ordinate bonds. There is vinyl group at carbon-2 position and methyl at carbon-3. The chlorophyll a absorbs violet blue and red wavelengths of the spectrum giving peaks at 430 and 662 nm respectively. It is found in all photosynthetic organisms except bacteria. Protoclorophyll is its precursor.

(b) Chlorophyll b : Its empirical formula is C55H70O6N4 Mg. It is similar to chlorophyll a except in having a formyl (CHO) group instead of methyl (CH3) at carbon-3 position of the tetrapyrrole ring. It absorbs blue and orange wavelengths giving peaks at 430 and 644 nm.

(c) Chlorophyll c: It lacks phytol esterification. It is found in brown algae, diatoms and dinoflagellates.

(d) Chlorophyll d: It has formyl group at carbon-2 position. It is reported from red algae.

(e) Chlorophyll e: It has been reported from Xanthophyceae members like Vaucheria.

(f) Bacteriochlorophyll: The molecular formula of bacteriochlorophyll-a is C55H74O6N4 Mg. Its structure is similar to chlorophyll a except presence of acetyl group instead of vinyl at carbon-2 position.

(g) Chlorobium chlorophyll: It has hydroxy-methyl group (CH3CHOH) at carbon-2 position in the tetrapyrrole nucleus.

(2) Carotenoid pigments (Accessory or shield pigments):

Carotenoids are yellow to orange lipid compounds which occur in almost all higher plants. Carotenoids are of two types-carotenes and xanthophylls.

(a) Carotenes are reduced molecules (C40 H56). They are of several types like a and b.

b-carotene is the most widespread and important carotene associated with chlorophyll in photosynthetic organisms. It is orange-yellow in colour. Vertebrates are able to break each molecule of b-carotene in two molecules of vitamin A during digestion. Red colour of tomatoes is due to a carotene called lycopene.

(b) Xanthophylls. These pigments, besides carbon and hydrogen also contain oxygen, e.g., lutein (C40H56O2), cryptoxanthin (C40H56O), etc.

Carotenoids have at least two major roles in photosynthesis: (i) they transfer the light energy absorbed by them to chlorophyll molecules, hence are called antenna molecules. They absorb light in blue violet range and (ii) they protect plant from excessive heat and prevents photooxidation (oxidative destruction by light) of chlorophyll.

(3) Phycobilins (Biliprotein):

Phycobilins are another group of accessory pigments found in red and blue-green algae. They have open chain tetrapyrrole structure and are hot water-soluble pigments. Phycobillins can be classified into two types: (i) Phycoerythrins, (ii) Phycocyanins.

Usually both these types occur together, but their proportion may vary according to species and environment.

Mechanism of Photosynthesis

The two major steps of photosynthesis are -Light or Hill reaction and dark or Blackman reaction.

Concept Builder

Flashing (Intermittent) light experiment was performed by Warburg. He established that the rate of photosynthesis was found to be greater in intermittent light as compared to continuous light. It indicates the existence of two steps in photosynthesis, a light dependent reaction and another light independent reaction (the dark reaction, enzymatic reaction).