Plant Physiology - Lecture Notes - O. M. Tarnopilska 2019

4. Photosynthesis
4.2 Plastid pigments - chlorophylls, carotenoids, phycobilins

Chlorophylls. METABOLISM/14.html">Chloroplasts of higher plants contain chlorophyll a and chlorophyll b. They were identified by the Russian scientist M. S. Tswett (1906) using his newly developed Chromatography method. The structural formula of chlorophyll, proposed by H. Fischer (1939), was finally confirmed in 1960 As a result of two independent studies conducted in the USA and the FRG on the artificial synthesis of chlorophyll a.

Chlorophyll is an ester of the dicarboxylic acid chlorophyllin, in which one carboxyl group is esterified with a methyl alcohol residue (СН₃ОН), and the other with a residue of the monohydric aliphatic alcohol phytol (С₂₀Н₃₉ОН).

All higher plants, Algae, and cyanobacteria contain chlorophyll a, whereas chlorophyll b is found in Higher Plants and green algae. Chlorophyll c, lacking phytol, is present in brown and diatom algae, and chlorophyll d in red algae. Photosynthetic green Bacteria contain bacteriochlorophyll c and d, and purple bacteria contain bacteriochlorophyll a and b.

In chlorophyll a, four pyrrole rings are interconnected by methine bridges (= СН-) to form a porphyrin ring. In addition, the nitrogen atoms of the pyrrole rings are bonded to a magnesium atom. A cyclopentane ring, formed by a ketopropionic acid residue and containing active carbonyl (С = О) and methylated carboxyl (О = С-О-СН₃) groups, is linked to the porphyrin core. The Structure consisting of the porphyrin core and the cyclopentane ring is called phorbin. The side chain, consisting of propionic acid and the unsaturated alcohol phytol, is attached to the carbon atom of pyrrole ring IV (Fig. 4.1). Chlorophyll lacking phytol is called chlorophyllide. If the magnesium atom is replaced by a proton, the resulting compound is called pheophytin.

The activity of chlorophylls, like that of other pigments, is due to the presence of numerous double bonds with delocalized electrons.

Chlorophylls are readily soluble in organic Solvents (ethyl ether, benzene, chloroform, acetone, ethyl alcohol) and insoluble in Water. They exhibit Light absorption maxima in the red and blue Regions of the spectrum.

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Figure 4.1 — Structural formulas of chlorophylls a and b (after V. V. Polovyi)

Solutions of chlorophylls exhibit Fluorescence and Phosphorescence.

Carotenoids. These are fat-soluble pigments of yellow, bright yellow, and red colors. They are constituents of chloroplasts and chromoplasts in the green parts of plants (flowers, fruits, ROOT crops). In non-green leaves, their color is masked by chlorophyll.

Carotenoids comprise three groups of compounds: 1) orange or red carotenes, 2) yellow xanthophylls, and 3) carotenoid acids. Carotene and xanthophyll consist of eight isoprene residues that form a chain of conjugated double bonds (Fig. 4.2). The main carotenoids are β-carotene, lutein, violaxanthin, and neoxanthin.

Figure 4.2 — Structural formulas of carotenoids and The sequence of their transformations (after V. V. Polovyi)

Carotene and xanthophyll are soluble in chloroform, benzene, carbon disulfide, and acetone. Carotenes dissolve well in ethers but poorly in alcohols, whereas xanthophylls show the opposite solubility. Carotenoids have absorption maxima in the violet-blue and blue regions of the light spectrum. They are incapable of fluorescence.

The primary Functions of carotenoids include acting as accessory pigments in light absorption, protecting chlorophyll molecules from irreversible photooxidation, scavenging reactive radicals, and participating in phototropism by helping to determine the direction of SHOOT growth.

Phycobilins. Blue-green and red algae, In addition to chlorophyll a and carotenoids, contain pigments called phycobilins. Their molecule consists of four consecutive pyrrole rings (Fig. 4.3).

Figure 4.3 — Structural formulas of phycobilins (after V. V. Polovyi)

Phycobilins serve as chromophore groups of globulin Proteins

called phycobiliproteins. They are divided into three groups: 1) phycoerythrins, which are red proteins; 2) phycocyanins, which are bluish-blue proteins; 3) allophycocyanins, which are blue proteins. All of them exhibit fluorescence and are water-soluble.

Phycobilins show absorption maxima in the orange, yellow, and green regions of the light spectrum. This enables algae to make more efficient use of the light penetrating into the water.

Water has light-absorbing properties. At a depth of about 30 m, red rays disappear completely; at about 180 m, yellow rays; at 320 m, green rays; and blue and violet rays do not penetrate to depths exceeding 500 m. Phycobilins are accessory pigments involved in the light-harvesting complex. About 90% of the light energy absorbed by phycobilins is transferred to chlorophyll a.

Plants also contain the phycobilin Phytochrome. It is not involved in Photosynthesis, but acts as a red and far-red light photoreceptor, performing regulatory functions in plant Cells.