Light Absorption:- Chlorophylls Absorb Light Energy for Photosynthesis
The most important light-absorbing pigments in the thylakoid membranes are the chlorophylls, green pigments with polycyclic, planar structures resembling the proto porphyrin of hemoglobin (see Fig. 5–1), except that Mg2+, not Fe2+, occupies the central position (Fig. 19–40). The four inward-oriented nitrogen atoms of chlorophyll are coordinated with the Mg2+. All chlorophylls have a long phytol side chain, esterified to a carboxyl-group substituent in ring IV, and chlorophylls also have a fifth five membered ring not present in heme. The heterocyclic five-ring system that surrounds the Mg2 has an extended polyene structure, with alternating single and double bonds. Such polyenes characteristically show strong absorption in the visible region of the spectrum (Fig. 19–41); the chlorophylls have unusually high molar extinction coefficients (see Box 3–1) and are therefore particularly well-suited for ab sorbing visible light during photosynthesis. Chloroplasts always contain both chlorophyll a and chlorophyll b (Fig. 19–40a). Although both are green, their absorption spectra are sufficiently different (Fig. 19–41) that they complement each other’s range of light absorption in the visible region. Most plants contain about twice as much chlorophyll a as chlorophyll b. The pigments in algae and photosynthetic bacteria include chlorophylls that differ only slightly from the plant pigments. Chlorophyll is always associated with specific binding proteins, forming light-harvesting complexes (LHCs) in which chlorophyll molecules are fixed in relation to each other, to other protein complexes, and to the membrane. The detailed structure of one light-harvesting complex is known from x-ray crystallography (Fig. 19–42). It contains seven molecules of chlorophyll a, five of chlorophyll b, and two of the accessory pigment lutein (see below).
FIGURE 19–40 Primary and secondary photopigments. (a) Chlorophylls a and b and bacteriochlorophyll are the primary gatherers of light energy. (b) Phycoerythrobilin and phycocyanobilin (phycobilins) are the antenna pigments in cyanobacteria and red algae. (c) β-Carotene (a carotenoid) and (d) lutein (a xanthophyll) are accessory pigments in plants. The areas shaded pink are the conjugated systems (alternating single and double bonds) that largely account for the ab sorption of visible light.
Cyanobacteria and red algae employ phycobilins such as phycoerythrobilin and phycocyanobilin (Fig. 19–40b) as their light-harvesting pigments. These open chain tetrapyrroles have the extended polyene system found in chlorophylls, but not their cyclic structure or central Mg2+. Phycobilins are covalently linked to specific binding proteins, forming phycobiliproteins, which associate in highly ordered complexes called phy cobilisomes (Fig. 19–43) that constitute the primary light-harvesting structures in these microorganisms.
FIGURE 19–41 Absorption of visible light by photopigments. Plants are green because their pigments absorb light from the red and blue regions of the spectrum, leaving primarily green light to be reflected or transmitted. Compare the absorption spectra of the pigments with the spectrum of sunlight reaching the earth’s surface; the combination of chlorophylls (a and b) and accessory pigments enables plants to harvest most of the energy available in sunlight.
The relative amounts of chlorophylls and accessory pigments are characteristic of a particular plant species. Variation in the proportions of these pigments is responsible for the range of colors of photosyn thetic organisms, from the deep blue-green of spruce needles, to the greener green of maple leaves, to the red, brown, or purple color of some species of multicellular algae and the leaves of some foliage plants favored by gardeners.
FIGURE 19–42 A light-harvesting complex, LHCII. The functional unit is an LHC trimer, with 36 chlorophyll and 6 lutein molecules. Shown here is a monomer, viewed in the plane of the membrane, with its three transmembrane -helical segments, seven chlorophyll a molecule (green), five chlorophyll b molecules (red), and two molecules of the accessory pigment lutein (yellow), which form an internal cross-brace.
FIGURE 19–43 A phycobilisome. In these highly structured assemblies found in cyanobacteria and red algae, phycobilin pigments bound to specific proteins form complexes called phycoerythrin (PE), phycocyanin (PC), and allophycocyanin (AP). The energy of photons absorbed by PE or PC is conveyed through AP (a phycocyanobilin binding protein) to chlorophyll a of the reaction center by exciton transfer, a process discussed in the text.
