Chlorophyll accessory pigments

Chlorophyll b occurs as an accessory pigment of the light-harvesting systems in land plants and green algae, and comprises one-third (or less) of total chlorophyll. The biosynthesis of chlorophyll b has been an area of active research particularly regarding its compartmentalization in chloroplast membranes, identification of the gene for chlorophyllide a oxidase, and characterization of the enzymes involved. ... 

Since the function of most of the chlorophyll and the accessory pigments is thought to be the harvesting of light, one would expect the action spectrum... 

Figure 10.16 Solar energy transfer from accessory pigments to the reaction centre, (a) The photon absorption by a component of the antenna complex transfers to a reaction centre chlorophyll, or, less frequently, is reemitted as fluorescence, (b) The electron ends up on the reaction centre chlorophyll because its lowest excited state has a lower energy than that of the other antenna pigment molecules. (From Voet and Voet, 2004. Reproduced with permission from John Wiley Sons., Inc.)...

The sunlight is absorbed by a number of pigments, of which chlorophyll a is the most important. The absorption spectrum of chlorophyll a is shown in Fig. 10.3. Apart from chlorophyll a there are accessory pigments which function as lightharvesting antennae and transfer the energy to the chlorophyll a. This is an elegant way to efficiently collect sunlight. 

Trees, C.C., D.K. Clark, R.R. Bidigare, and M.E. Ondrusek. 2000. Chlorophyll a versus accessory pigment concentrations within the euphotic zone A ubiquitous relationship Limnology and Oceanography 45 1130-1143. 

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 a-helical segments, seven chlorophyll a molecules (green), five chlorophyll b molecules (red), and two molecules of the accessory pigment lutein (yellow), which form an internal cross-brace. 

FIGURE 19-45 Organization of photosystems in the thylakoid membrane. Photosystems are tightly packed in the thylakoid membrane, with several hundred antenna chlorophylls and accessory pigments surrounding a photoreaction center. Absorption of a photon by any of the antenna chlorophylls leads to excitation of the reaction center by exciton transfer (black arrow). Also embedded in the thylakoid membrane are the cytochrome bkf complex and ATP synthase (see Fig. 19-52). 

Figure 23-22 Absorption spectra of chlorophylls and accessory pigments compared. Redrawn from G. and R. Govindjee,265 and from J. J. Wolken 266...

This website provides an overview of photosynthesis, including leaf structure, the nature of light, chlorophyll and accessory pigments, chloroplast, and stages of photosynthesis. Several links to other photosynthesis resources are also provided. 

In other words, the direction of excitation transfer or migration is from the accessory pigments to Chi a (Eq. 5.3) and from Chi a to the special trap chlorophylls (Eq. 5.4) where the photochemical reactions take place. Hence, the overall effect of the steps described by Equations 5.2 to 5.4 is to funnel the excitations caused by the absorption of light to the trap chi. 

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