The Accessory Pigments

Carotenoid absorbance extends from the UV to about 550 mn in the visible, as illustrated by the characteristic three-peaked spectrum of p-carotene shown in Fig. 9 below. The short wavelength peak in a carotenoid is usually attenuated if there is a P-end group present. In any event, the characteristic 

In addition to the two well-established roles, recent reports have suggested a structural role for some carotenoids in green plants. Havaux reported that carotenoids of the xanthophyll family and some other terpenoids may stabilize and photoprotect the lipid phase of thylakoid membranes. Available evidence 

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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 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. 

Groot ML, Yu JY, Agarwal R, Norris JR, Fleming GR. Three-pulse photon echo experiments on the accessory pigment in the reaction center of Rhodobacter sphaeroides. J Phys Chem B 1998 102 5923-5931. 

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. 

In addition to the transfer from one Chi a molecule to another, excitations can also migrate by resonance transfer from the accessory pigments to Chi a. The transfers of excitation among Chi a s can be nearly 100% efficient (i.e., transfer rate constants for competing pathways see Eq. 4.16), whereas the... 

For each photon absorbed by any of the accessory pigments or Chi s whose excitations are funneled into a reaction center, one electron can be removed from its trap chi. Because four electrons are involved per 02 derived from water, the evolution of this molecule of 02 requires the absorption of four photons by Photosystem II or the light-harvesting antennae feeding into it (see Eq. 5.8 and Fig. 5-15). An additional four photons whose excitations arrive at the trap chi of Photosystem I are required for the reduction of the two molecules of NADP+ to NADPH necessary for the subsequent reduction of one C02 molecule (Eq. 5.9 Figs. 5-1 and 5-15). Hence eight photons are needed for the evolution of one molecule of 02 and the fixation of one molecule of C02. (In Chapter 6, Section 6.3D, we will consider how many photons are used to provide the ATP s required per C02 fixed.) The series representation (Fig. 5-15) proposed by Hill and Fay... 

Figure 5-15. Schematic model for a series representation of the two photosystems of photosynthesis, indicating the stoichiometry of various factors involved in the reduction of C02 to a carbohydrate ( CH20)). Some of the photons (hv) are captured by the accessory pigments and Chi a in the light-harvesting antennae these excitations are then fed into the two photosystems, but primarily to Photosystem II.

Other pigments, called antenna pigments, or accessory pigments, absorb light at other wavelengths. The accessory pigments are responsible for the brilliant colors of plants in the autumn (in the Northern Hemisphere). The breakdown of chlorophyll allows us to see the colors of the accessory pigments. 

These organisms may exhibit 680-nm fluorescence either by the absorption of the short blue band of chlorophyll and/or the transfer from the accessory pigment phycoerythrin. 

Coming off from Da-Db are the accessory pigments, monomeric Ba and Bg, and BOa and Bd>B, followed by a quinone molecule, Qa or Qb- The center of either monomeric BChl- molecule is 11 A from that of the nearest D, and the corresponding edge-to-edge distance is 4 A. Each BO is 10 A from the nearest monomeric BChl and 12 A from the nearest quinone. The Mg atoms of Ba and Bb are ligated by L-His 153 and M-His 180, respectively, as marked in Fig. 3 (A). The Mg atoms of aU four BChl molecules (Da, Db, Ba and Bb) are thus penta-coordinated. 

In Figure 17.7c, the accessory pigments / -carotene and phycocyanin are also shown. 

The thylakoid membranes of the chloroplast hold the chlorophyll and some of the accessory pigments. Their membranes contain only a small fraction of the common phospholipids, but are rich in glycolipids. They also contain much protein, and some of the photosynthetic pigments are attached to certain of these proteins. 

In addition, the accessory pigment function of carotenoid is depicted by the direct excitation of CAR by light and the transfer of this radiant energy to SENS [reprinted, with permission, from N. I. Krinsky in Carotenoids (0. Isler, Ed.), Birkhauser Verlag, Basel, 1971, p. 684]. 

The absorption of light by accessory pigments (superimposed on the absorption of chlorophylls and ). The accessory pigments absorb light and transfer their energy to chlorophyll. 

In the fall, the chlorophyll in leaves is lost, and the red and yellow colors of the accessory pigments become visible, accounting for fall foliage colors. 

Only a relatively small portion of the visible spectrum is absorbed by chlorophylls. The accessory pigments absorb light at additional wavelengths. As a result, most of the visible spectrum can be harnessed in light-dependent reactions. 

Section Rhodophyta (red algae), the color is caused by the accessory pigment phycoerythrin which is localized in die chloroplast, see teurilene (Laurencia spp.), obtusallene I ... 

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