Light-harvesting pigment molecules accessory pigments

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. 

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... 

The first part of the process occurs in light-harvesting complexes. Each multisubunit protein complex contains multiple antenna pigment molecules, chlorophylls and some accessory pigments, and two chlorophyll molecules that act as the reaction center. The reaction center traps energy quanta excited by the absorption of light. 

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