Antenna chlorophyll

Such a process can naturally be expected to play a certain part in the mechanism of directed energy transport in biological systems, in particular, in the transfer of absorbed energy from the antenna chlorophyll molecules to the reactive center in the photosynthetic system of plants. In Ref. [30], energy exchange between molecules of the photosynthetic pigments chlorophyll a and pheophytin a was studied experimentally with pigments introduced into the polar matrix. 

Figure 12.7 The photosynthetic unit, in which an antenna chlorophyll molecule is excited by photon absorption and the energy is transferred to the chlorophyll dimer at the reaction centre...

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). 

Antenna Chlorophylls Are Tightly Integrated with Electron Carriers... 

The electron-carrying cofactors of PSI and the lightharvesting complexes are part of a supramolecular complex (Fig. 19-51a), the structure of which has been solved crystallographically. The protein consists of three identical complexes, each composed of 11 different proteins (Fig. 19-51b). In this remarkable structure the many antenna chlorophyll and carotenoid molecules are... 

Figure 5.7 Electron transfer processes in the first stages of photosynthesis. The energy of light E absorbed by the antenna chlorophylls is transferred to the special pair (BChl)FC is the ferrocytochrome, BPh the bacteriopheophytin and QFe, Q are quinones...

Reaction (14) and hence the overall rate of electron phototransfer across the membrane can be enhanced by providing additional excitation of the ZnTPPin molecules on the inner membrane surface. It could be done by virtue of energy transfer from some antenna collecting light and then transferring the excitation to the reaction centers , i.e. the ZnTPPin molecules embedded into the membrane this approach reproduces the action of a pull of the antenna chlorophyll in chloroplasts. In corresponding experiments (System 14 of Table 1) a water-soluble... 

FIGURE 7. Tentative model for the organisation of photosystem II, showing the relationship between the antennae chlorophyll-proteins and the reaction centre. The dotted lines show bonds stabilised by the presence of Mg++. The diagram is drawn parallel to the plane of the membrane, so that the components extend above and below the plane of the page. 

This potential danger of electron transfer out into the antenna is there because the midpoint potential of an antenna chlorophyll molecule is not that much higher than that of the previous donor in the RC, and in PSII it is clearly well below that of P680. 

Polypeptides of 43 and 47 kDa are thought to be the components of the reaction centre and antenna chlorophylls complex, also binding pheophytin and Q, with Qa and the Fe centre on the outer side of the membrane. Polypeptides of 24, 18 and 33 kDa seem to be on the internal side (exposed to the lumen of thyla-koids), while a 34 kDa polypeptide which was co-isolated with a 31 kDa component [32] seems to bind Mn in a cleft facing the thylakoid lumen (see model in Ref. 10). 

A Scherz and WW Parson (1986) Interactions of bacteriochlorophylls in antenna chlorophyll-protein complexes of photosynthetic bacteria. Photosynthesis Res 9 21-32... 

---