Chlorophyll electron release from

Although LHC antenna chlorophylls can transfer light energy absorbed from a photon, they cannot release an electron. As we ve seen already, this function resides in the two reaction-center chlorophylls. To understand their electron-releasing ability, we examine the structure and function of the reaction center in bacterial and plant photosystems In the next section. 

After the primary electron acceptor, Qb, in the bacterial reaction center accepts one electron, forming Qb , it accepts a second electron from the same reaction-center chlorophyll following its absorption of a second photon. The quinone then binds two protons from the cytosol, forming the reduced quinone (QHb), which is released from the reaction center (see Figure 8-36). QHb diffuses within the bacterial membrane to the Qo site on the exoplasmic face of a cytochrome bci complex, where it releases its two protons into the periplasmic space (the space between the plasma membrane and the bacterial cell wall). This process moves protons from the cytosol to the outside of the cell, generating a proton-motive force across the plasma membrane. Simultaneously, QHb releases its two electrons, which move through the cytochrome bci complex exactly as depicted for the mito-... 

Chlorophyll a fluorescence induaion is a widespread method to evaluate the photosynthetic activity. This method is noninvasive, highly sensitive, fast, and easily measured. When chlorophyll molecules in photosystem II absorb light, that light may be assimilated into the hght reactions of photosynthesis or may be released as fluorescence or heat energy. In vivo fluorescence increases when photosynthesis declines or is inhibited. Numerous environmental f ors can affect the rate of electron transport between photosystem II and photosystem I due to interference with electron carriers between the two photosystems. For example, when the diuton is added in the measured sample, electron transport from photosystem II to photosystem I is blocked resulting in maximum fluorescence. This method was often employed to detect the photosynthetic activity of immobilized photosynthetic material. ... 

In the bacterial reaction center the photons are absorbed by the special pair of chlorophyll molecules on the periplasmic side of the membrane (see Figure 12.14). Spectroscopic measurements have shown that when a photon is absorbed by the special pair of chlorophylls, an electron is moved from the special pair to one of the pheophytin molecules. The close association and the parallel orientation of the chlorophyll ring systems in the special pair facilitates the excitation of an electron so that it is easily released. This process is very fast it occurs within 2 picoseconds. From the pheophytin the electron moves to a molecule of quinone, Qa, in a slower process that takes about 200 picoseconds. The electron then passes through the protein, to the second quinone molecule, Qb. This is a comparatively slow process, taking about 100 microseconds. 

All photoeffects involve the absorption of photons to produce an excited state in the absorber or liberate electrons directly. With the direct release of electrons, photoemission may occur from the surface of solids. While the excited state may revert to the ground state, it may proceed further to a photochemical reaction to provide an electron-hole pair (exciton) as the primary photoproduct. The exciton may dissociate into at least one free carrier, the other generally remaining localized. In an externally applied electric field, photoconduction occurs. Photomagnetic effects arise in a magnetic field. Absorption of photons yield photoelectric action spectra which resemble optical absorption spectra. Photoeffects are involved in many biological systems in which charge transfer takes place (e.g., as observed in the chlorophylls and carotenoids) [14]. 

The chlorophyll-protein complexes are oriented in the lamellar membranes in such a way that the electron transfer steps at the reaction centers lead to an outward movement of electrons. For instance, the electron donated by Photosystem II moves from the lumen side to the stromal side of a thylakoid (see Figs. 1-10 and 5-19). The electron that is donated back to the trap chi (Pgg0) comes from H20, leading to the evolution of 02 by Photosystem II (Eq. 5.8). The 02 and the H+ from this reaction are released inside the thylakoid (Fig. 5-19). Because 02 is a small neutral molecule, it readily diffuses out across the lamellar membranes into the chloroplast stroma. However, the proton (H+) carries a charge and hence has a low partition coefficient (Chapter 1, Section 1.4A) for the membrane, so it does not readily move out of the thylakoid lumen. 

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