Chlorophyll functions

In contrast to a conventional p-n-junction-type solar cell, the mechanism of the DSSC does not involve a charge-recombination process between electrons and holes because electrons are injected from the dye photosensitizers into the semiconductor, and holes are not formed in the valence band of the semiconductor. In addition, electron transport takes place in the Ti02 film, which is separated from the photon absorption sites (i.e., the photosensitizers) thus, effective charge separation is expected. This photon-to-current conversion mechanism of the DSSC is similar to that for photosynthesis in nature, where chlorophyll functions as the photosensitizer and electron transport occurs in the membrane. 

Triazines, ureas, carbamates and acylanilides block light reaction II, stopping transport of the electrons to the chlorophyll functional units. Therefore, after a certain time the chlorophyll is oxidised and decolourises. The visible effect of this herbicidal action is chlorosis of the leaves. 

Resonance transfer is important for the way chlorophylls function in cells. Only about 1 02 molecule is produced for every 2500 chlorophylls. Instead, most of the chlorophyll molecules act as antenna molecules of the light-harvesting complexes. Antenna molecules absorb photons and the energy is passed by resonance transfer to specific chlorophyll molecules in a relatively few reaction centers. The path the energy takes to arrive at the energy center is random (Figure 17.11). 

Katz, J.J., Chlorophyll function in photosynthesis, in The Chemistry of Plant Pigments, Chichester, C.O. (Ed.), Academic Press, New York, 1972, 103. 

All organisms seem to have an absolute need for magnesium. In plants, the magnesium complex chlorophyll is the prime agent in photosynthesis. In animals, magnesium functions as an enzyme activator the enzyme which catalyses the ATP hydrolysis mentioned above is an important example. 

This symmetry is important in bringing the two chlorophyll molecules of the "special pair" into close contact, giving them their unique function in initiating electron transfer. They are bound in a hydrophobic pocket close to the symmetry axis between the D and E transmembrane a helices of both... 

Hemin is shown on the right in Figure 22-7. It is shown beside the model of chlorophyll A to emphasize the astonishing similarity. The portions within dotted lines identify the differences. Except for the central metal atom, tl.e differences are all on the periphery of these cumbersome molecules. We cannot help wondering how nature managed to standardize on this molecular skeleton for molecules with such different functions. We cannot avoid a feeling of impatience as we await the clarification of the possible relationship, a clarification that will surely be provided by scientists of the next generation. 

Recently, the ocean-basin distribution of marine biomass and productivity has been estimated by satellite remote sensing. Ocean color at different wavelengths is determined and used to estimate near-surface phytoplankton chlorophyll concentration. Production is then estimated from chlorophyll using either in situ calibration relationships or from empirical functional algorithms (e.g., Platt and Sathyendranth, 1988 Field et al., 1998). Such studies reveal a tremendous amount of temporal and spatial variability in ocean biological production. 

This chapter brings together information concerning structural features, spectral characteristics, distributions, and functions of major chlorophylls in photosynthetic organisms. Other topics discussed include biosynthesis and degradation in senescent plants and ripening fruits and potential biological properties of chlorophylls. 

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