Transport light-driven active

Figure 1).1>2 ye have found that the rate of K " transport is significantly enhanced by u.v. light irradiation or by alternate irradiation of u.v. and visible light.Unfortunately, this system cannot be used to effect "light-driven active transport", because the reverse transport cannot be suppressed to zero. On the other hand, the photo-and pH-dependent inter conversion between trans-l cis-lH+ may be useful to achieve "active BC -transport", because the extractabilltles of cis-lH (n=6) and cis-lH+(n=10) for K" are almost close to zero (Table III). 

As an attempt to connect the first discussion, which was concerned with diffusion-reaction coupling, with Dr. Williams presentation of enzymes as dynamic systems, I wanted to direct attention to a number of specific systems. These are the energy-transducing proteins that couple scalar chemical reactions to vectorial flow processes. For example, I am thinking of active transport (Na-K ATPase), muscular contraction (actomyosin ATPase), and the light-driven proton pump of the well-known purple... 

The simplest ATP-driven reaction, ATP-driven proton uptake, has already been discussed. After activation the membrane-bound ATP synthase pumps protons into the inner thylakoid space, coupled to ATP hydrolysis. Both 4pH and Aip are produced with magnitudes similar to those produced by light-driven proton transport [51.77]. 

Geue et al. investigated the thermal erasure of SRGs obtained via light-driven mass transport, at various temperatures, which allowed them to estimate the activation energy for erasure to be 2.6 eV. For erasing at T < Tg, the SRG was erased by flow of polymer material perpendicular to the initial surface, accompanied by the formation of an intrinsic density grating, as commented on in the work of reference 95. At T > Tg, the lateral density modulation was equalized by a lateral flow of material. 

Hydrogen ions are often needed in biochemical reactions carried out by enzymes. Several such enzymes use water wires to shuttle the ions from the solvent through the protein and into the active site where the reaction takes place. This happens, for example, in cytochrome and peroxidase enzymes, and in bacterio-rhodopsin, a light-driven proton pump used by some Archaea to transport protons across a membrane during the conversion of light energy into chemical energy. 

The thylakoids and stroma are the sites of the so-called light and dark reactions of photosynthesis, respectively. This compartmentalization of photosynthetic functions was recognized by Park and Pon when they broke open the chloroplasts, separated the contents into thylakoid and stroma fractions and examined their properties. The specific activities of the thylakoids include photochemical reactions, electron transport, oxygen evolution, ATP synthesis and NADP reduction, while the stroma contains enzymes for CO2 fixation driven by ATP and NADPH and other biochemical reactions in the dark. Our understanding and appreciation of the detailed structure and organization of the thylakoid membranes has increased tremendously in recent years. Further discussion of thylakoid structure will be continued in section VII on page 26. 

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