Transport pathways

The first study was conducted to determine whether carotenoids and cholesterol share common pathways (transporters) for their intestinal absorption (During et al., 2005). Differentiated Caco-2 cells on membranes were incubated (16 h) with a carotenoid (1 pmol/L) with or without ezetimibe (EZ Zetia, an inhibitor of cholesterol transport), and with or without antibodies against the receptors, cluster determinant 36 (CD36) and scavenger receptor class B, type I (SR-BI). Carotenoid transport in Caco-2 cells (cellular uptake + secretion) was decreased by EZ (lOmg/L) as follows P-C and a-C (50% inhibition) P-cryptoxanthin and LYC (20%) LUT ZEA (1 1) (7%). EZ reduced cholesterol transport by 31%, but not retinol transport. P-Carotene transport was also inhibited by anti-SR-BI, but not by anti-CD36. The inhibitory effects of EZ and anti-SR-BI on P-C transport... 

Transport of pheromones. Two major routes for translocation of pheromones have been considered in this chapter (a) from the secretory cell directly through the cuticle overlying it and (b) indirectly through the hemolymph. Cockroaches share with even the most studied lepidopterans an almost complete lack of information on the former pathway. Transport of hydrocarbons and contact sex pheromones, on the other hand, has been extensively studied in cockroaches, commencing with the work of Chino and colleagues. It has... 

Except for the conversion of glyceraldehyde 3-phosphate to 1,3-bisphosphoglycerate occurring in the cytoplasm, all of the above reactions take place in the mitochondria. Since mitochondria are not permeable to NADH, two shuttle pathways transport the reducing equivalents of cytoplasmic NADH into the mitochondria. 

Koushik K, Bandi N, Kompella UB. Respiratory delivery of a peptide drug degradation pathways, transport mechanisms, and sustained release large porous microparticles prepared using supercritical fluid technology. AAPS Pharm Sci 2002 4, (Abstract R6117). 

Vesicular Traffic in the Secretory and Endocytic Pathways Transport from the Plasma Membrane via Endosomes Endocytosis—... 

Two translocation pathways are operative in eukaryotes, i.e. co-translational and post-translational. In the co-translational pathway, transport occurs while the polypeptide chain is being synthesized on a membrane-bound ribosome. In the post-translational pathway, the polypeptide chain is completed in the cytoplasm before being transported. In prokaryotic, ribosomes do not seem to be tightly bound to the membrane and most proteins may be transported post-translationally or after much of the chain has been synthesized. 

The extent of a hazard is dependent on the mechanism of the toxicity, which involves the metabolic pathway, transport, distribution and interaction processes of the pesticide or metabolites in the different human tissues. This hazard must be assessed not only from isolated studies, but also by considering common mechanisms of toxicity that different pesticides may share. 

Figure C2.12.7. Channel system of MFI (top) and MEL (bottom). The linear channels are interconnected by zigzag channels in ZSM-5 while exclusively straight miming channels are present in ZSM-11 - larger internal openings are present at the chaimel intersections - the arrows indicate the pathways for molecular transport tlirough the channel system.

Traditionally, the electron and proton transport pathways of photosynthetic membranes (33) have been represented as a "Z" rotated 90° to the left with noncycHc electron flow from left to right and PSII on the left-most and PSI on the right-most vertical in that orientation (25,34). Other orientations and more complex graphical representations have been used to depict electron transport (29) or the sequence and redox midpoint potentials of the electron carriers. As elucidation of photosynthetic membrane architecture and electron pathways has progressed, PSI has come to be placed on the left as the "Z" convention is being abandoned. Figure 1 describes the orientation in the thylakoid membrane of the components of PSI and PSII with noncycHc electron flow from right to left. 

The documented occurrence of pesticides in surface water is indicative that mnoff is an important pathway for transport of pesticide away from the site of appHcation. An estimated 160 t of atra2ine, 71 t of sima2ine, 56 t of metolachlor, and 18 t of alachlor enter the Gulf of Mexico from the Mississippi River annually as the result of mnoff (47). Field appHcation of pesticides inevitably leads to pesticide contamination of surface mnoff water unless mnoff does not occur while pesticide residues remain on the surface of the soil. The amount of pesticides transported in a field in mnoff varies from site to site. It is controUed by the timing of mnoff events, pesticide formulation, physical—chemical properties of the pesticide, and properties of the soil surface (48). Under worst-case conditions, 10% or more of the appHed pesticide can leave the edge of the field where it was appHed. 

Eor pesticides to leach to groundwater, it may be necessary for preferential flow through macropores to dominate the sorption processes that control pesticide leaching to groundwater. Several studies have demonstrated that large continuous macropores exist in soil and provide pathways for rapid movement of water solutes. Increased permeabiUty, percolation, and solute transport can result from increased porosity, especially in no-tiUage systems where pore stmcture is stiU intact at the soil surface (70). Plant roots are important in creation and stabilization of soil macropores (71). 

A simplified diagram representing the various reservoirs and transport mechanisms and pathways involved in the cycles of nutrient elements at and above the surface of the Earth is given in Eigure 1. The processes are those considered to be the most important in the context of this article, but others of lesser significance can be postulated. Eor some of the elements, notably carbon, sulfur, chlorine, and nitrogen, considerable research has been done to evaluate (quantitatively) the amount of the various elements in the reservoirs and the rates of transfer. 

Eig. 1. Generalized cycle of the various reservoirs and transport mechanisms and pathways involved in the circulation of nutrient elements. The numbered arrows represent processes by which elements transfer among the reservoirs. Processes shown are those considered to have the most important influence... 

In polycrystalline materials, ion transport within the grain boundary must also be considered. For oxides with close-packed oxygens, the O-ion almost always diffuses much faster in the boundary region than in the bulk. In general, second phases at grain boundaries are less close packed and provide a pathway for more rapid diffusion of ionic species. Thus the simplified picture of bulk ionic conduction is made more complex by these additional effects. 

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