Cells transport pathways

The mutant L37 cbrA21 is affected as regards to its iron uptake pathway mediated by the siderophore achromobactin. Because this mutation results in derepression of the chrysobactin mediated iron transport pathway, the mutant is probably less susceptible to iron deprivation than wild-type cells are, when entering the host. This results in a delay in Pels production thus leading to delayed symptoms, as reported by Sauvage and Expert (1994). 

Stevenson M. Portals of entry uncovering HIV nuclear transport pathways. Trends Cell Biol 1996 6(1) 9-15. 

Figure 14 Ion transport pathways responsible for water flux across intestinal epithelia. Sodium absorption in villus tip cells (left) stimulates water absorption, while chloride channel exit in crypt cells (right) stimulates water secretion.

Let us systematically delineate the transport pathways of the nondissociated and protonated species of the P-blockers by applying Eq. (82). The insignificance of the mass transfer resistance of the ABL on the overall transport process, as evidenced by the lack of influence of stirring on Pe, indicates that the passive diffusional kinetics are essentially controlled by the cell monolayer and filter. Therefore, Eq. (82) simplifies to... 

S Muallem. (1989). Calcium transport pathways of pancreatic acinar cells. Annu Rev Physiol 51 83-105. 

There are two pathways by which a drug molecule can cross the epithelial cell the transcellular pathway, which requires the drug to permeate the cell membranes, and the paracellular pathway, in which diffusion occurs through water-filled pores of the tight junctions between the cells. Both the passive and the active transport processes may contribute to the permeability of drugs via the transcellular pathway. These transport pathways are distinctly different, and the molecular properties that influence drug transport by these routes are also different (Fig. 

A combinatorial library of fluorescent NBD molecules was used to visualize subcellular transport pathways in living cells, using a kinetic, high content imaging system to monitor spatiotemporal variations of intracellular probe distribution <2004MI414>. 

The transport behavior of Li+ across membranes has been the focus of numerous studies, the bulk of which have concentrated upon the human erythrocyte for which the Li+ transport pathways have been elucidated and are summarized below. The movement of Li+ across cell membranes is mediated by transport systems which normally transport other ions, therefore the normal intracellular and subcellular electrolyte balance is likely to be disturbed by this extra cation. Additionally, Li+ has been shown to increase membrane phospholipid unsaturation in rat brain, leading to enhanced fluidity in the membrane, which could have repercussions for membrane-associated proteins and for membrane transport properties. 

Erythrocytes share some transport mechanisms with many cell types although, surprisingly, the Li+ transport pathways in other cells have not... 

Pelkmans L, Kartenbeck J, Helenius A. Caveolar endocytosis of simian virus 40 reveals a new two-step vesicular-transport pathway to the ER. Nat Cell Biol 2001 3(5) 473 83. 

Taiz and Zeiger (2002) give a full account of this topic. Mineral ions absorbed from solution outside the root surface must be transported across the root to the main long-distance transport vessels in the xylem, through which they reach the shoot. This process is highly specific for different ions and molecules and is closely regulated. The regulation is in part a fnnction of the anatomy of the varions root tissues and in part a fnnction of active transport processes in root cells. The pathways and transport processes are affected by root adaptations to anoxia. 

Two particular aspects of the transport of degradable contaminants were considered in laboratory experiments that used soil originating from the field experiments described in the previous sections. Studies on diffnsion of degradable insecticides were performed in diffusion cells, while the spatial redistribntion of pesticides from a point source was measured in specially designed pans (60 cm high, 40 cm diameter). Periodic sampling and contaminant analysis enabled visnaUzation of the contaminant transport pathway. 

Polar Cell Systems for Membrane Transport Studies Direct current electrical measurement in epithelia steady-state and transient analysis, 171, 607 impedance analysis in tight epithelia, 171, 628 electrical impedance analysis of leaky epithelia theory, techniques, and leak artifact problems, 171, 642 patch-clamp experiments in epithelia activation by hormones or neurotransmitters, 171, 663 ionic permeation mechanisms in epithelia biionic potentials, dilution potentials, conductances, and streaming potentials, 171, 678 use of ionophores in epithelia characterizing membrane properties, 171, 715 cultures as epithelial models porous-bottom culture dishes for studying transport and differentiation, 171, 736 volume regulation in epithelia experimental approaches, 171, 744 scanning electrode localization of transport pathways in epithelial tissues, 171, 792. 

INTESTINE Characterization of a membrane potassium ion conductance in intestinal secretory cells using whole cell patch-clamp and calcium-sensitive dye techniques, 192, 309 isolation of intestinal epithelial cells and evaluation of transport functions, 192, 324 isolation of enterocyte membranes, 192, 341 established intestinal cell lines as model systems for electrolyte transport studies, 192, 354 sodium chloride transport pathways in intestinal membrane vesicles, 192, 389 advantages and limitations of vesicles for the characterization and the kinetic analysis of transport systems, 192, 409 isolation and reconstitution of the sodium-de-pendent glucose transporter, 192, 438 calcium transport by intestinal epithelial cell basolateral membrane, 192, 448 electrical measurements in large intestine (including cecum, colon, rectum), 192, 459... 

Ion transport pathways across the luminal and basolateral membranes of the thick ascending limb cell. The lumen positive electrical potential created by K+ back diffusion drives divalent (and monovalent) cation reabsorption via the paracellular pathway. NKCC2 is the primary transporter in the luminal membrane. 

Ion transport pathways across the luminal and basolateral membranes of collecting tubule and collecting duct cells. Inward diffusion of Na+ via the epithelial sodium channel (ENaC) leaves a lumen-negative potential, which drives reabsorption of and efflux of K+. (R, aldosterone receptor.)... 

HDL may be taken up in the liver by receptor-mediated endocytosis, but at least some of the cholesterol in HDL is delivered to other tissues by a novel mechanism. HDL can bind to plasma membrane receptor proteins called SR-BI in hepatic and steroidogenic tissues such as the adrenal gland. These receptors mediate not endocytosis but a partial and selective transfer of cholesterol and other lipids in HDL into the cell. Depleted HDL then dissociates to recirculate in the bloodstream and extract more lipids from chylomicron and VLDL remnants. Depleted HDL can also pick up cholesterol stored in extrahepatic tissues and carry it to the liver, in reverse cholesterol transport pathways (Fig. 21-40). In one reverse transport path, interaction of nascent HDL with SR-BI receptors in cholesterol-rich cells triggers passive movement of cholesterol from the cell surface into HDL, which then carries it back to the liver. In a second pathway, apoA-I in depleted HDL in-... 

The uptake of potassium by microorganisms has been well studied. In the case of E. coli, kinetic investigations on different strains have demonstrated the presence of three or four transport systems. The presence of inducible pathways with widely different Km values for binding K+ allows the cell to accumulate K+ to a constant level under different environments. These transport pathways include those linked to the proton circuit and an example linked to a pump and ATP hydrolysis. Thus the Kdp system is a high affinity pathway with Km = 1 mol dm-3, and involves three proteins in the inner membrane of E. coli, including a K+-stimulated membrane ATPase. The KHA (i.e. K+-H+ antiport) path is driven by proton motive force, while the low affinity system TrKA depends on both ATP and the proton motive force.80,81,82 S. cerevisiae accumulates K+ by K+/H+ exchange.83 Potassium transport may thus be used to control intracellular pH. 

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