Transmembrane transport water

Kutchai HC. Cellular membranes and transmembrane transport of solutes and water. In Berne RM, Levy MN, eds. Principles of Physiology. 3rd ed. St Louis, MO CV Mosby 2000. 

Propose the key elements of a transmembrane protein that transports water through a lipid bilayer (consider the aquaporins). Draw a picture. 

One of the strategies described in Sect. 3 for BLM stabilization, mixing water-insoluble, nonlipid monomers with nonpolymerizable lipids, has also been applied to liposomes. Meier and coworkers [21] created stabilized, nanoscale bioreactors (Fig. 20) by incorporating OmpF, a channel-forming protein, into POPC vesicles to provide for passive transmembrane transport of low molecular weight compounds. p-Lactamase was entrapped during liposome formation, followed by addition of... 

A countercurrent plate-and-frame dialyzer is to be sized to process 1.0 m3/h of an aqueous solution containing 25 wt% H2S04 and smaller amounts of copper and nickel sulfates. A wash water rate of 1000 kg/h is to be used, and it is desired to recover 60% of the acid at 298 K. From laboratory experiments with an acid-resistant vinyl membrane, a permeance of 0.03 cm/min for the acid and a water-transport number of +0.8 were reported. Transmembrane transport of copper and nickel sulfates is negligible. For these operating conditions, it has been estimated that the combined external mass-transfer coefficients will be 0.02 cm/min. Estimate the membrane area required. 

Fig. 18.4 Phosphoric acid dilution, transmembrane transport, and water evaporation in a HT-PEM fuel cell...

The theoretical description of the kinetics of transmembrane transport through a liquid membrane should be based on the principles of solvent extraction kinetics. It should be determined by the processes at both water/membrane interphases and should also involve the intermediate step of diffusion in the membrane. Thus the existence of all these three steps makes the membrane system and its description much more complicated than the relatively simple water/organic phase. However, even the kinetics mechanism in simpler extraction systems is often based on the models dealing only with some limiting situations. As it was pointed out in the beginning of this paper, the kinetics of transmembrane transport is a fimction both of the kinetics of various chemical reactions occurring in the system and of diffusion of various species that participate in the process. The problem is that the system is not homogeneous, and concentrations of the substances at any point of the system depend on the distance from the membrane surface and are determined by both diffusion and reactions. The solution of a system of differential equations in this case can be a serious problem. 

This selective transport across cellular membranes is carried out by two broad classes of specialized proteins, which are assodated with or embedded in those lipid bilayers channels and transmembrane transporters. They work by different mechanisms Whereas channels catalyze the passage of ions (or water and gas in the case of the aquaporin channel) (Agre, 2006) across the membrane through a watery pore spanning the membrane-embedded protein, transporters are working via a cycle of conformational changes that expose substrate-binding sites alternately to the two sides of the membrane (Theobald Miller, 2010). 

Solution—Diffusion Model. In the solution—diffusion model, it is assumed that (/) the RO membrane has a homogeneous, nonporous surface layer (2) both the solute and solvent dissolve in this layer and then each diffuses across it (J) solute and solvent diffusion is uncoupled and each is the result of the particular material s chemical potential gradient across the membrane and (4) the gradients are the result of concentration and pressure differences across the membrane (26,30). The driving force for water transport is primarily a result of the net transmembrane pressure difference and can be represented by equation 5 ... 

Unphosphorylated functioning according to Fig. 5 catalyzes facilitated diffusion of mannitol across the membrane. The same process has been reported for purified II reconstituted in proteoliposomes [70]. The relevance of this activity in terms of transport of mannitol into the bacterial cell is probably low, but it may have important implications for the mechanism by which E-IIs catalyze vectorial phosphorylation. It would indicate that the transmembrane C domain of Il is a mannitol translocating unit which is somehow coupled to the kinase activity of the cytoplasmic domains. We propose that the inwardly oriented binding site which is in contact with the internal water phase (Ecyt Mtl, see Fig. 5) is the site from where mannitol is phosphorylated when transport is coupled to phosphorylation. Meehan-... 

The bonding of K+ and Na+ to A-methylacetamide is of interest64 in studies of the interaction of these ions with peptides and proteins, and particularly studies of the ion transport through transmembrane channels such as the gramicidin channel. Roux and Karplus35 have used the complexation of the given alkali ion with two N-methylacetamide molecules and two water molecules as a model for interactions occurring in transmembrane channels. 

The recent crystallization of the small water-soluble transcriptional regulator of Bacillus subtilis multidrug transporter Bmr, BmrR, which binds hydrophobic cations from the cytosol [64, 65] provides a good example for an interaction determined primarily by van der Waals interactions. Interestingly, the same drugs, which bind to the water-soluble BmrR are also substrates for the transmembrane multidrug transporter Bmr. As will be discussed below, the latter interactions could well be of different nature. 

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