Diffusion, cell membranes

Side-by-side diffusion cell (membrane method) Diffusion coefficient determination mass transport studies 10-14... 

AletabolicFunctions. The chlorides are essential in the homeostatic processes maintaining fluid volume, osmotic pressure, and acid—base equihbria (11). Most chloride is present in body fluids a Htde is in bone salts. Chloride is the principal anion accompanying Na" in the extracellular fluid. Less than 15 wt % of the CF is associated with K" in the intracellular fluid. Chloride passively and freely diffuses between intra- and extracellular fluids through the cell membrane. If chloride diffuses freely, but most CF remains in the extracellular fluid, it follows that there is some restriction on the diffusion of phosphate. As of this writing (ca 1994), the nature of this restriction has not been conclusively estabUshed. There may be a transport device (60), or cell membranes may not be very permeable to phosphate ions minimising the loss of HPO from intracellular fluid (61). 

Materials may be absorbed by a variety of mechanisms. Depending on the nature of the material and the site of absorption, there may be passive diffusion, filtration processes, faciHtated diffusion, active transport and the formation of microvesicles for the cell membrane (pinocytosis) (61). EoUowing absorption, materials are transported in the circulation either free or bound to constituents such as plasma proteins or blood cells. The degree of binding of the absorbed material may influence the availabiHty of the material to tissue, or limit its elimination from the body (excretion). After passing from plasma to tissues, materials may have a variety of effects and fates, including no effect on the tissue, production of injury, biochemical conversion (metaboli2ed or biotransformed), or excretion (eg, from liver and kidney). 

Fig. 5. Tentative mixed potential model for the sodium-potassium pump in biological membranes the vertical lines symbolyze the surface of the ATP-ase and at the same time the ordinate of the virtual current-voltage curves on either side resulting in different Evans-diagrams. The scale of the absolute potential difference between the ATP-ase and the solution phase is indicated in the upper left comer of the figure. On each side of the enzyme a mixed potential (= circle) between Na+, K+ and also other ions (i.e. Ca2+ ) is established, resulting in a transmembrane potential of around — 60 mV. This number is not essential it is also possible that this value is established by a passive diffusion of mainly K+-ions out of the cell at a different location. This would mean that the electric field across the cell-membranes is not uniformly distributed.

However, there are disadvantages to using immobilised cells. The cell may contain numerous catalytically active enzymes, which may catalyse unwanted side reactions. Also, the cell membrane itself may serve as a diffusion barrier, and may reduce productivity. The matrix may sharply reduce productivity if the microorganism is sensitive to product inhibition. One of the disadvantages of immobilised cell reactors is that the physiological state of the microorganism cannot be controlled. 

Acetylcholine serves as a neurotransmitter. Removal of acetylcholine within the time limits of the synaptic transmission is accomplished by acetylcholinesterase (AChE). The time required for hydrolysis of acetylcholine at the neuromuscular junction is less than a millisecond (turnover time is 150 ps) such that one molecule of AChE can hydrolyze 6 105 acetylcholine molecules per minute. The Km of AChE for acetylcholine is approximately 50-100 pM. AChE is one of the most efficient enzymes known. It works at a rate close to catalytic perfection where substrate diffusion becomes rate limiting. AChE is expressed in cholinergic neurons and muscle cells where it is found attached to the outer surface of the cell membrane. 

Due to their physicochemical properties trace amines can pass the cell membrane to a limited extent by passive diffusion, with the more lipophilic PEA and TRP crossing membranes more readily than the more polar amines TYR. and OCT. In spite of these features, trace amines show a heterogeneous tissue distribution in the vertebrate brain, and for TYR. and OCT storage in synaptic vesicles as well as activity-dependent release have been demonstrated. So far, trace amines have always been found co-localized with monoamine neurotransmitters, and there is no evidence for neurons or synapses exclusively containing trace amines. 

Fluorescein absorbance is sensitive to pH. This property is utilized to measure cytosolic pH changes. Fluorescein derivatives that contain esters on the carboxyl groups have been constructed. These compounds partition through the cell membrane and, once inside the cell, the esters are cleaved by nonspecific esterases in the cytosol, leaving free carboxyl groups thus the probe cannot diffuse out of the cell (or at least does so slowly). Commonly used derivatives are 6-carboxyfluorescein (21-23) and the more recently developed probe 2, 7 -bis(2-carboxyethyl)-5(and-6)-carboxyfluorescein (13, 24,25). 

W. R. Lieb and W. D. Stem, Non-stochesian nature of the transverse diffusion within hmnan red cell membranes. J. Membr. Biol. 1986, 92, 111-110. 

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