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A question on dialysis tubing...help appreciated.

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A question on dialysis tubing...help appreciated.

Postby persian » Sun Jul 22, 2007 4:29 pm

Hi,

I wanted to know 2 differences and a similarity of dialysis tubing compared to the cell membrane.

Thanks in advance.
persian
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Postby blcr11 » Mon Jul 23, 2007 4:27 pm

The most obvious difference is composition. I’m not that involved with things like kidney dialysis membranes, so I can’t say too much about their composition, though I presume they are polymers of one sort or another. Dialysis tubing used in the lab to purify or desalt or concentrate proteins are generally made of cellulose. The pore size determines the molecular weight cut-off of the tubing; molecules that are smaller than the cut-off size can freely diffuse across the tubing while molecules larger than the cut-off stay on whatever side they’re put—usually the inside. Unless the polymer used to make the tubing affords some kind of selection (and I’m not aware of any typical laboratory-type membranes that do), there is no preferential transport of ions across dialysis tubing; that is, sodium, potassium, calcium, sulfate, carbonate—anything small like that can freely cross a dialysis membrane with little if any charge discrimination. The driving force for transport is strictly diffusional. Transport occurs in the direction from highest to lowest concentration of diffusible substance. No active transport is possible. What selectivity there is is based on molecular weight alone.

Biological membranes are lipid bilayers dotted with proteins of one sort or another. There are ion-selective pores that can discriminate between ions, allowing potassium or chloride ions to pass, but not sodium or sulfate, to name two. Active transport is possible for biological membranes so that substances can be transported against a concentration gradient (i.e, in the opposite direction that simple diffusion would have things move). Both dialysis membranes and biological membranes can discriminate between diffusible and non-diffusible substances, but the mechanism of discrimination for dialysis tubing is almost purely mechanical—does it fit through the hole or not—while the mechanism for biological membranes is usually more chemical—does it bind to the receptor, or the ion-selective channel. And then there’s the issue of active transport (which only biological membranes can do) versus passive diffusion (which both membranes can do).
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