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"The knowledge about semi-permeable membranes is applied for various purposes. We know that semi-permeable membrane is capable of separating substances when a driving force is applied across it. Artificially synthesized semi-permeable membranes are used for separation of bacteria from viruses, because bacteria cannot cross a semi-permeable membrane."

My Question is Why Bacteria cannot cross a semi-permeable membrane? Reason?

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In my opinion, it should work by size exclusion. Bacteria are on the size of few microns, whereas the pores in semi-permeable membranes are on the size of nanometers. Therefore, bacteria will not pass through the membrane.

Viruses are usually smaller than bacteria, and so maybe more likely to pass through such a membrane. However, you have not included the pore size of the semipermeable membrane in question, and so I am unable to give a more specific answer at this point. If the pores of the membrane are very small, even viruses will not be able to pass through it.

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    $\begingroup$ Please add some references. $\endgroup$
    – L.B.
    Dec 4, 2016 at 19:59
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A semipermeable membrane is like a sieve. It has pores on the surfaces and tunnels that connect them. Molecules may pass through depending on their size. The pores in semipermeable membranes may be highly elliptical and the shape of molecules branched and complex rather than spherical.

However, the pore size (for example in the Polyflux 210H commonly used in hemodialysis) is 34 and 45 nm. There are spherical bacteria but often not and their size is usually greater than 0.5 micro meters. That is more than 10 times the size of pore in a Polyflux 210H semipermeable membrane.

Some viruses however are 20nm and may pass through though typically viruses are in the 100nm range.

Here is a paper on the morphology of the 210H with SEM images.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3502870/

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