Pore and Charge Question

[Sammy1024]

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I was doing a TBR where the question stated that the pore distinguished by size and charge.

I was wondering why is it that a larger charge would get through a pore slower? I know that's what happens but not sure why.

 

[sillyjoe]

Is this in reference to nanopore sequencing?

 

[Sammy1024]

No, it was about a semipermeable membrane (tbr gchem atomic theory) and how they can distinguish between size, shape and charge. And then the question was something like which of the following would be the slowest if the membrane distinguishes between size and charge.

I used the table to answer but got it wrong, even though my first instinct was that larges charges would be slower. I just wanted to know why a larger charge would be slower, just the reasoning behind it.

 

[sillyjoe]

Can you post the problem please?

 

[Sammy1024]

34. If the pore of the membrane used in the experiment associated with Table 1 were to distinguish by both size and charge, the SLOWEST rate would occur with which cation?

A. Li+
B. K+
C. Mg2+
D. Ca2+

 

[sillyjoe]

Well both size and charge are inversely proportional to diffusion time through a lipid membrane due to the repulsive nature of the semipermeable membrane.

 

[Sammy1024]

So it's basically like larger charge, less likely to pass through because the semi-permeable membrane repulses it. Kind of like it wants to only let in +1 charge so a +2 charge would have a harder time trying to get through since it has a preference.

 

[sillyjoe]

It's all relative. Cellular membranes for example don't want any ions passing through it due to the repulsive nature of the lipophilic portion of the membrane. That said, relatively to a +1 charge, a +2 charge would have a harder time getting through the membrane. At the same time, a smaller molecule/atom would have a relatively easier time than an molecule/atom which is bigger.

 

[Sammy1024]

Oh okay I get it now. Thanks! :]

 

[bmd7]

Quick questions here - doesn't it seem from the table given in this question that for atoms of comparable size (Na+ and Mg2+) and (K+ and Ca2+) that the rate of diffusion is greater for the 2+ charged atoms? Ex rate for K+ = .0035 m/s while for Ca2+it's .0083 m/s?? Any ideas...

 

[victorias]

I was doing a TBR where the question stated that the pore distinguished by size and charge.

I was wondering why is it that a larger charge would get through a pore slower? I know that's what happens but not sure why.

Pores are often lined with calcium so molecules with larger positive charges are going to be repelled. For molecules with negative charges, they are going to engage in greater bonds and associations, so will have a slower diffusion rate. In terms of size, the lager the molecule, the greater the frictional force it will experience as it passes through the pores.

 

[victorias]

Quick questions here - doesn't it seem from the table given in this question that for atoms of comparable size (Na+ and Mg2+) and (K+ and Ca2+) that the rate of diffusion is greater for the 2+ charged atoms? Ex rate for K+ = .0035 m/s while for Ca2+it's .0083 m/s?? Any ideas...

Na+ and Mg2+ are isoelectronic (have the same electron configuration) but since Mg2+ has a greater number of protons, it exerts greater attractive forces on its electrons and thus is smaller - that's why it's rate of effusion is faster than the rate of effusion for Na+. Also refer to the Graham's Law of effusion formula.