permeable and semi permeable membranes?

[purple_rose]

permeable and semi permeable membranes? What's the difference? I've tried searching the net but theres a lot on semi permeable membranes but not that much on plain permeable membranes.

[DrowNz]

For the purpose of VCE Biology, we do not tend to deal with 'plain' permeable membranes.

A 'plain' permeable membrane would apply less selectivity pressure and will be more open for free passage of a variety of molecules and materials.

A semi-permeable membrane, also selectively-permeable membrane, only allows a distinct, selected materials to pass through (by diffusion/osmosis) such as small molecules, ions and etc.

[chana]

The Cell Membrane acts as a semi-permeable membrane. This means that it is selective to what enters and leaves the cell.

[bucket]

Glucose cannot pass through a semi-permeable membrane surrounded by h2o, can it?

[nancy]

glucose is a large molecule it is to big to pass through the membrane, it requires active transport!!!

[bucket]

In a prac i just did, glucose solution was put into a dialysis bag(semi permeable membrane) and the bag was put into water, after 1hour we tested the water for glucose and there WAS glucose present in the water, why is that?
It surely can't be due to error can it? The whole class got the same results, wtf!
Any reason other than error that glucose may have passed through the membrane?
We did the same test with starch solution and it wasn't able to get through the membrane. Starch is a bigger molecule then glucose, so is it possible that glucose is small enough to pass through the membrane or should I just rule it out as error?

[Toothpaste]

Glucose is polar therefore it's insoluble in the membrane's bilayer (which also means that it's hydrophilic - H2O is polar and "like-dissolves-like"). This rules out simple diffusion.

Glucose relies on facilitated transport, which is passive, to diffuse through the membrane from an area of high to low concentration.

Active transport of glucose would mean that it is travelling from an area of low concentration to an area of high concentration. I'm pretty sure this happens with liver cells where there is a higher concentration of glucose, and it takes in more glucose against a concentration gradient (from blood).

In a prac i just did, glucose solution was put into a dialysis bag(semi permeable membrane) and the bag was put into water, after 1hour we tested the water for glucose and there WAS glucose present in the water, why is that?
It surely can't be due to error can it? The whole class got the same results, wtf!
Any reason other than error that glucose may have passed through the membrane?
We did the same test with starch solution and it wasn't able to get through the membrane. Starch is a bigger molecule then glucose, so is it possible that glucose is small enough to pass through the membrane or should I just rule it out as error?

Ah, I remember doing that same practical in unit 1 biology!

Well starch molecules are large polysaccharides, so they can't fit through the differentially permeable membrane. The starch solution in the dialysis bag should have turned dark blue/blackish. Iodine molecules are smaller than starch, and the dialysis bag/membrane prevented the starch from leaving it.

Yes, glucose diffused through the membrane and into the water, from a high to low concentration. That's meant to happen so it's not an error. Size determines whether something passes through or not.

Did you measure the thistle funnel with the glucose solution? The volume should have dropped gradually as time passed (we did this over two days, so there was significant change), also supporting the fact that glucose diffused out.
With the starch solution the water level in the funnel should've risen, due to more molecules diffusing into the bag.

Hope that helped. If this is wrong ... I'll just go cry.

[bucket]

Ahh yes thanks a lot toothpick, that helped quite a bit lol.
We did use a thistle funnel with the starch and yes, the level raised, and the starch solution changed colour very slightly indicating that there had been osmosis and diffusion into the bag as you said :p

Thanks again =]

[beezy4eva]

bucket did yous do the potato cubes in salt solution prac as well?

[bucket]

Nah we didn't do that one, it was on the sheet though.

[Saraime2003]

I did all of these pracs... lol i only realised in the potato chips one that i was thinking of diffusion in the wrong way, like i thought the other thing was diffusing into the potato than what it actually was, quite nervous now, i haven't got that sac back yet >< lol