An Easy Bio-Diffusion Question

[jamzincanada]

Hi!! I'm trying to help my wife who is Korean and talking a Bio 12 course. She only came to Canada 3 years ago. She's working really hard to understand the concepts, so I'm trying to help her too. I wonder if you folks could offer an explanation for this problem.

Our question is based on a simple Bio Diffusion Experiment. Here it is.

A 2% glucose and 3% sucrose solution is added to a bag made from a membrane that is permeable only to water and glucose. The bag was then placed in a beaker of water containing a 1% glucose and 1% sucrose solution. What would happen to the concentration levels of sucrose and glucose inside the bag after one hour? (My wife thinks that sucrose is still going out of the bag somehow. I don't think so though, but I'm not a bio expert.

How would you describe the concentrations of sucrose and glucose in the bag after one hour?

Thanks a million from Kelowna, BC.

[canalon]

The bag is permeable to 2 molecules, not only glucose. Molecules (all types, not just sugar) can diffuse.
I guess nothing was said about the size of the bag?
So no elbow room added: concentration affects only the volume of solution (between 1 and 3% of any sugar that would be minuscule) and the molecule you are measuring. The movement of the other molecules do not affect it significantly. And profs are not mean, they do not make exercises with secretly leaky bag, that would be counter productive. At least not until grad school, where learning to correct the failures of your experiments and troubleshooting is a highly valuable skill. But the question would be different, because it would be all about controls, calibration and trouble shooting measures.

[jamzincanada]

Are you saying that sucrose is also going out of the bag? The experiment specifically states that the membrane is only permeable to water and glucose.

Thanks a zillion.

[JackBean]

no, I think he's trying to say, it's permeable to water and glucose, but not sucrose

[jamzincanada]

Thanks for that answer.

I also still need to hear in simple terms what is happening to the concentrations of glucose and sucrose inside the bag after one hour and why. Specifically is the concentration of Sucrose also going down, if so and why? The experiment says the membrane is only permeable to water and glucose.

Thanks..

[JackBean]

Yes. We know that. But you obviously don't.

If the sucrose cannot can in or out. How can you achieve equilibrium?

[canalon]

no, I think he's trying to say, it's permeable to water and glucose, but not sucrose

I guess nothing was said about the size of the bag?

Mmm.. If the sucrose cannot move, I wonder what could then other than glucose....

[JackBean]

LOL, I didn't want to write it that way...

[Darby]

Just figure that, whatever CAN move through the barrier, the net movement will be from the higher concentration area to the lower concentration area, until the concentrations are equal (which might happen in an hour, but you can't really be sure with what you've been given.

Sucrose can't change, since it can't get through. For every 2 molecules of glucose going out, only one is entering, so the concentrations will gradually shift until balanced.