Really confused with this question. Please help!

Question

The following diagram (sorry, cant put in the diagram)shows what happened 10 minutes after the start of an experiment. The starch suspension in the membrane turned blue-black while the iodine solution remained brown. 

 

(at the start, there's a beaker with a membrane filled with white starch suspension. There's also pale brown iodine surrounding the membrane. After 10 minutes, the starch suspension in the membrane turned from white to blue-black and the membrane increased in size. The iodine surrounding it is still brown.)

 

Which of the following statements describe the following results?

 

1. starch molecules in the membrane are too big to pass through the membrane but the iodine molecules are small enough to enter. 

2. Iodine molecules enter the membrane by diffusion. 

3. Iodine molecules enter the membrane by osmosis

4. The membrane is fully permeable. 

 

A  1,3,4 only

B  2,4 only

C  1,2,4 only

D  1,2 only (This is the correct answer)

 

 

I am am very very confused by this question. I know that I can eliminate point 4 since it's totally wrong. So the only answer left is D.

 

However, all I want to know is:

 

1. why is it diffusion that is occurring? Not osmosis?! Doesn't osmosis occur with a partially permeable membrane?

 

2. And I thought that diffusion does not occur if there is a partially permeable membrane. Is it true? And does diffusion occur in liquids? 

 

3. Does osmosis only involve the movement of water molecules? If it involves the movement of other molecules through a partially permeable membrane, (eg. Iodine), it would be diffusion and not osmosis right?

 

Please help to answer all my three questions. Thank you! I really need clarification for this. So confused. 

 

 

Answer 1

Hi Amy!

 

Perhaps someone who has taught/tutored this subject more can give a more finely detailed answer.  But, from what I remember and can see in references, osmosis -- which does involve a semi-permeable membrane -- involves the movement of solvent, not solute, through the membrane.  In this case, it looks like the iodine is dissolved in the suspension inside the membrane, so it is the solute, not the solvent.  That means this process falls under the broader category of diffusion.

 

Diffusion is just the process of random movement through a space, including a semi-permeable membrane.

 

And the key to osmosis, as I mentioned, is that it involves the movement of solvent through a membrane to a region of higher solute concentration.  The key here is that iodine is the solute, being dissolved IN the interior suspension.

 

I hope that helps!  If you have other questions, I or another tutor on here can try to answer them.