| MadSci Network: Biochemistry |
Hello Kailey!
Sorry for the delay in answering, Now, let's solve this problem..... Well, we know that osmosis ONLY involves the movement of water; and that water moves from "an area of higher water potential to an area of lower water potential". That's easy enough to remember - water flows downhill!
In simple English, water goes from a weak solution to a strong solution. Now, dialysis tubing only allows water and other small molecules to pass through it, so first you make up a STRONG solution of corn syrup (say 20%) and put that IN the dialysis tubing. Tie a knot at one end, and make the tubing about 6" long (This is easier if the tubing is wet first). We use a 10ml syringe to put the solution into the tubing with our students here in the UK, but you could use your funnel. Then rinse the outside of the tubing off, and tie the neck with a piece of cotton, You should now have a 'sausage' about 5" long, BUT IT SHOULD ONLY BE ABOUT HALF FULL.
Place this in a boiling-tube (or beaker) of plain water for about 2 hours or so and you will find that the 'sausage' fills up and becomes TURGID - almost like a balloon. If you are able to, you could set up the apparatus shown here, but you need a length of glass tubing to do that. see: http://www.nicksnowden.net/images/Membranes_and_Transport/ osmosis_demo.JPG
OK, so that dealt with aspect (1) of your problem! Now to deal with aspect (2)...
Dialysis tubing only allows water and SMALL molecules through. So..... Make up one solution of about 20% corn syrup and a second of about 2% starch. To make the 2% starch solution, you need to get the water boiling. Add the starch to the cold water, then use the hotplate to heat it as you stir - or you will get lumps! It's EXACTLY what happens when you make gravy, custard (from powder, not with eggs and milk), or white sauce. See...biology in the kitchen too!
Allow the hot starch solution to cool, then mix it 50:50 with the 20% corn syrup. If you mix equal volumes of BOTH solutions, BOTH will be diluted by 50%, so you will now have a mix of 1% stach and 10% corn syrup - see?
Now, make a 'sausage', just as you did before, and fill it with your solution mixture.
WASH THE OUTSIDE CAREFULLY!!
Now place the sausage into a boiling-tube or beaker of water, and leave it for an hour or so.
Test the liquids inside and outside the 'sausage' for stach (an iodine solution turns blue-black in the presence of starch) and for sugar (Benedict's solution goes orange when you boil it in the presence of sugar).
AT THE START, you get BOTH starch AND sugar INSIDE the sausage (since you put them there!), BUT you get NO REACTION AT ALL with with water on the outside . That is why you need to rinse the outside of the 'sausage' well!.
At the end of an hour or so, you once again test the liquids inside and outside ....but you can guess what will NOW happen....can't you? see http://central.saisd.org/dpts/science/biologyap/student/unit2/Unit%202%20Labs/Osmosis%20and% 20Diffusion%20Lab.htm and http://www.indiana.edu/~phys215/lecture/lecnotes/diff.html and http://www.biologycorner.com/bio1/diffusion.html
Have fun - this is good practical to test how careful you are......
HD2
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