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Biology H-Mr.Willets
Lab- Osmosis
It is vital to cells that certain substances be allowed to enter and leave and other substances not be able to do so. This is how a cell maintains homeostasis. The cell membrane is the structure that controls this. It is semi-permeable and has pores that allow certain size and type molecules to easily pass through.
In this lab, we will use an artificial cell membrane to demonstrate how a real one operates. We will also demonstrate the effects of hypertonic and hypotonic solutions on certain cells.
Part One-
a) Get two slices of potato
b) Place one slice in a small plastic jar and cover with distilled (100%) water.
c) Place the other in a small plastic jar and cover with salt solution (85% water, 15% salt).
d) Label each jar (a piece of paper underneath will do) and set aside until later.
Part Two-
a) Get a section of dialysis tubing. (dialysis- the diffusion of molecules other than water through a semi-permeable membrane) This tubing will act as our cell membrane.
b) Wet it to open it as shown in class and tie a knot in one end.
c) Use the pipette to add glucose solution to the tubing. Leave enough room in the tubing to tie off
the open end.
d) Tie off the open end and rinse the tubing in water to remove any glucose that may have gotten on the outside of the tubing. Be particularly careful about rinsing the ends.
e) Place the tubing in a plastic jar and cover with warm water from either of the two front sinks. It is OK if you bend the tubing to get it into the jar.
f) Place this jar on a piece of paper with "Glucose -Part Two" written on it.
Part Three-
a) Repeat Part Two with these changes:
1) Place starch solution in the tubing instead of glucose.
2) Add several drops of iodine to the water in the jar. Enough to make it light brown.
3) Place this jar on a piece of paper with "Starch-Part Three" written on it.
Part Four-
a) Fill a section of dialysis tubing (same procedure as before) with protein solution.
b) Place this in a jar of water and label- "Protein"
Part Five-
Before large molecules can enter or leave cells, they must be broken down into smaller molecules. This is accomplished by means of hydrolytic enzymes. One such enzyme, diastase, breaks down starch molecules into individual glucose molecules. (Remember that starch is a polysaccharide made of many glucose molecules bonded together)
a) Place a small amount of starch solution in a test tube (about 1/4 inch will do)
b) Add one drop of iodine and mix. What color did the starch become?______________
c) Add some water to the test tube to dilute the starch so that the blue color is medium rather than dark. Check with your friendly teacher about the color.
d) Pour about 1/2 inch of this solution into another test tube and discard the rest.
e) Bring this test tube to your friendly teacher who will add some diastase powder
f) Place this test tube in the rack. We will observe it later.
We will now go to second lunch. This will give everything time to work. When we return, follow the instructions on the back.
Observations and Conclusions-
For each part, you will write down your observations and conclusions as outlined below. Today, you should concentrate on the observations and do the conclusions later. Think before you write your conclusions. Make sure that what you say is clear and in a logical order. This should be done on a separate sheet.
Part One-
Remove the two potato slices and compare their firmness. In which solution was the firmer slice ? ______________________
Explained what happened for each slice of potato. Why was one firmer and the other less firm? Use all the big words you learned in class to describe the two solutions and what happened in each.
Part Two-
Take a small amount of the water from the jar used in part two and place in a clean test tube. Add 5 drops of Benedict's solution and place the test tube into the warm water bath. What color did the solution change to?_________ This tells you that_____________ is present in the water in the jar.
Part Three-
Examine the jar and tubing from part three. What color appears inside the dialysis tubing? __________ What color is the water in the jar? ____________
Using your observations from Parts Two and Three- In this part, ignore the movement of water and deal only with the glucose, starch and iodine. Describe both situations in terms of concentration gradients and what movement of molecules would be expected. Did the molecules move as expected? Which ones passed through our membrane and which ones did not? Give evidence to support your statements. Why do you think they were or were not able to diffuse through?
Part Four-
Take some water from the jar and place in a test tube. Add 5 drops of Biuret A. Does a violet color appear? What did or did not happen and why?
Part Five-
Examine the liquid in the test tube from Part Four. There is probably some sediment at the bottom. Try not to disturb it. Is the solution still a blue color?_________ Pour half of this liquid into a clean test tube. Try to leave the sediment behind. Add a drop of iodine to this new test tube. Is there starch present? _____
Now add 5 drops of Benedict's solution to the solution left in the original test tube. Place the test tube in the warm water bath. What color did it change to? _______________ This tells us that there is ____________ present.
Explain what happened as a result of adding the diastase to the starch solution, giving evidence to prove your point.