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Biology CP - 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 while preventing others from going through.
In this lab, we will use an artificial cell membrane (called dialysis tubing) 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)
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. 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 pipettes 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 spilled glucose from the outside.
Be especially careful to rinse the ends.
e) Place the tubing in a plastic jar and cover with warm water. I will leave a warm water faucet running near the front of the room. 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 iodine to the water in the jar to make it a 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 which is made of many glucose molecules bonded ).
a) Place a small amount of starch solution in a test tube (about 1/4 inch will do)
b) Add a 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
d) Pour about 1/2 inch of this medium blue solution into a clean 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.
Part Five- Go to second lunch to give everything enough time to work. When we come back, go to the back of this sheet for instructions.
Name______________________________ Period______ Date _____________
Observations and Conclusions-
Part One-
Remove the two potato slices and compare their firmness. The one in the ______________water was less firm. This means its cells _________________ (lost or gained) water. Because of this, the _____________ pressure decreased and the cells lost rigidity This happens in _________________ (hypotonic or hypertonic) solution.
Part Two-
Take a small amount of the water from the jar from 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. Observe any color changes that occur. What is present in this water? ____________________ This means the glucose went from _____________________ the tubing to the water ____________________. In other words, it went from ______________ concentration to ____________________________
Part Three-
What color is now present inside the tubing? ____________________ This tells us that the _________________ entered the tubing. Is this same color present outside the tubing? _________ This tells us that the _______________ did not leave the tubing. Which molecules are bigger- starch or iodine?_______________ How do you know? ________________________________________
_______________________________________________________________
Which is a bigger molecule- starch or glucose? ________________ What evidence from today's lab proves this? ________________________________________________________________________
Part Four-
Take some water from the jar and place in a clean test tube. Add 5 drops of Biuret A. Does a lavender color appear? ________This tells you that the protein did not _______________________________. Make a guess why this did not happen. _________________________________________________
Part Five-
Examine the liquid in the test tube from Part Four. Is it still blue? ________ Pour half of this into a clean test tube and add a drop of iodine. Did it turn blue?_____ Is there starch present? ______
Now add 5 drops of Benedict's solution to what you left in the original test tube. Place this test tube in the warm water bath and observe any color changes that occur. This tells us that _____________ is now present in the test tube. What did the diastase do to the starch? (be specific)_____________________
____________________________________________________________________________