Name ___________________________________________________ Period Date

Osmosis and Diffusion Lab ActivityEXERCISE A: DIALYSIS BAG DIFFUSION

Background:Now that we’ve studied the cell membrane you know that molecules like water and

glucose, among other things, must make their way across the cell membrane. How do cells control which substances are allowed to move across the cell membrane and which cannot? This lab will focus on understanding how the cell membrane controls what enters and leaves the cell.

In this lab experiment, we’re going to create a model of a cell using dialysis tubing as the cell membrane. Dialysis tubing is very similar to a real cell membrane because it only allows certain things (tiny molecules) to pass through it. Inside of our model cell, we’ll put some nutrients that you might find inside the digestive system – starch and glucose, mixed with water.

Diffusion is the random movement of molecules from an area of higher concentration of those molecules to an area of lower concentration. For example, if one were to open a bottle of hydrogen sulfide (H2S has the odor of rotten eggs) in one corner of a room, it would not be long before someone in the opposite corner would smell rotten eggs. The bottle has a higher concentration of H2S than the room does, so the gas diffuses from the area of higher concentration to the area of lower concentration. The question in this lab is, which of the nutrients- glucose or starch- will be able to diffuse through our model cell membrane?

Osmosis is a process by which water moves through a selectively permeable membrane. The osmotic process is a special case of diffusion involving the diffusion of a solvent, such as water, rather than the diffusion of substances (solutes) dissolved in the solvent.

Osmosis takes place whenever there is an unequal concentration of water on either side of a selectively permeable membrane. Water will continue to move across the concentration gradient until equilibrium occurs.

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Model cell with dialysis tubing membrane

Starch & glucose solution

Predictions:

1. Remind yourself what glucose and starch are by filling out the below table.

Carbohydrate, Lipid, or Protein?

Monomer or Polymer?

GlucoseStarch

2. I believe that glucose will/will not (circle one) cross the membrane because _____________________________________________________________________________________________________________________

3. I believe that starch will/will not (circle one) cross the membrane because ______________________________________________________________________________________________________________________

4. To test your hypothesis, we need to be able to see if starch and/or glucose moves into the beaker.

a) To test for STARCH, we will put iodine in the beaker. Iodine is normally orange but turns blue/purple when it touches starch.

If starch crosses the membrane, how will you know? _________________________________________

b) To test for GLUCOSE, we will use a glucose test strip. This is a plastic strip with a chemical indicator at the end. It is normally a bright turquois but turns a different color when it touches glucose.

If glucose crosses the membrane, how will you know? ________________________________________

Procedure:

1. Fill a plastic cup approximately 2/3 full with water. Add 2 eye-droppers full of IKI (Potassium Iodine) to the water in the cup. Gently stir the solution until you have an even mixture.

****The IKI solution is an irritant; if affects skin and eyes, and can stain clothing. Handle the solution with caution. Wash off spills and splashes with water.****

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Water

2. Determine if glucose is present in the water/IKI solution by dipping a glucose indicator strip into the cup. Record the data in Table 1 (write “yes” if glucose is present or “no” if glucose is not present) in the “Glucose – Beaker Before 20 min” box.

3. Determine if iodine AND starch are present in the water/IKI solution by analyzing the solution color and recording it in Table 1. Dark blue means that both starch and iodine are present, and orange means that one or both is not present.

4. Pour 15 ml of prepared glucose/starch solution into a graduated cylinder.

5. Determine if glucose is present in the prepared solution by dipping a glucose indicator strip into the solution. Record the data in Table 1 (write “yes” if glucose is present or “no” if glucose is not present) in the “Glucose – Dialysis Bag”.

6. Determine if iodine and starch are present in the glucose/starch solution by analyzing the color and recording it in Table 1.

7. Obtain a piece of dialysis tubing from the beaker of water. Fold one end of the dialysis tubing and place the dialysis tube clip over the folded end of the tubing and clip into place.

****The pores of the dialysis tubing are extremely small, and can be easily clogged by any oil or dirt on your fingers and hands. Keep physical contact with the tubing to a minimum.****

8. Open the tubing by rubbing the unclipped end between your fingers. Pour 15 ml of the glucose/starch solution into the tubing.

9. Fold the open end of the tubing and place another dialysis tube clip as before. Be sure to leave enough room for expansion.

10. Completely immerse the dialysis bag in the solution in the cup & wait 20 minutes.

11. While you are waiting, move onto Exercise B: Diffusion of potato cores. Complete steps 1-6 for the beaker contents that you have been assigned.

12. After 20 minutes remove the dialysis bag from the cup. Record the final color of the solutions in the bag and the beaker in Table 1.

13. Determine the glucose content in the beaker and in the dialysis bag by using glucose indicator strips. To test the solution in the bag, unclip the bag, and insert the indicator strip into the solution. Record the data in Table 1.

14. Use your results to infer the final location(s) of each molecule and fill out Table 2.

15. Using your results, complete the analysis section for Exercise A.

Results:

Table 1: Initial Observations for Glucose & Starch LocationsLocation Glucose Test (+ or -)

Before 20 min After

Starch-Iodine Test (dark blue = positive, orange = negative)

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20 min Before 20 min After 20 min

Beaker

Dialysis Bag

Table 2: Experimental ResultsCompound

Chemical Formula

Initial Location

Final Location

Did it diffuse through the membrane? How do you know?

Glucose

Starch (C6H10O5)1000

Iodine IKI

Water

Analysis:

1. Below is a diagram of a chamber separated by a semi-permeable membrane. One side contains a mixture of molecules in water and the other side contains only water. In the space next to the picture, draw what you think the chamber would look like after an extended period of time.

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2. IKI (iodine potassium iodide), when mixed with starch, reacts to cause a color change from clear to a blue/black. Glucose test strips are impregnated with a chemical that will change color when exposed to glucose. Given this information, answer the following questions:a. Did the IKI enter, leave, or stay outside the “cell”? What evidence do you have to

support your explanation?

b. Did the starch enter, leave, or stay inside the “cell”? What evidence do you have to support your explanation?

c. Did the glucose enter, leave, or stay inside the “cell”? What evidence do you have to support your explanation?

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3. What is osmosis? Was there any evidence that osmosis occurred through the dialysis tubing? If yes, what evidence do you have to support your explanation? If no, explain how the experiment could be changed to allow for evidence to be collected to show osmosis occurred.

4. Molecules of a similar substance are about the same size, whereas molecules of different substances are known to have different sizes. By looking at your results, can you determine the relative sizes of molecules that did or did not diffuse across the dialysis membrane? If yes, list them in order from largest to smallest.

5. Can it be said that the dialysis membrane is similar to a plasma membrane? Explain.

6. Predict what would happen if you allowed the dialysis tubing to sit in the cup overnight. Explain.

Exercise B: Diffusion in Potato CoresPurpose: The purpose of this exercise is to extend your knowledge of osmosis and diffusion. You

will do this by making predictions about, and observing the result of, how concentration can affect potato core mass.

Materials:

Potato Cores Electronic ScaleCup Solutions

0% Sucrose (distilled water)2% Sucrose4% Sucrose

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6% Sucrose8% Sucrose10% Sucrose

Procedure:

1. Pour 50 mL of each of the above solutions into the appropriately labeled plastic cups.

2. Collect 24 potato cores. You need four potato cores for each cup.

4. Determine the mass of the four potato cores together and record your initial mass in Table 2. Put the four potato cores into the beaker of sucrose solution.

5. Cover the beaker with a plastic lid to prevent evaporation. Take note of what group number you are that is written on the top of the plastic lid.

6. Let it stand overnight.

7. Remove the potato cores from the beakers, blot them gently on a paper towel, and determine their total mass.

8. Record the final masses in Table 2.

9. Calculate the change in mass using the following equation:

Final Mass – Initial Mass= Change in Mass

10. Graph the change in mass data

Table 2: Potato Core Data% Sucrose

in CupInitialMass

FinalMass

MassDifference

0% Sucrose

2% Sucrose

4% Sucrose

6% Sucrose

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8% Sucrose

10% Sucrose

Change in Mass of Potato Cores at Different Concentrations of Sucrose

1. Explain the relationship between the change in mass and the concentration of sucrose.

2. Predict what would happen to the mass of a potato cores in this experiment if they were placed in 0.5 M sucrose solution. Explain your response.

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Exercise C: Naked Eggs! Demo

BACKGROUND: Underneath their shells, all eggs have a semi-permeable membrane, which allows water to permeate (or, pass through) but not other things. The egg is made up of mostly water and the rest is particles found mostly in the yolk (like proteins, lipids, carbohydrates). If the shell is removed from an

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I’m keeping my clothes on!

egg using vinegar, the cell membrane is exposed and we can observe how diffusion of water works. Because water is so important in our bodies, the diffusion of water across a semi-permeable membrane has a special name, called osmosis.

MATERIALS: “naked” eggs, cups, balance, distilled water, corn syrup

PROCEDURES: Naked eggs will be covered in distilled water or corn syrup and left overnight.

HYPOTHESES: Fill out a hypothesis for each egg. Show the direction of water by drawing an arrow for each egg.

DATA TABLE:

Qualitative Observations of eggs

Mass on Day

1 (g)Qualitative

Observations of eggs

Mass on Day

2 (g)

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Egg in Corn Syrup

Egg in dH2O

Control Egg

ANALYSIS QUESTIONSDIRECTIONS: AFTER Day 2, answer the following in complete sentences unless otherwise noted.1. Draw what each egg looked like. (Were your hypotheses correct???)

2. What type of solution was the egg soaked in? Circle one for each:Water: [ hypertonic / hypotonic / isotonic ]

Corn Syrup: [ hypertonic / hypotonic / isotonic ]

3. Did any of the solutes from the yolk (protein, lipids, etc.) move through the egg membrane? What is it called when a membrane will let some particles pass but not others?

4. What would you expect to happen if you decided to put the egg that was in corn syrup into a beaker of water overnight?

5. What would you expect to happen if you decided to put the egg that was in water into a beaker of corn syrup overnight?

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Review Questions1. Fresh fruits and vegetables are often sprinkled with water at the supermarket. Explain why.

2. If a lawn is fertilized and not watered, the grass often dies. Explain why?

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