Lab #2: Osmosis Pre-Lab Exercise

Transcription

1 Lab #2: Osmosis Pre-Lab Exercise Name 1. Using your own words, define the following terms: a. Osmosis b. Concentration gradient: c. Hypertonic solution: d. Isotonic solution: e. Hypotonic solution: 2. What is the solvent inside the raw, decalcified egg? 5. What hypothesis is being tested in this activity? 6. Predict the outcome of the experiment in terms of your hypothesis. Explain which results would support your hypothesis.

2 Lab #2: Osmosis Work in groups of four This lab is designed to illustrate the principle of osmosis. Summary of Activities 1. Observe osmosis using a decalcified egg as a model "cell". I. Osmosis Osmosis is the movement of water across a selectively permeable membrane. A selectively permeable membrane allows water, but not most solutes, to freely pass. The plasma membrane of a cell is a good example of a selectively permeable membrane. When solutes cannot freely move in the presence of a concentration gradient across a membrane, water will move instead in an attempt to equalize the concentration of solutes on both sides of the membrane. Another way of thinking about osmosis is the diffusion of water from a high concentration of water (what we usually think of as a dilute solution) to a low concentration of water (a more concentrated solution). For example, if a red blood cell (RBC) is placed in a solution that contains a higher concentration of solutes than what is normally found inside the cell (a hypertonic solution), water will move from inside the cell in an attempt to dilute the solutes outside the cell. The RBC will shrivel up as a result of the loss of water. Conversely, if a RBC is placed in a solution containing a lower concentration of solutes (a hypotonic solution), water will move from the outside to the inside of the cell. This will result in the cell bursting from the excess of water moving in. It is important to use a solution that contains the same solute concentration as blood (an isotonic solution) when administering fluids intravenously to prevent bursting or shriveling the cells. In the following experiment, you will be using decalcified eggs as model cells. The eggs have been treated with vinegar to remove the calcium from the shell, leaving behind a membrane that is permeable to water (solvent), but not to other molecules (solutes). Materials: 4 decalcified eggs 4 weigh boats, one for each egg Beakers containing solutions A, B, & C Beaker containing solution X Paper towel Gram scale Procedure IA: 1. Obtain four decalcified eggs. Gently dry and weigh each egg before immersing it in a solution. Dry the egg by gently rolling it on a paper towel. Do not dry the egg for too long because the paper towel will begin to draw out water from inside the egg and will change the weight of the egg. Record the weight of the egg in Table IA.

3 2. Place one egg into each of the three solutions; A, B, and C. Let these three eggs soak for 20 minutes. Go on to Procedure IB while you are waiting. 3. After 20 minutes, dry and weigh each egg. Record the results in Table IA. 4. The change in weight reflects the movement of water into or out of the egg. Based on the movement of water, determine if the Solutions A, B, and C are hypo-, iso-, or hypertonic. Procedure IIIB: 1. Gently dry and weigh your fourth egg, egg X. Record the results in Table IB. 2. Immerse the egg in solution X. Dry and reweigh this egg every two minutes for about a half hour. Record the results in Table IB. 3. Plot the results on a graph to show the rate of water movement into or out of the egg.

4 Lab #5: Data Sheet: Turn in with lab report Table IA: Determining the Tonicity of Solution A, B, & C Weight Before Soaking Weight After Soaking Difference in Wt. Tonicity of Solution (hyper-, hypo-, or iso-) Egg in Solution A Egg in Solution B Egg in Solution C Procedure IIIB: Egg in Solution X Time (minutes) Weight (grams) Change in Weight (grams) Time (minutes) Weight (grams) Change in Weight (grams)

5 Diameter (mm) Time (minutes)

6 Lab Questions 1. Define osmosis in your own words. 2. What is the source of energy that moves water molecules around? 3. What causes the water to move in a particular direction, into or out of the egg? 4. How does the change in weight of the egg when it is placed in a solution tell you the tonicity of that solution? 5. What is the tonicity of the fluid surrounding your body cells? 6. What might happen to your body cells if the solution surrounding them was very hypertonic? Very hypotonic? 7. What is the tonicity of solution X? 8. What does the rate of weight change of the egg in solution X tell you about the process of osmosis and the membrane of the egg?