Name



Name Date Period

HBiology

Diffusion and Osmosis Lab

NOTE: BE SURE TO ANSWER THE PRELAB QUESTIONS BEFORE COMING TO LAB ON DAY ONE OF THE EXPERIMENT!!!

Introduction:

Many aspects of the life of a cell depend on the fact that atoms and molecules have kinetic energy and are constantly in motion. Diffusion and osmosis are two processes exemplifying this principle. Diffusion is the random movement of molecules from areas of high concentration to areas of low concentration; specifically, down the concentration gradient. Over time, the molecules will reach equilibrium and there will be no NET movement of molecules from one area to another. Osmosis is a special case of diffusion. Osmosis describes the tendency of water to move from an area high in water potential to an area lower in water potential. Osmosis and diffusion describe transport without the use of cellular energy called passive transport. Another type of transport, active transport, requires the use of energy because substances are traveling up the concentration gradient.

Part 1: Diffusion (Demonstration)

In this experiment, we will measure the diffusion of small molecules through dialysis tubing, which will simulate the plasma membrane in an animal cell. Small solute molecules can travel freely through the selectively permeable membrane, but larger particles will pass through much slower, or perhaps not at all.

A solution of 1% starch and 15% glucose is placed inside a sealed bag of dialysis tubing which is placed inside a beaker containing distilled water. Both the solution inside the bag and inside the beaker are tested for the presence of mono- and polysaccharides using Benedict’s solution and Lugol’s Iodine. After 20-30 minutes, a sample from both the bag and the beaker are tested again for the presence of monosaccharides and polysaccharides.

Fill in the ‘Hypothesis’ and ‘Data/Results/Conclusions’ tables as the demo is completed.

Part 2: Osmosis

In this experiment you will use dialysis tubing to investigate the relationship between solute concentration and the movement of water through a selectively permeable membrane. You will be assigned one of the six concentrations of solute to perform the experiment. As a class, we will collect and analyze data for all solute concentrations.

Procedure:

1. Obtain a 30-cm strip of presoaked dialysis tubing.

2. Double knot and tie a string around one end of the dialysis tubing to form a bag. To open the other end, rub the unknotted end while holding the tubing under running water.

3. Use a syringe to transfer approximately 20-mL of one of the following solutions into the bag. (Your group will be assigned ONE of the following solutions.)

a. distilled water

b. 0.2 M sucrose

c. 0.4 M sucrose

d. 0.6 M sucrose

e. 0.8 M sucrose

f. 1.0 M sucrose

Remove most of the air form each bag by drawing the dialysis bag between two fingers. Twist, flip and tie off the other end of the bag. Leave sufficient space for the expansion of the contents in the bag; the solution should only fill about 1/3 to 1/2 of the piece of tubing.

4. Rinse each bag with distilled water to remove any sucrose spilled during the filling.

5. Carefully blot the outside of each bag and record the initial mass of each bag in grams.

6. Place each bag into an empty 250-mL beaker or cup; label the cup or beaker with the molarity of the solution in the bag and your names.

7. Fill each beaker or cup with distilled water. Be sure to completely submerge each bag.

8. Let stand overnight. Next period, carefully blot and determine the mass of each bag.

9. Record your group’s data in the table below, filling in only the appropriate row for your molarity. As a class, we will share and compile data to completely fill in the chart titled ‘Dialysis Bag Results- Group Data’.

Part 3: Water Potential

In this experiment you will be using potato cores in different molar concentrations of sucrose to determine the water potential of potato cells. Water potential is used to predict the movement of water into or out of plant cells, in this case, the potato. Water will always move from an area of high water potential (more water molecules) to an area of lower water potential (fewer water molecules). Movement of water into and out of a cell is influenced by the amount of solute on each side of the cell membrane. If there are low solute concentrations in the cell, water moves out of the cell and the cell will shrink. If there are high solute concentrations in the cell, water moves into the cell and the cell will swell. If high enough solute concentrations exist outside the cell, the water in the cells (high water potential) will move out of the cell, down the concentration gradient towards an area where the water potential is much lower.

Procedure:

1. Pour 50 mL of the solution you were assigned in Part 2 into a labeled 250-mL beaker.

2. Use a “corer” to produce four equal sized cores of potato that are approximately 2 cm thick. Do not include any skin on the slivers.

3. Determine the mass of the four slivers together and record the mass in the table below. Put the four cylinders in to the beaker of sucrose solution.

4. Cover the beaker with ParaFilm or aluminum foil to prevent evaporation.

5. Let stand overnight. Next period, remove the cores from the beaker, blot them gently with a paper towel and determine the total mass.

6. Record your group’s data in the table below, filling in only the appropriate row for your molarity. As a class, we will share and compile data to completely fill in the chart titled ‘Potato Core Results- Group Data’.

Name:_____________________________________________________Date:______ Period______

HBio: Diffusion and Osmosis Lab

PRE-Lab Questions:

1. What is the dependent variable in this experiment (parts 2 and 3)?

2. What is the independent variable in this experiment (parts 2 and 3)?

3. What is your hypothesis for part 2 (include dependent and independent variables)?

4. What is your hypothesis for part 3 (include dependent and independent variables)?

Part 1: Diffusion (Demonstration)

Hypothesis: (What do you think is going to happen???)

| |Benedict’s |Iodine |

| |Inside Bag |Outside Bag |Inside Bag |Outside Bag |

|Initial | | | | |

|Final (after 30 minutes) | | | | |

Data/Results/Conclusions:

| |Benedict’s |Iodine |

| |Inside Bag |Outside Bag |Inside Bag |Outside Bag |

|Final (after 30 minutes) | | | | |

Part 2: Dialysis Bag Results – Group Data (please put an * next to your group)

|Contents in Bag |Initial Mass |Final Mass |Mass Difference |% Change in Mass* |

|a.) distilled H2O | | | | |

|b.) 0.2 M sucrose | | | | |

|c.) 0.4 M sucrose | | | | |

|d.) 0.6 M sucrose | | | | |

|e.) 0.8 M sucrose | | | | |

|f.) 1.0 M sucrose | | | | |

* % change in mass = [(final mass - initial mass)/initial mass] x 100

Part 3: Potato Core Results – Group Data (please put an * next to your group)

|Contents in Bag |Initial Mass |Final Mass |Mass Difference |% Change in Mass* |

|a.) distilled H2O | | | | |

|b.) 0.2 M sucrose | | | | |

|c.) 0.4 M sucrose | | | | |

|d.) 0.6 M sucrose | | | | |

|e.) 0.8 M sucrose | | | | |

|f.) 1.0 M sucrose | | | | |

* % change in mass = [(final mass - initial mass)/initial mass] x 100

POST-Lab Questions (Answers to be typed and attached to this sheet along with graphs)

1. Graph your data for parts 2 and 3 on two separate graphs. Be sure to graph PERCENT CHANGE IN MASS as it relates to sucrose molarity. Attach graphs to the lab.

2. For part 2, explain the relationship between the change in mass and the molarity of sucrose within the bag (not just for your experiment, for ALL data.)

3. Restate your original hypothesis for part 2. Was your hypothesis supported or rejected for part 2?

4. Why did we calculate percent change in mass instead of just the mass of the bags?

5. Predict what would happen to the bags in part 2 if we placed them all into a 0.4M sucrose solution. Explain.

6. For part 3, explain the relationship between the change in mass and the molarity of sucrose surrounding the potatoes (not just for your experiment, for ALL data.)

7. Restate your original hypothesis for part 3. Was your hypothesis supported or rejected for part 3?

8. Errors are common in science experiments, especially those which rely on many group’s data. Explain what errors (at least three) may have occurred within this experiment (not necessarily just YOUR group, but among everyone). Also explain how these may have affected the data collected in this experiment.

OSMOSIS LAB – GROUP DATA

Dialysis Bag Results – Group Data

|Contents in Bag |Initial Mass |Final Mass |Mass Difference |% Change in Mass* |

|a.) distilled H2O | | | | |

|b.) 0.2 M sucrose | | | | |

|c.) 0.4 M sucrose | | | | |

|d.) 0.6 M sucrose | | | | |

|e.) 0.8 M sucrose | | | | |

|f.) 1.0 M sucrose | | | | |

Potato Core Results – Group Data

|Contents in Bag |Initial Mass |Final Mass |Mass Difference |% Change in Mass* |

|a.) distilled H2O | | | | |

|b.) 0.2 M sucrose | | | | |

|c.) 0.4 M sucrose | | | | |

|d.) 0.6 M sucrose | | | | |

|e.) 0.8 M sucrose | | | | |

|f.) 1.0 M sucrose | | | | |

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