Topic: Osmosis

Part One: Teacher Notes

Sate and National Standards:

Missouri Department of Elementary and Secondary Education:

Strand 3: Characteristics and Interactions of Living Organism

Section 2) Living organisms carry out life processes in order to survive:

• Explain the significance of semi-permeability to the transport of molecules across cellular membranes.

• Relate the role of osmosis to the movement of molecules across a semi-permeable membrane.

Concept:

The concept being addressed in this lesson is osmosis. Osmosis is the diffusion of water across a membrane. Water will move in the direction where there is a high concentration of solute and a lower concentration of water to where there is a low concentration of solute and higher concentration of water. Osmosis is an example of passive transport whereby ions or molecules move down their concentration gradient.

Exploration:

The students will be assigned into groups of four and will be given directions to perform the experiment. In this experiment, the students will be observing potatoes soak in two different dishes, one with water and the other with salt water. They will be recording their observations, answering the questions provided and state a hypothesis about what they think is happening.

Concept Introduction:

Through osmosis, water naturally moves from areas of low salt concentrations to areas of high salt concentrations. Adding salt to the water creates a higher salt concentration in the water in the dish than in the water contained in the potato. Consequently, the water in a potato that is soaking in salt water will migrate out of the potato leaving a rather disgusting, mushy spud. Cell membranes are completely permeable to water; therefore, the environment the cell is exposed to can have a dramatic effect on the cell.

Concept Introduction Notes:

• Osmosis is a specialized case of diffusion that involves the passive transport of water.

• In osmosis water moves through a selectively permeable membrane from a region of its higher concentration to a region of its lower concentration. The membrane selectively allows passage of certain types of molecules while restricting the movement of others.

• Selectively Permeable is a membrane that allows some things through, the cell membrane is selectively permeable.

• Osmosis allows water molecules to pass easily through most biological membranes.

• Hypertonic Solutions: contain a high concentration of solute relative to another solution (e.g. the cell's cytoplasm). When a cell is placed in a hypertonic solution, the water diffuses out of the cell, causing the cell to shrivel.

• Hypotonic Solutions: contain a low concentration of solute relative to another solution (e.g. the cell's cytoplasm). When a cell is placed in a hypotonic solution, the water diffuses into the cell, causing the cell to swell and possibly explode.

• Isotonic Solutions: contain the same concentration of solute as a solution (e.g. the cell's cytoplasm). When a cell is placed in an isotonic solution, the water diffuses into and out of the cell at the same rate. The fluid that surrounds the body cells is isotonic.

Application:

The experiment I have picked for the application part is a well-known experiment that is done in many biology classes to explain the concept of osmosis. In this experiment, students will be providing 6 different dialysis tubes, where one is filled with water and the other five are filled with different molarity of sucrose solution that will be provided by the teacher. The students are to measure the mass of the tubes and immerse them each individually in a beaker that is ¾ full with water. After thirty minutes, the students will take the tubes out of the beakers and measure their final mass.

After the data collection, the students should describe what the result of the experiment is by answering the questions provided. The sheet with the table and the questions should be collected upon completion at the end of the class.

Material List:

For the exploration part the supplies each group will need are: 2 potatoes, 2 dishes, salt and water.

For the concept application part the supplies each group will need are: A beaker, 6 strips of dialysis tubing, 10mL of distilled water, 10mL of 0.2-M sucrose, 10mL 0.4-M sucrose, 10mL of 0.6-M sucrose, 10mL of 0.8-M sucrose, and 10mL of 1.0-M Sucrose.

History of Concepts:

Osmosis is a form of diffusion:

In 1829, T. Graham studied diffusion of gases from a closed vessel through a small tube into the surrounding air.

In 1855, A. Fick formulated his mathematical statement of the law of diffusion by analogy with Fourier’s law of heat conduction.

Related Websites:

http://www.phschool.com/science/biology_place/labbench/lab1/concepts.html

http://www.tvdsb.on.ca/westmin/science/sbi3a1/Cells/Osmosis.htm

http://arbl.cvmbs.colostate.edu/hbooks/cmb/cells/pmemb/osmosis.html

Miller, levine. Biology. Pearson Education, Inc., New Jersey:2002.

References:
Miller, levine. Biology. Pearson Education, Inc., New Jersey:2002.

http://www.phschool.com/science/biology_place/labbench/lab1/concepts.html

Part Two: Students Pages

Exploration:

Supplies each group will need:

• 2 potatoes

• 2 dishes

• salt and

• water

Procedure:
1. Fill both of the dishes with water.
2. Slice the potato lengthwise into several pieces that each have two flat sides.
3. Add about two tablespoons of salt to one of the dishes.
4. Put half of the pieces in the dish that contains plain water.
5. Put the other half of the pieces in the dish that contains the salt water.
6. Let the potatoes soak for 20 minutes.

Fill out the following table as you are letting the potatoes soak; explain the changes you might see in the different dishes over time.

After you have completed the table, answer the following questions:

1.  Where do you see the most changes occur between the dishes?

2.  Where do you think the least changes occur between the dishes?

3.  Describe the difference between the two dishes at minute 20?

4.  What causes such difference at minute 20?

5.  State your hypothesis:

Concept Application:

1.  You will need 6 strips of dialysis tubing.

2.  Tie off one end of your piece of dialysis tubing to form a bag. Pour 10 mL of one of the following solutions into this bag.

3.  Remove most of the air from the bag by drawing the dialysis bag between two fingers. The solution should fill about one-third to one-half of the piece of tubing with each solution and tie off the other end of the bag.

4.  Rinse each bag gently with tap water to remove any sucrose spilled during filling

5.  Carefully mark the outside of the bag and record its initial mass (grams) in table provided.

6.  Fill a 100-mL beaker about two-thirds full with distilled water.

7.  Immerse the bag in the beaker of distilled H2O and label the beaker to indicate the molarity (M) of the solution in the dialysis bag.

8.  Wait for thirty minutes, and then remove the bags from the water. Carefully mark and determine the mass the bag.

9.  Record data in your table and graph the group results on a separate piece of paper. 

Table: Dialysis Bag Results

To calculate percent change in Mass = (Final Mass – Initial mass  * 100) / Initial Mass

As a group the students will be responsible for answering the following questions:

1. Explain the relationship between the increase in mass and the molarity of sucrose within the dialysis bags?

2. Predict what would happen in an experiment if all the bags were placed in a 0.4-M sucrose solution instead of distilled water?

3. Why did you calculate the percent change in mass rather than simply using the change in mass? 

4.  How can you explain what is happening in this experiment?

5.  What causes the change in the mass of the different tubes?

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