Diffusion & Osmosis Lab

Diffusion & Osmosis Lab

Recap/Review of Information:

The cell membrane determines what substances can diffuse into a cell. Diffusion is the random movement of molecules from an area of higher concentration to an area of lower concentration. Eventually equilibrium will be achieved. Molecules can still move through the membrane but do so at equal rates. Osmosis is the diffusion of water through a membrane. Plasmolysis is the shrinking of the cytoplasm in plant cells in response to diffusion of water out of the cell and into a hypertonic solution surrounding the cell.

Purpose:

1.  To understand the mechanisms of diffusion and osmosis and their importance to cells.

2.  To determine the effect of time and concentration on the diffusion of Potassium Permanganate (KMnO4).

3.  To investigate the effects of a selectively permeable membrane on diffusion and osmosis between two solutions separated by the membrane

Materials:

·  Beaker

·  Stop Watch, Ruler, Razor blade, Forceps

·  Potassium Permanganate Solution (5%, 1%, .1%)

·  Potato cubes (1cm), Red Onion

·  Dialysis tubing

·  Sucrose Solution (.2M, .4M, .6M, .8M, 1.0M)

·  15% NaCl solution

·  Distilled Water, Microscope, Microscope Slide/Coverslip

Procedure:

Part A:

·  Place four potatoes cubes in a beaker of 5% Potassium Permanganate. Note the time the cubes were added to the solution. Every 10 minutes, remove 1 cube from the solution using forceps.

·  CAREFULLY slice the cube with the razor blade. Dry the blade before each cube. Measure the distance in millimeters that the solution diffused into each potato cube. Distances may not be very large. Record the information in table 1.

·  CAREFULLY slice the cube that you did not put in the solution. This is your control. Consider it as the zero minutes in the table.

·  Place 1 new potato cube in a beaker of 5%, 1% and .1% Potassium Permanganate. Note the times that they were placed in the beakers. After 40 minutes, remove the cubes and CAREFULLY slice each cube. Dry the blade before slicing each cube.

·  Measure the distance in millimeters that the Potassium Permanganate has diffused into each cube. Record information in Table 2.

Part B:

·  Obtain a strip of dialysis tubing. One end has already been tied off in a knot for you. Pour 15 mL of the solution told to you by the teacher. Once this is done. Tie off the other end of tubing. Make sure to leave space between the liquid in the bag and the knot you tied.

·  Gently wash the bag with distilled water to remove any excessive sugar from the outside of the bag. Blot with paper towels. Record the initial mass of your bag and enter this into Table 3. The remaining information will be filled in as other groups complete this task.

·  Once all groups are done filling dialysis tubing, all bags will be placed in a container of distilled water for 30 minutes. After 30 minutes, remove your bag from the container, blot it dry and weigh the bag to determine the mass. Record data in Table 3.

·  Graph data recorded in Table 3. Remember to Title the Graph, and label the x axis (independent variable) and the y axis (dependent variable)

Part C:

·  Prepare a wet mount slide of a small piece of red onion skin. Place a thin piece of skin on the slide followed by one drop of distilled water. Carefully place the cover slip over the skin and water. Using 100x magnification, observe the slide. Sketch and describe the appearance of the onion cells.

·  Add several drops of 15% NaCl to one edge of the cover slip. Draw this solution across the slide by placing a paper towel on the other side of the slip and allow the paper towel to wick the fluid from the opposite side. Observe the slide. Sketch and describe the appearance of the onion cells.

·  Remove the cover slip, and flood the area with fresh water. Observe the slide. Sketch and describe the appearance of the onion cells.

Table 1: Potato Cubes in Solutions for Different Lengths of Time

Cube / Time in Solution (min) / Distance of Diffusion (mm)
1 / 0
2 / 10
3 / 20
4 / 30
5 / 40

Table 2: Potato Cubes in Solutions of Different Concentrations

Cube / Concentration of Chemical / Distance of Diffusion (mm)
1
2
3

Table 3: Dialysis Bag

Contents in Bag / Initial Mass / Final Mass / Mass Difference / Percent Change in Mass*
Distilled water
.2M
.4M
.6M
.8M
1.0M

*percent change in Mass= (final mass-initial mass/initial mass) x 100

Analysis Questions:

1.  Explain the relationship between the change in mass and the concentration (molarity) of the sucrose within in the dialysis bags.

2.  Predict what would happen to the mass of each bag in this experiment if all the bags were placed in .4 M sucrose solution instead of distilled water. Explain your response.

3.  What is plasmolysis and why did the onion cells plasmolyze?

4.  In the winter, grass often dies near roads that have been salted to remove ice. What causes this to happen?