Cell Membrane: Diffusion & Osmosis
For each of these labs you should devise a data table to record your results. Analyse your results and present your
processed data in an appropriate manner. Attempt to explain your results for each section based on your knowledge
of cell membranes.
Purpose: The purpose of this experiment is to observe plamolysis in cells.
Materials: microscope, slide and coverslip, salt solution, water, red onion.
Procedure: Obtain a piece of red onion epidermis, make a wet mount and observe under the microscope. Remove
the slide from the microscope and add several drops of a salt solution. Allow the specimen to sit
several minutes, then observe under the microscope. Remove the slide again and add water. Observe.
Observation: Draw a diagram to indicate the relative proportions of salt and water within the onion cells and
outside the onion cells when they were placed in the saline solution and one for the distilled water
solution. Also use an arrow to properly indicate the direction of osmosis
Discussion: Red blood cells (and other animal cells) placed in a distilled water solution usually swell up and burst.
Why the red onion cells prevented from swelling up and bursting when they were placed in the
distilled water? Why do grocery store owners spray fresh fruits and vegetables with water?
2. Selectivity of Membranes
Purpose: The purpose of this experiment is to observe diffusion through a selectively permeable membrane.
Materials: beakers, test tubes, narrow diameter dialysis tubing, thread, starch suspension, glucose solution,
Lugol’s iodine solution, Benedict’s solution, hot plate.
Testing for Starch & Glucose:
The presence of starch is determined by adding iodine solution. A positive test turns the
rusty-brown iodine a blue-black colour.
The presence of glucose is determined by adding Benedict’s solution to the solution and
heating the mixture for 20 minutes. A positive test turns the solution red.
PRE-LAB: What reactions are responsible for the colour changes in the positive tests described
Procedure: Cut off 2 pieces of dialysis tubing, long enough to more than fill a test tube. Tie off one end of the
tubing. Moisten each piece until you can open it into a tube shape. Pour starch suspension into one
‘tube’ and glucose solution into the other. Leave the other end of the tubing untied, place the tubing
with the starch into a test tube filled with water. Enough Lugol’s iodine should be added to the water
to turn it a rusty colour. Add the dialysis tubing containing glucose solution to a test tube containing
water. Sufficient Benedict’s solution should be added to this water to turn it pale blue. Leave both
tubes for 20 minutes. Place the tube with the glucose and Benedict’s solution into a water bath and
heat on a hot plate.
Using your observations describe which molecules passed through the membrane. Explain how your
observations support the conclusion that dialysis tubing is a selectively permeable membrane.
3. Osmosis (dry lab) to be done as homework
Purpose: The purpose of this experiment is to observe the rate of osmosis over a 24 hour period.
Materials: dialysis tubing (large diameter), molasses, elastic bands, basin, water, balance.
Procedure: Tie one end of the dialysis tubing and fill it with molasses. Tie off the other end to make a “sausage”.
Determine the mass of the sausage. Place the tubing into a basin of water. At 10 minute intervals,
remove the dialysis tubing, dry it off and determine its mass.
4. Osmosis and Turgor in Living Cells
Purpose: The purpose of this experiment is to observe the relationship between osmosis and turgidity of
Materials: potato, 1 M (mol/L) sucrose solution, cork borer, balance, distilled water, test tubes, graduated
PRE-LAB: 1. How will you dilute the 1M sucrose solution to the desired concentrations?
Procedure: Use the stock 1 mol/L sucrose solution and dilute to obtain 10 mL of sucrose of the following
concentrations: 0.1 M, 0.2M, 0.4 M, 0.6M, 0.8 M, and 1.0 M. Use a cork borer and knife to obtain 6
sections of potato, each 3-5 cm in length. Determine the mass of each piece of potato. Place one
piece of potato into each test tube and leave overnight. Remass the potatoes.
In the conclusion, you may wish to explain how the data you gathered relates to the qualitative changes you observed
in the potato cores. You can also estimate the percentage of a potato core is water based on your data.
5. Effects of Temperature on Membrane Permability
Purpose: The purpose of this experiment is to determine the effects of temperature on membrane
permeability of beetroot cells.
Materials: cork borer, beet, ruler, knife, beakers, test tubes, test tube rack, distilled water, hot plate,
thermometer, forceps, spectrophotometer.
Background: The cells of beet root contain a large amount of a red pigment called betacyanin. This pigment is
contained within a large central vacuole. If the membranes are damaged by the application of
physical or chemical stress, the betacyanin will leak out of the cells into the surrounding medium.
PRE-LAB: Find out what a spectrophotometer is and how it works. Why is the spectrophotometer set up to
measure absorbance of the solution at 460nm?
Procedure: 1. Cut 6 cyliners of beet using the cork borer and trim each to a length of 15mm. Place the
cylinders in a beaker and rinse them with tap water. Place a beet in each of the 6 test tubes.
2. Place one tube in the refrigerator and another in the freezer for 30 minutes. Remove the tubes
from the refrigerator and freezer and add 10 mL of room temperature distilled water to each
tube. Let these pieces of beet soak in the water for 20 minutes, then discard the beets.
3. Place the beet section from the third tube into a beaker of hot water at 70ºC for exactly 1
minute. Handle the beet carefully with forceps to avoid rupturing the cells. Remove the beet
from the beaker, return it to the test tube and add 10 mL of room temperature distilled water.
Allow the beet to soak for 20 minutes, then remove and discard the beet.
4. Immerse the beet from the fourth test tube in a beaker of water at 55ºC for exactly 1 minute.
Return the beet to the test tube and add 10 mL of room temperature distilled water. Allow the
beet to soak for 20 minutes, then remove and discard the beet.
5. Repeat step 4 with water at 40ºC and 20 ºC respectively.
6. For all 6 tubes, use the spectrophotometer to determine the absorbance of the solution at 460