Membrane Diffusion Kit
Student Laboratory Kit
Materials Included In Kit
Sucrose, 1 kg
Cork borer, 7-mm i.d.
Dialysis tubing, 33-mm, 10 feet
Pipet, serological, 5-mL, 3
Additional Materials Required
(for demonstration and 15 groups)
Water, distilled or deionized (DI)
Water, distilled or deionized*
Balance, 0.01-g precision†
Balance, 0.1-g precision*
Beakers, 1-L, 6*
Beakers or cups, 250-mL, 30
Beakers or Erlenmeyer flasks, 600-mL or larger, 3*
Beakers, tall-form, 500-mL, 3
Clamps (to hold pipets—buret or thermometer type), 3
Cork borer, 7 mm, or French fry slicer†
Flasks, volumetric, 6*
Marker or waxed pencil
Pipets, disposable, 3
Plastic wrap or Parafilm®†
Pipets, serological, 5-mL, 3
Potatoes, baking or red
Potatoes, sweet (optional)
Rubber bands, small, 3
Ruler, centimeter, 15
Scalpel or razor blade, 15
Stirring rods, 8*
Support stands, 3
†May be shared
Prepare the 15% sucrose solution by weighing out 90 g of sucrose and placing it into a graduated beaker or Erlenmeyer flask (600-mL or larger). Fill to the 600-mL mark with DI water and stir to dissolve sucrose. Prepare the 30 % sucrose solution in a similar fashion with 150 g of sucrose and DI water up to the 500-mL mark.
Cut three 6–7" lengths of dialysis tubing and soak in DI water for about 10 minutes.
The material quantities listed above presume a class of 30 students working in pairs, with each pair of students working with two sucrose solutions. Table 1 chart details the proportions for making each of the sucrose solutions. The procedure for each is: add approximately 500 mL of distilled or deionized water to a 1000-mL volumetric flask, weigh out required amount of sucrose and add it to the flask. Stir or shake to dissolve sucrose. Fill to 1000-mL mark with water and invert several times to mix.
Although the materials in this activity are considered nonhazardous, please use all normal laboratory safety precautions. Use caution when cutting. Remind students to wash their hands thoroughly with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information.
Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. The sucrose solutions may be disposed of by disposing of down the drain with plenty of excess water according to Flinn Suggested Disposal Method #26b. The leftover potatoes and the cores may be disposed of in the regular trash.
- Enough materials are provided in this kit for 30 students working in pairs or for 15 groups of students plus one demonstration. Both parts of this laboratory activity can reasonably be completed in one 50-minute class period. The Membrane Diffusion Worksheet may be completed the day after the lab.
- In Part I. Initiate the demonstration at the beginning of class and allow it to continue while you discuss the concepts and principles of osmosis and diffusion.
- In Part I. A drop of food dye placed in the 15% sucrose solution used to fill the dialysis bags will enhance visibility of the liquid levels in the pipets.
- In Part I. Concurrent with this demonstration, fill two beakers with hot and cold water respectively. Allow the water to become still and gently add a drop of food dye to each beaker. Observe as the dye diffuses throughout the water in each beaker. Placing the beakers in front of a white background will make it easier for students to observe the results.
- In Part II. To save class time, use a cork borer to pre-cut cores from the potatoes. Cores should be longer than 4 cm so students can trim them to length. Seal the cores in an airtight bag or small container until class.
- In Part II. To see if sweet potatoes live up to their name, assign three teams of students (covering all six sucrose concentrations) cores from a sweet potato.
Correlation to Next Generation Science Standards (NGSS)†
Science & Engineering Practices
Asking questions and defining problems
Developing and using models
Planning and carrying out investigations
Analyzing and interpreting data
Using mathematics and computational thinking
Engaging in argument from evidence
Obtaining, evaluation, and communicating information
Disciplinary Core Ideas
MS-LS1.A: Structure and Function
HS-LS1.A: Structure and Function
Cause and effect
Scale, proportion, and quantity
Systems and system models
Structure and function
Stability and change
MS-LS1-2. Develop and use a model to describe the function of a cell as a whole and ways parts of cells contribute to the function.
HS-LS1-3. Plan and conduct an investigation to provide evidence that feedback mechanisms maintain homeostasis.
HS-LS1-2. Develop and use a model to illustrate the hierarchical organization of interacting systems that provide specific functions within multicellular organisms.
Answers to Questions
- Calculate the Percent Change in Mass for the two samples. The Percent Change in Mass is calculated using Equation 1.
See Data Tables.
- Obtain data from other members of the class and average all the values for each sucrose solution. Record these values in the data table.
See Data Tables.
- Plot a graph of Percent Change in Mass vs. Sucrose Solution Concentration. The graph should have positive and negative values on the y-axis with the zero line (no change in mass) serving as the x-axis.
See Discussion for Part II.
- Review the graph created in Question 3. How did soaking the potato cores in a sucrose solution affect the mass of the potato cores? Is there a correlation between the molarity of the sucrose solution and the change in mass? If so, how was the mass affected. Using the information in the Background section, construct a hypothesis as to what is occurring within the cells of the potato.
See Discussion for Part II.
The demonstration presents the three possible scenarios that an organism immersed in a liquid medium might face. In one case, the surrounding medium has a higher concentration of water; in the second, the surrounding medium has an equal concentration of water; in the third, the surrounding medium has a lower concentration of water.
The results of the demonstration show that the net movement of water is to the location where it is less concentrated. In the beaker of DI water, the net movement is into the dialysis bag, causing the liquid level in the pipet to rise. In the beaker of 30% sucrose, the net movement is out of the dialysis bag, causing the liquid level in the pipet to drop. In the beaker of 15% sucrose, there is little or no net movement evident as water is equally concentrated on both sides of the membrane. The case of the Paramecium is analogous to the beaker of distilled water in that the concentration of water within the Paramecium cell membrane is less than that outside the membrane. Saltwater fish present the opposite case as water is more concentrated in their body fluids than in the surrounding water. Saltwater fish must take in water while they simultaneously rid themselves of excess salts.
Biologists have terms for each of the three scenarios detailed above. In the case where the fluids inside and outside the cell are equally (or nearly so) concentrated, the surrounding fluid is said to be isotonic to the cell cytoplasm. If the surrounding fluid has a higher concentration of water (therefore a lower solute concentration) it is hypotonic to the cell cytoplasm. If the surrounding fluid has a lower water concentration (therefore a higher solute concentration), it is hypertonic to the cell cytoplasm. The series of diagrams below represents a graphic summary of what we’ve discussed. Each circle represents a cell and the arrows show the movement of water into and out of the cell. The double arrow represents a high rate of water movement, the single arrow a low rate. In the first diagram the net movement is into the cell, in the second there is no net movement, and in the third the net movement of water is out of the cell. With no corrective mechanism what is the ultimate fate of each cell?
This far we’ve spoken mostly in terms of water concentration. The concentration of water (the solvent) is dependent on the concentration(s) of materials (solutes) dissolved in it. A solution that is 10% salt and 90% water has a higher salt concentration and a lower water concentration than pure (i.e., 100% water). Chemists, by convention, look at, and define, solutions with respect to solute concentration and, as such, never speak in terms of water concentration. As defined by a chemist, osmosis is the movement of water (or other solvent) through a membrane in such a direction as to equilibrate the concentration of solutes on either side of that membrane.
Part II shows the effects of a range of solutions of different concentrations on a natural membrane system. The range of solutions was selected so that some would be hypotonic to the potato cores and some would be hypertonic to the potato cores (except the sweet potato). Some would gain water (those showing a positive change in mass) and some would lose water (those showing a negative change in mass). If you draw a best-fit line on the graph (from Post-Lab Question 3), you will see that there is a point where the line crosses the x-axis (the zero line, representing no change in mass). This value (in molarity of sucrose solution) represents the isotonic point (i.e., the point at which the concentration of water is equal inside and outside of the potato cells). Presumably a potato placed in this concentration of sucrose will neither gain nor lose water.
It is possible to precisely measure the length of the potato cores before and after soaking and see the same affect. If the potato cells are losing water or gaining water, the length of the cores should change as the cells shrink or expand. By graphing change in length, a similar value for the isotonic point should be found.
It may also have been observed that the texture of the potato cores varied, depending on which solution they were soaked in. Cores in hypotonic solutions (lower sucrose concentration) gained water and remained firm and crisp. Cores in hypertonic solutions (higher sucrose concentration) lost water and became shriveled and limp. Plant cells rely on absorbed water to retain their shape. This provides insight into why plants wilt if they are deprived of water. Plants do not have skeletons like we do and actually rely on water pressure (turgor in plant cells) to remain erect and keep their leaves oriented to the light.
Abramoff, P.; Thomson, R. G. Laboratory Outlines in Biology–V; W. H. Freeman: New York, 1991; pp 111–118.
Stern, K. R. Introductory Plant Biology, 7th ed.; Wm. C. Brown: Dubuque, IA, 1997; pp 144–146.
Green, N. P. O.; Stout, G. W.; Taylor, D. J. Biological Science: Vol. 2, 2nd ed.; Soper, R., Ed.; Cambridge University: Cambridge, 1990; pp 931–932.
||Membrane Diffusion—Student Laboratory Kit
||Diffusion Demonstration Model
||Water, Distilled, 4 L
||Water, Deionized, 4 L
||Beakers, Polymethylpentene (PMP), 250 mL
||Beakers, Borosilicate Glass, Berzelius, 500-mL
||Cork Borer, 7-mm i.d.
||Parafilm® M, 20" x 50', Roll
||Flinn Scientific Electronic Balance, 1000 x 0.1-g
||Beakers, Polymethylpentene (PMP), 1000 mL
||Serological Pipet, 5 mL