Membrane Diffusion Lab
In order to survive, all organisms need to move molecules in and out of their cells. Molecules such as gases (e.g., O2, CO2), water, food, and wastes pass across the cell membrane. There are two ways that the molecules move through the membrane: passive transport and active transport. While active transport requires that the cell uses chemical energy to move substances through the cell membrane, passive transport does not require such energy expenditures. Passive transport occurs spontaneously, using heat energy from the cell's environment.
Diffusion is the movement of molecules by passive transport from a region in which they are highly concentrated to a region in which they are less concentrated. Diffusion continues until the molecules are randomly distributed throughout the system, thus reaching equilibrium. Osmosis, the movement of water across a membrane, is a special case of diffusion. Water molecules are small and can easily pass through the membrane. Other molecules, such as proteins, DNA, RNA, and salts are too large to diffuse through the cell membrane. The membrane is said to be semipermeable, since it allows some molecules to diffuse through but not others.
If the concentration of water on one side of the membrane is different than on the other side, water will move through the membrane seeking to equalize the concentration of water on both sides. When water concentration outside a cell is greater than inside, the water moves into the cell faster than it leaves, and the cell swells. The cell membrane acts somewhat like a balloon. If too much water enters the cell, the cell can burst, killing the cell. Cells usually have some mechanism for preventing too much water from entering, such as pumping excess water out of the cell or making a tough outer coat that will not rupture. When the concentration of water inside of a cell is greater than outside, water moves out of the cell faster than it enters, and the cell shrinks. If a cell becomes too dehydrated, it may not be able to survive. Under ideal conditions, the water concentration outside is nearly identical to that inside.
- Diffusion: The spontaneous tendency of a substance to move down its concentration gradient from a more concentrated to a less concentrated area. Osmosis is the diffusion of water.
- Hypotonic Solution: In comparing two solutions, it is the one with the lower solute concentration
- Isotonic Solution: Having the same solute concentration as another solution.
- Hypertonic Solution: In comparing two solutions, it is the one with the higher solute concentration.
- Particles move across membranes by simple diffusion, facilitated diffusion osmosis and active transport
- Skill: Estimation of osmolarity in tissues by bathing samples in hypotonic and hypertonic solutions (Practical 2)
Materials: (for each group)
- Six 50 or 100ml beakers (all six need to be of same size)
- 75 ml each of distilled water, 0.2 Molar, 0.4 Molar, 0.6 Molar, 0.8 Molar and 1.0 Molar Saline solutions
- 6 2cm3 potato cubes
- Label each beaker with it’s appropriate solution concentration
- Pour 50 ml of distilled water into the beaker marked “0.0.”
- Repeat this for the remaining saline solutions in the 5 beakers with their respective saline solutions.
- Cut 6 potato cubes to a length of 2cm3 (be as accurate as possible!). Remove any skin from the cubes.
- Find and record the mass for each potato cube and record in Table 1 under “Initial Mass.”
- Place 0.0 Molar potato cube in its’ respected beaker.
- Repeat Steps 5 and 6 for each of the remaining saline solution beakers.
- Ensure that all potato cubes are completely submerged; add an equal amount of solution to all beakers if one cube is not submerged.
- After 48 hours, record the temperature of the solutions in each of the beakers in Table 1.
- Remove the potato cube out of the 0.0 Molar Solution cup and carefully blot dry with a paper-towel.
- Find and record the 0.0 Molar potato cube mass under Final Mass in Table 1.
- Repeat Steps 11 and 12 for each of the remaining saline solutions.
- Calculate the percent change in mass for each of the solutions:( (Final Mass-Initial Mass)/Initial Mass) x 100%
- Turn in group data for Percent Change in Mass for each solution concentration here.
Table 1: Raw Data Individual Mass Change in Potato Tissue in Various Salt Solutions
Initial Mass (g)
Final Mass (g)
% Change in Mass
Table 2. Raw Data Class Mean % Change in Mass in Potato tissue in various Saline Concentrations
*Each Trial represents the data from a different class group as found on the class data spreedsheet.
Percent Change of Potato Trials
Molarity of Solutions
Table 3. Summarized Data Class Mean % Change in Mass in Potato tissue in various Saline Concentrations
Percent Change of Potato Trials
Molarity of Solutions
Copy and paste these tables into Excel in order to complete the calculations, then copy it back into this document (no need to fill it out twice).
Table 4. Measurement Uncertainties of Temperature and Potato Cube Size
Mass Measurement Uncertainty
Table 5. Qualitative Observations of salt Concentration Effect on Potato Cube Mass
Please complete the following conclusion & evaluation questions on a separate piece of paper (please type) and staple to your lab packet.
- Create a graph from the summarized class data in Table 3. Place the percent change in mass on the Y-Axis and Salt molarity on the X-Axis. Insert a best fit linear regression line, R2 value, and show the degree of uncertainty for each concentration as measured by 95% Confidence Interval.
Upload an image of your graph here
- Calculate an estimate of the osmolarity in the potato cube (meaning, what solution would create a balanced equilibrium/isotonic solution) based on the X-intercept of your graph from the best fit line of the class average.
Conclusion & Evaluation:
- Restate the question and hypothesis.
- Describe if the hypothesis was supported or not supported using data and statistical analysis to support your explanation and analysis of the hypothesis.
- Explain the results obtained in the experiment in terms of diffusion and solution types (hypotonic, hypertonic, isotonic).
- Justify your calculation of the osmolarity in the potato cube using data and statistical analysis.
- Describe the importance of conducting statistical tests (standard deviation, linear regression, and graph error bars).
- Explain and justify your confidence in the reliability of your data (class data) based on the statistical tests completed. Justify your explanation using statistical analysis data.
- Analyze the measurement uncertainties (not error) associated with your experiment.
- Evaluate and explain the influence of procedural errors of this experiment..
- Discuss in as much detail as possible, ways that the errors and uncertainties discussed in question nine could be minimized or eliminated in future experiments. Also discuss any other ways you can think of that the reliability of the data could be improved.
- The report includes sufficient relevant quantitative and qualitative raw data that could support a detailed and valid conclusion to the research question.
- Appropriate and sufficient data processing is carried out with the accuracy required to enable a conclusion to the research question to be drawn that is fully consistent with the experimental data.
- The report shows evidence of full and appropriate consideration of the impact of measurement uncertainty on the analysis.
- The processed data is correctly interpreted so that a completely valid and detailed conclusion to the research question can be deduced.
- The report includes relevant but incomplete quantitative and qualitative raw data that could support a simple or partially valid conclusion to the research question
- Appropriate and sufficient data processing is carried out that could lead to a broadly valid conclusion but there are significant inaccuracies and inconsistencies in the processing
- The report shows evidence of some consideration of the impact of measurement uncertainty on the analysis
- The processed data is interpreted so that a broadly valid but incomplete or limited conclusion to the research question can be deduced
- The report includes insufficient relevant raw data to support a valid conclusion to the research question
- Some basic data processing is carried out but is either too inaccurate or too insufficient to lead to a valid conclusion
- The report shows evidence of little consideration of the impact of measurement uncertainty on the analysis
- The processed data is incorrectly or insufficiently interpreted so that the conclusion is invalid or very incomplete
The student’s report does not reach a standard described by the descriptors above.
- A detailed conclusion is described and justified which is entirely relevant to the research question and fully supported by the data presented.
- A conclusion is correctly described and justified through relevant comparison to the accepted scientific context.
- Strengths and weaknesses of the investigation, such as limitations of the data and sources of error, are discussed and provide evidence of a clear understanding of the methodological issue involved in establishing the conclusion.
- The student has discussed realistic and relevant suggestions for the improvement and extension of the investigation.
- A conclusion is described which is relevant to the research question and supported by the data presented
- A conclusion is described which makes some relevant comparison to the accepted scientific context
- Strengths and weaknesses of the investigation, such as limitations of the data and sources of error, are described and provide evidence of some awareness of the methodological issues* involved in establishing the conclusion
- The student has described some realistic and relevant suggestions for the improvement and extension of the investigation
- A conclusion is outlined which is not relevant to the research question or is not supported by the data presented
- The conclusion makes superficial comparison to the accepted scientific context.
- Strengths and weaknesses of the investigation, such as limitations of the data and sources of error, are outlined but are restricted to an account of the practical or procedural issues faced.
- The student has outlined very few realistic and relevant suggestions for the improvement and extension of the investigation.
- The student’s report does not reach a standard described by the descriptors above.
A full explanation of the lab standard rubric can be found here
 This lab is primarily adapted from Mr. Herbstritt’s IB Biology Diffusion Lab. The background information is from Vernier’s Osmosis Lab, #22.