Membrane Diffusion Lab^[1]

In order to survive, all organisms need to move molecules in and out of their cells. Molecules such as gasses (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.

Key Terms:

• 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.

Objectives:

• 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)

Research Question:

Hypothesis:

Materials: (for each group)

• Six 50 or 100ml beakers (all six need to be of same size)
• 75 ml of distilled water 0.0M, 0.2 Molar, 0.4 Molar, 0.6 Molar, 0.8 Molar and 1.0 Molar Saline solutions
• 6 2cm3 potato cubes

Procedure:

1. Label each beaker with its appropriate solution concentration.
2. Pour 50 ml of distilled water into the beaker marked “0.0M.”
3. Repeat this for the remaining saline solutions in the 5 beakers with their respective saline solutions.
4. Cut 6 potato cubes to a length of 2cm3 (be as accurate as possible!). Remove any skin from the cubes.
5. Find and record the mass for each potato cube and record in Table 1 under “Initial Mass.”
6. Place 0.0 Molar potato cube in its’ respected beaker.
7. Repeat Steps 5 and 6 for each of the remaining saline solution beakers.
8. Ensure that all potato cubes are completely submerged; add an equal amount of solution to all beakers if one cube is not submerged.
9. After 48 hours, remove the potato cube out of the 0.0 Molar Solution cup and carefully blot dry with a paper-towel.
10. Find and record  the 0.0 Molar potato cube mass under Final Mass in Table 1.
11. Repeat Steps 11 and 12 for each of the remaining saline solutions.
12. Calculate the percent change in mass for each of the solutions:( (Final Mass-Initial Mass)/Initial Mass) x 100%
13. Turn in group data for Percent Change in Mass for each solution concentration here.

Data Tables:

Table 1: Raw Data Individual Mass Change in Potato Tissue in Various Salt Solutions

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 spreadsheet.

Table 3. Summarized Data Class Mean % Change in Mass in Potato tissue in various Saline Concentrations

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 Potato Cube Size

Table 5. Qualitative Observations of salt Concentration Effect on Potato Cube Mass

Data Analysis

• Create data tables for each of the different sections of the lab (qualitative data, raw data, summarized data)
• Submit your individual group data to the following form; class data can be found here.
• Use the class data for all statistical analysis to analyze the raw data
• Perform statistical tests/analysis for the data.
• Create a graph to visualize the data
• 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.

Evaluation

• Using your graphs and statistical analysis of the class data, analyze the experiment results and your results given the Conclusion & Evaluation rubric by writing a conclusion & evaluation in paragraph form.

Turn-In:

Turn in via Canvas

Assessment: