Introduction: The human body is about as salty as seawater. If we take seawater as an example of a solution, the salt is called the solute and the water is the solvent. Osmosis is the movement of water across a membrane from an area of lower solute concentration to an area of higher solute concentration. Cells tend to lose water (their solvent) in hypertonic environments (where there are more solutes outside than inside the cell) and gain water in hypotonic environments (where there are fewer solutes outside than inside the cell). When solute concentrations are the same on both sides of the cell, there is no net water movement, and the cell is said to be in an isotonic environment. In this lab we will test samples of potato tissue to see how much water they absorb or release in salt solutions of varying concentrations. This gives us an indirect way to measure the osmotic concentration within living cells.

Materials:

• electronic balance (0.01 mg range)
• 6 dishes or pans
• potatos (1 or 2 per class)
• single edged razor or knife
• paper towels
• watch or clock
• table salt, tap water
• 6 beakers (250 ml or larger)

Methods:

1) Pre-mix 6 beakers of salt solutions (0.0001%, 0.001%, 0.01%, 0.1%, 1%, 10%) in water.

2) Prepare six potato slices that are the same thickness (approximately 5 mm cubes) and blot them dry on a paper towel.

3) Mass (weigh) each to the nearest 0.1 grams, keeping them separate, and record as initial mass.

4) Fill each pan with one of the 6 stock solutions, keeping track of which is which!

5) Leave one of the potato slices in each of the salt solutions for at least 15 minutes so that they may gain (or lose) water by osmosis. (Keep them all in the salt water the same amount of time-leaving them overnight is likely to give the best results).

6) Remove the slices, blot them dry on a paper towel, carefully re-weigh them and record in the data table as final mass.

Results:

1) Record your actual results below:

2) On the reverse, prepare a graph showing change in mass as a function of % salt. Note that the graph is log scaled (each salt concentration is 10 times the previous one) on the x-axis. The y-axis has a zero line half way up, indicating whether the samples lost or gained weight. You will have to scale the y-axis according to your greatest and smallest changes in mass.

3) When completed, use a ruler to draw a straight line of best fit through your six data points, or use the computer to graph your data and calculate the line of best fit. Where the line of best fit crosses the horizontal zero line, draw a vertical line down to the x-axis. This is the point at which the potato is isotonic with its surroundings, and is the estimated salt concentration of the potato.

Figure 1: Change in mass of potato (g) as a function of salt concentration.

Questions:

1) Why did some potato samples gain water and others lose water? Was there any pattern?

2) When you drew the best fit line through your data and dropped the vertical line to the x-axis, what salt concentration did you obtain (Estimate if it is between numbers)? What does this mean for the potato?

3) Why cant we save water by using seawater to irrigate our crops?

4) What happens when a thirsty person drinks salt water to try to quench their thirst?

5) Why does salted popcorn dry your lips?

6) Challenge question: Saltwater fish are hypotonic to their surroundings while freshwater fish are hypertonic to their surroundings. What must each fish do with fluids in order to compensate for the difference in salinity between the body and the surrounding environment?

Download the program "MacCurveFit" or "WinCurveFit" and use it to graph and fit a curve to your data.