Determining the Tonicity of Solutions Relative to a Potato

The term used to described the concentration of solutes in a given solution is tonicity (Lab 6-7). Three states of tonicity exist: hypotonicity, hypertonicity, and isotonicity. In a hypotonic solution, one side of a membrane, such as the potato cells, has a lower solute concentration than the other side (Lewis 963). In a hypertonic solution, the solute concentration is greater than on the other side (Lewis 963). In an isotonic solution, the amount of solute both inside and outside of membrane is the same; this state is referred to as dynamic equilibrium. This lab determined the tonicity of Solution A (5% NaCl), Solution B (0% NaCl), Solution C (0.9% NaCl), and Solution D (2% NaCl) relative to a potato.

Hypothesis

Solution A is hypertonic to the potato; the potato submerged in Solution A will weigh more than the potatoes in the hypo/isotonic solutions. Solution B is hypotonic to the potato, so the potato will weigh less comparatively. Solution C is isotonic, so the potato will remain approximately the same weight throughout the lab.

Materials and Methods

This experiment was followed as stated in General Biology: Study Guide and Laboratory Manual:(Lab 6-7), with one exception: the actual concentrations of NaCl in Solutions A, B, C, and D were provided. Their concentrations were the ensuing: Solution A (5% NaCl), Solution B (0% NaCl), Solution C (0.9% NaCl), and Solution D (2% NaCl). Four 50-ml beakers were labeled A, B, C, and D. Then four potato cubes were cut at approximately 1.0 cm per side and weighed to the nearest 0.01 g using a tared weighing boat. Once the weights were recorded, one potato cube was placed in each beaker. Enough of Solutions A, B, C, or D was poured over each cube to fully submerge it. Each cube was weighed at fifteen minutes, up to seventy-five minutes. The potato was blotted on a paper towel to remove excess water before it was weighed. The weight was recorded and the cube was returned to its respective beaker. Once the weighing was complete, the percent weight change from the original weight was calculated.

Results

Originally, the potato placed in Solution A weighed 1.71 grams, 1.25 grams in Solution B, 1.33 grams in Solution C, and 1.38 grams in Solution D. After fifteen minutes, Solution A's potato exhibited 1.75% change. After thirty minutes, it exhibited 2.34% change; -5.26% change after forty-five minutes; -11.11% change after sixty minutes; and -10.53% change after seventy-five minutes. A change of 8.8% occurred in Solution B's potato after fifteen minutes. 8.8% change occurred after thirty minutes; 16.8% after forty-five minutes; 8% after sixty minutes; and 9.6% change after seventy-five minutes. Solution C's potato exhibited 2.26% change after fifteen minutes, -2.26% change after thirty minutes; 0% change after forty-five minutes; -1.5% change after sixty minutes; -3% change after seventy-five minutes. After fifteen minutes, Solution D's potato exhibited a 1.45% change; -0.72% change after thirty minutes; -5.8% change after forty-five minutes; a -5.8% change after sixty minutes; and -8/7% change after seventy-five minutes.

Discussion of Results

Based upon the data, it appears that Solution A is hypertonic to the potato; Solution B is hypotonic; Solution C is isotonic; and Solution D is slightly hypertonic. The reason why A is hypertonic is because it initially has a higher concentration of NaCl than the potato before the potato and solution achieve dynamic equilibrium sometime between thirty and forty-five minutes. The weight of the potato remained between 1.53 and 1.75 grams. The greatest change occurred between thirty and forty-five minutes, when the potato and the solution were exiting dynamic equilibrium. Solution B is hypotonic because it initially has a lower solute concentration than the potato until the potato and solution achieve dynamic equilibrium between fifteen and thirty minutes. The weight of the potato remained between 1.25 and 1.46 grams. The greatest weight change happened between thirty and forty-five minutes when, it too, exited dynamic equilibrium. Solution C is isotonic because it contains the same amount of NaCl as the potato and therefore has already achieved dynamic equilibrium. The potato did not experience any significant weight change, remaining between 1.29 and 1.36 grams. Exact dynamic equilibrium occurred at forty-five minutes. Solution D is mildly hypertonic because it initially contains a slightly higher concentration of NaCl than the potato, but, around fifteen to thirty minutes, it achieved dynamic equilibrium. The weight remained between 1.26 and 1.40 grams. Given that the solutions were possessed the predicted tonicity, data supports the hypothesis.

Conclusion

The data supports the hypothesis that Solution A is hypertonic, Solution B is hypotonic, and Solution C is isotonic. Although not mentioned in the hypothesis, Solution D is slightly hypertonic.

Literature Cited

King, Betty; Wise, Mary; Erdahl, Emma; West, George; Parker, David. General Biology: Study Guide and Laboratory Manual: Tenth Edition (NVCC-Alexandria). New York: Custom Publishing, 1980.

Lewis, Ricki; Parker, Bruce; Gaffin, Douglas; Hoefnagels, Marielle. Life, Sixth Edition. Boston: McGraw Hill Higher Education, 2007.