Potato Osmosis Lab With a Reversed Data Point
During a 50-minute block, 28 students in six lab groups measure osmosis in potato cores. Each group cuts six cores from the same potato, weighs them to 0.01 g, soaks one core in 0.0, 0.2, 0.4, 0.6, 0.8, or 1.0 M sucrose for 20 minutes, blots, and reweighs. Group 4 reports its 0.4 M core going from 2.14 g to 2.26 g, while the class mean change at 0.4 M is −0.19 g. The same group's 0.2 M core lost 0.11 g. They conclude that potato cell sap is between 0.2 and 0.4 M and submit the lab as valid. You notice some cores have skin, students blot for different times, the 0.6 M core was left on the bench for 9 minutes before weighing, and the balance drifts by 0.03 g when the door is left open.
1. Identify the procedural errors and explain how each could bias the group's estimate of the isotonic sucrose concentration.
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The isotonic point is where net mass change is zero, so every error that shifts measured mass moves the apparent crossing point. Uneven cores and retained skin change surface area-to-volume ratio and diffusion distance; skin also slows water movement, making a core behave as if the external solution were more dilute. Variable blotting adds or removes water before weighing, and because the error differs between initial and final masses, a single core's zero point shifts unpredictably. The 9-minute delay lets the 0.6 M core evaporate, lowering its final mass and exaggerating water loss. Balance drift of 0.03 g is about 1.5% of a 2 g core, enough to move a single core across the zero line. Group 4's pattern-gain at 0.4 M but loss at 0.2 M-reverses the expected monotonic trend, which suggests a sample mix-up, inconsistent blotting, or a recording error rather than a real biological result. The conclusion should rest on class means with uncertainty, not one group's crossing point.
2. Rewrite the procedure and state a defensible rule for deciding whether to exclude Group 4's data.
Read the worked answer
Use one potato and a cork borer to cut uniform cores, remove skin, trim to equal length, rinse, and blot each core the same way for a fixed 3 seconds on paper towel. Weigh immediately with the balance draft shield closed, randomize the order of concentrations, run each concentration in triplicate, and keep all cores in the same water bath at the same temperature. Record percent mass change rather than raw mass, plot mean ± standard deviation, and estimate the isotonic concentration from where the fitted line crosses zero. A data point may be excluded only for a documented procedural deviation, such as a dropped core, a missed timing, or a balance error-not because it contradicts the hypothesis. If Group 4's values are statistical outliers by a pre-set rule, such as more than 2 standard deviations from the class mean, report them as excluded with the reason and show the analysis both with and without them. Otherwise retain the data and discuss the discrepancy.