Aim for FiveBiology

Interpreting and evaluating experimental results with graphing

Sweet potato cores in sucrose

  • Unit 2
  • 9 points
  • About 25 minutes

You can use a calculator on this question, just like on exam day.

A long question with an experiment and a data table that you turn into a graph. On this site you describe the graph you'd draw (type, axes, units, scale, points and error bars), then analyze the data and make and justify a prediction. On the exam: Question 2 of 6 in Section II (90 minutes, 50% of the exam score); about 25 minutes suggested.

The question and its sources

Water moves across cell membranes by osmosis. Students investigated the water potential of sweet potato tissue.

Experiment

Students cut cylinders (cores) of sweet potato of the same size, blotted them dry and recorded each core's mass. They placed the cores in open beakers of sucrose solution at six concentrations at 22°C, with four beakers per concentration and three cores per beaker. After 24 hours, they blotted and reweighed the cores and calculated the percent change in mass.

Source: Hypothetical study

Table 1. Percent change in mass of sweet potato cores

Sucrose concentration (M)Percent change in mass, mean±2SE
0.018.01.5
0.29.01.3
0.42.41.1
0.6−6.21.2
0.8−12.51.4
1.0−16.11.6

Source: Hypothetical data

Suggested time: 25 minutes

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Part (a)

1 point

Describe the net movement of water by osmosis in terms of water potential.

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Part (b)

4 points

(i) Describe the graph you would construct to represent the data in Table 1. In your description, name the type of graph, give the label and units for each axis, describe the scale for each axis, list the values you would plot, and describe the error bars. (ii) Based on the data, estimate the sucrose concentration at which the cores would show no change in mass.

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Part (c)

2 points

(i) Calculate the solute potential of the 0.4 M sucrose solution at 22°C. Use Ψs = −iCRT, where R = 0.0831 L·bar/(mol·K). Include units. (ii) Explain why the cores in the 0.0 M solution gained mass.

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Part (d)

2 points

A student repeats the experiment for one group of cores, using 0.2 M NaCl instead of 0.2 M sucrose. NaCl separates into two ions in water. (i) Predict how the percent change in mass of the cores in 0.2 M NaCl would compare with that of the cores in 0.2 M sucrose. (ii) Justify your prediction.

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