Learning focus
Develop accurate biological vocabulary, explain structure–function relationships, apply ideas to unfamiliar contexts, and use practical evidence.
Potato-cylinder method
Cut cylinders with the same cork borer and trim them to equal length. Measure initial mass, place them in known solution concentrations for the same time, blot consistently and measure final mass. Percentage change = change in mass ÷ initial mass × 100. Percentage change permits fair comparison when starting masses are not exactly equal.
Dialysis-tubing method
Soak the tubing, tie one end, add a solution and securely tie the other end. Rinse the outside, blot and measure mass before immersion in a surrounding solution. Water moves according to the water-potential gradient, while large solute molecules may be retained. A control using equal concentrations helps show that mass change is caused by the gradient.
Graph interpretation
Plot solution concentration on the x-axis and percentage mass change on the y-axis. Draw a smooth best-fit line or curve. The x-intercept, where percentage change is zero, estimates the concentration isotonic with the tissue. Interpolation within the data range is more reliable than extrapolation beyond it.
Sources of error
Unequal cylinder dimensions alter surface area and diffusion distance. Inconsistent blotting leaves different amounts of surface liquid. Evaporation changes concentration, and short immersion may not allow sufficient movement. Biological variation between pieces of potato also contributes.
Improving reliability and validity
Repeat each concentration and calculate a mean, use pieces cut from the same potato, control temperature and time, cover containers, and use a wide but sensible range of concentrations with smaller intervals near the x-intercept. Reliability concerns consistency; validity concerns whether the method tests the intended relationship.
Practical or data skill
Carry out either potato or dialysis-tubing osmosis. Present a full results table with units in headings, calculate percentage change and plot a graph. Identify an anomaly and decide whether it should be repeated rather than automatically removed.
Examination tip
Do not say ‘repeat to make it accurate’. Repeats identify anomalies, reduce random error and allow a mean, improving reliability.
Review questions and suggested answers
Question 1
Why calculate percentage change rather than only change in mass?
Suggested answer
It accounts for differences in initial mass and allows fair comparison.
Question 2
What does the x-intercept represent?
Suggested answer
The solution concentration at which there is no net water movement and no percentage mass change.
Question 3
Give one reason for blotting potato cylinders.
Suggested answer
To remove surface solution so measured mass change reflects water movement within the tissue.
