4.6.3Additional lessons
Osmosis- Skills — IB Biology HL Revision Notes & Topical Questions
5 min read 6 parts Lesson 53 of 101
Part 1 of 6Experimental Design
Quantitative data allow stronger conclusions than observations such as softer or firmer.
Independent variable
- The concentration of the sucrose or salt solution in each tube, chosen to span a wide range including pure water.
Dependent variable
- The change in mass or length of tissue cylinders after a fixed time in the solution.
Controls
- Use the same plant, cylinder size, volume of solution, temperature and time, so that only concentration differs.
Repeats
- Use several cylinders per concentration and calculate a mean to reduce the effect of random error.
Part 2 of 6Method with Potato Cylinders
1
Cut equal cylinders of potato with a cork borer, trim to the same length and remove the skin.
2
Blot each cylinder dry and record its initial mass on a balance to two decimal places.
3
Place cylinders in test tubes containing different solution concentrations and leave them for a set time, often at a constant temperature.
4
Remove, blot dry in the same way, and measure the final mass of each cylinder.
5
Calculate the percentage change in mass for each, and plot a graph against solution concentration.
Part 3 of 6Analysis
Percentage change
- Change in mass divided by initial mass, times 100;
- percentage allows cylinders of different starting mass to be compared.
Graph
- Plot concentration on the x-axis and percentage change on the y-axis, drawing a line or curve of best fit.
Intercept
- Where the line crosses zero change, tissue and solution are isotonic, so this concentration equals the osmolarity of the tissue.
Gains and losses
- Positive values show water uptake from hypotonic solutions, and negative values show loss in hypertonic solutions.
| Concentration | % change in mass | Interpretation |
|---|---|---|
| 0.0 mol dm⁻³ | +15 | Hypotonic, water enters |
| 0.3 mol dm⁻³ | 0 | Isotonic |
| 0.6 mol dm⁻³ | -12 | Hypertonic, water leaves |
Part 4 of 6Sources of Error
Blotting
- Different amounts of surface water left on the cylinders alter the mass, so blot consistently.
Cylinder variation
- Cylinders from different parts of a potato may have different solute concentrations, so use one potato for each set.
Preliminary trials
- Trials help choose a sensible range of concentrations and a suitable time.
Part 5 of 6Worked Example
Question
- A potato cylinder has an initial mass of 2.50 g and a final mass of 2.80 g after 60 minutes in a solution.
- A second cylinder starts at 2.40 g and finishes at 2.16 g.
- Calculate the percentage changes and state what they show.
- Cylinder 1: (2.80 - 2.50) / 2.50 x 100 = 12.0%.
- Cylinder 2: (2.16 - 2.40) / 2.40 x 100 = -10.0%.
- A gain shows water entered; a loss shows water left.
Answer
- Cylinder 1 gained 12.0% (solution hypotonic to the tissue);
- cylinder 2 lost 10.0% (solution hypertonic to the tissue).
Part 6 of 6Exam Checklist & Tips
4.6.3 – Key Points for the Exam
- Osmolarity The total concentration of solute particles in a solution.
- Percentage change Change in a quantity as a percentage of its starting value.
- Isotonic point The concentration at which tissue shows no change in mass.
- Control variable A factor kept constant so it does not affect the result.
Common Mistakes
- Calculating percentage change using the final mass as the denominator.
- Giving a negative percentage a positive sign.
- Reading the isotonic point from the highest or lowest concentration instead of the intercept.
- Failing to blot cylinders in the same way each time.
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