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4.5.3Additional lessons

Osmosis: Skills — IB Biology SL Revision Notes & Topical Questions

5 min read 6 parts Lesson 115 of 142

📚 IB Biology SL Hub / Topical Revision
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Part 1 of 6Planning the Investigation

4.5.3 Osmosis: Skills
1
Prepare a range of sucrose solutions of known concentration, including distilled water as the lowest concentration, using accurate dilution from a stock solution.
2
Cut identical cylinders of tissue from one potato with a cork borer and trim them to the same length, so that surface area is equal.
3
Blot each piece dry, record its initial mass to a fixed number of decimal places, and place one piece in each solution.
4
Leave all tubes for the same time at the same temperature, then remove, blot dry and record the final mass.
5
Repeat with several pieces per concentration so a mean can be calculated and anomalies spotted.

Part 2 of 6Variables and Controls

Quantitative data allow stronger conclusions than observations such as a piece looking softer.

Independent
  • The concentration of the sucrose solution, which is deliberately changed in equal steps.
Dependent
  • The percentage change in mass of the tissue, which reflects the net water gained or lost.
Controlled
  • Tissue source, piece size, volume of solution, temperature, and time of immersion all have to be kept constant.
Pilot study
  • A short trial run to choose a suitable time and concentration range so that data are useful and reagents are not wasted.

Part 3 of 6Processing the Results

Processing the Results
Percentage change
  • Use (final mass − initial mass) ÷ initial mass × 100, which allows pieces of different starting mass to be compared fairly.
Graph
  • Plot concentration on the x-axis and percentage change in mass on the y-axis, with a smooth best-fit line and error bars where possible.
Intercept
  • The concentration where the line crosses zero change is the point of no net water movement, so tissue and solution are isotonic.
Interpretation
  • Left of the intercept the solution is hypotonic and tissue gains mass;
    • to the right it is hypertonic and tissue loses mass.
Uncertainty
  • Balance precision gives a ± value for each mass, and spread between repeats can be shown as standard deviation error bars.

Part 4 of 6Evaluating the Method

Drying
  • Inconsistent blotting leaves different amounts of surface liquid and can alter mass readings.
Equilibrium
  • If time is too short the tissue has not reached balance, so the intercept is underestimated or unreliable.
Tissue variation
  • Pieces from different potatoes differ in solute content, so using one potato improves validity.
Improvement
  • Use more concentrations near the intercept and more repeats to narrow the estimate.

Part 5 of 6Worked Example

Question

  • Potato cylinders lose or gain mass in sucrose solutions.
  • At 0.2 mol dm⁻³ a cylinder rises from 2.50 g to 2.65 g;
    • at 0.4 mol dm⁻³ another falls from 2.40 g to 2.28 g.
  • Estimate the concentration that is isotonic with the tissue, assuming a straight line between these points.

  1. At 0.2 mol dm⁻³: (2.65 − 2.50) ÷ 2.50 × 100 = +6.0%.
  2. At 0.4 mol dm⁻³: (2.28 − 2.40) ÷ 2.40 × 100 = −5.0%.
  3. Across this 0.2 mol dm⁻³ interval the change is 11.0 percentage points, so zero change occurs 6.0 ÷ 11.0 of the way from 0.2 to 0.4.
  4. Concentration = 0.2 + 0.2 × (6.0 ÷ 11.0) = 0.2 + 0.109 = 0.309 mol dm⁻³.

Answer About 0.31 mol dm⁻³ sucrose is isotonic with the potato tissue.

Part 6 of 6Exam Checklist & Tips

4.5.3 – Key Points for the Exam

  • Osmotic concentration Total solute concentration of a solution or tissue, which determines water movement by osmosis.
  • Isotonic point Concentration at which tissue shows no net change in mass.
  • Percentage change Change in a quantity divided by the starting value, multiplied by 100.
  • Control variable A factor kept constant so that only the independent variable affects the result.
  • Pilot study Preliminary trial used to refine the method before the main investigation.

Common Mistakes

  • Plotting raw final mass instead of percentage change, which hides the effect of different starting masses.
  • Extrapolating the line beyond the range of concentrations tested when reading off the isotonic point.
  • Forgetting to blot the tissue, which makes the measurements inconsistent.
  • Using a different potato for every concentration, so tissue differences are mixed with the effect of the solution.

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