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Prepare several known solute concentrations from a stock solution using measured volumes and a consistent final volume. Include an appropriate range spanning expected mass gain and mass loss. Use the same tissue source where possible and cut comparable pieces so surface area and diffusion distance do not vary unnecessarily.
Video by Malmesbury Education. Use the demonstration alongside this lesson’s specification reference and written explanations.
Viewing focus: Prepare and label a solute dilution series
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: Why use percentage rather than only raw mass change?
To account for different starting masses. The response is scaled to the initial mass.
After watching 2: The zero-change crossing indicates approximately:
Similar tissue and external water potentials. Net movement is near zero in the model.
What you'll be able to do
Prepare several known solute concentrations from a stock solution using measured volumes and a consistent final volume. Include an appropriate range spanning expected mass gain and mass loss. Use the same tissue source where possible and cut comparable pieces so surface area and diffusion distance do not vary unnecessarily.
Measure initial masses, immerse pieces in equal treatment volumes for an appropriate controlled time, and keep temperature consistent. Use independent pieces as replicates. Before final weighing, blot each piece consistently to remove surface solution without squeezing out tissue water; inconsistent blotting can imitate an osmotic mass change.
Percentage mass change is (final−initial)/initial×100. This normalises moderate differences in starting mass better than comparing raw gram changes alone. Positive values indicate mass gain and negative values loss under the measurement model.
Plot mean percentage mass change against solute concentration or the known water potential of each standard, with suitable variation information. The zero-change crossing estimates the treatment with similar water potential to the tissue at the relevant conditions. Interpolate within a bracketed range rather than guessing from only solutions that all caused mass loss.
A zero-change concentration in mol dm⁻³ is not itself a water potential in kPa. To report water potential, use a supplied concentration–water-potential relation at the relevant temperature or standards already labelled with water potential. Do not attach kPa to a molarity number without conversion evidence.
The estimate assumes mass change mainly reflects net water movement and that the tissue approaches a suitable equilibrium without substantial damage or solute exchange. Biological variation, incomplete equilibration and damaged cells can affect it. Repeat over a narrower range near the crossing if greater precision is needed.
Try each question before opening the worked response. Use precise biological terms, connect each cause to its effect, and check units and assumptions when interpreting data.
Common mistakes to avoid
Key takeaways
Question 1: A zero-change crossing occurs at 0.30 mol dm⁻³. A supplied calibration gives −490 kPa at 0.20 and −980 kPa at 0.40, approximately linear between them. Estimate tissue water potential.
Approximately −735 kPa using the supplied calibration and interpolation assumption.
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.