H2 Biology Osmosis and Diffusion Practical: Potato Cores and Agar Blocks

Safety: Use cut-resistant mat; handle scalpels carefully; NaOH is corrosive-wear goggles and rinse spills immediately.

3 Osmosis: serial dilution and setup

Prepare 6 sucrose concentrations (10 mL each) using volumetric glassware. Label clearly and keep all at room temperature.

1. Cut potato cores to a uniform length (e.g., $\pu{30.0 mm}$) and diameter; trim ends square. Blot gently.
2. Record initial mass for each core ($m_i$). Assign one core per concentration.
3. Immerse cores for a fixed period (e.g., 20-30 min) without stirring; keep temperature constant.
4. Remove, blot consistently, and record final mass ($m_f$).

Quick calculation: $\%,\Delta m = \frac{m_f - m_i}{m_i} \times 100\%$.

Graph: plot $\%,\Delta m$ (y) vs sucrose concentration (x). The x-intercept where $\%,\Delta m = 0$ estimates the isotonic concentration.

Controls/consistency

• Use cores from the same potato where possible; avoid air gaps; standardise blotting.
• Keep soak time identical; avoid convection by spacing tubes.

4 Diffusion: agar block decolourisation

1. Prepare phenolphthalein agar blocks in alkaline solution (pink). Cut three sizes (e.g., $\pu{1.0, 1.5, 2.0 cm}$ cubes) with clean edges.
2. Rinse briefly, then immerse in neutral water (or mild acid) and start timing.
3. At intervals, bisect a block to inspect the colourless penetration depth, or end each run when fully decolourised and note the time.

Outputs

• Table: block size, estimated surface area/volume, time to full decolourisation (or rate proxy such as depth per unit time).
• Graph: time vs size (or inverse with SA/V). Expect larger blocks to take longer; discuss diffusion limits and why cells are small.

5 Data treatment (PDO)

• Show significant figures consistently between raw and processed values.
• Add lines of best fit where appropriate; do not force through origin without reasoning.
• Include error bars on repeated measurements (standard deviation) if time permits.

6 Evaluation (ACE)

Common uncertainties

• Potato cores: variable tissue density, blotting inconsistency, surface damage-standardise cutting and blotting; run duplicates for key concentrations.
• Agar blocks: imprecise endpoint when judging colour; use consistent lighting and an agreed endpoint criterion; consider photographing slices.
• Temperature drift: diffusion and osmosis rates are temperature-sensitive; measure and report ambient temperature.

Improvements

• Replace manual timing with video timestamps or a data-logger where permitted; increase repeats at the isotonic range for a better intercept.
• For diffusion, switch to thinner slabs and measure penetration depth vs time with calipers for a richer dataset.

7 60-minute pacing guide

8 Links & next steps

• If you want the full 9477 Paper 4 roadmap around this experiment, start with the broad practical guide: https://eclatinstitute.sg/blog/h2-biology-experiments/H2-Biology-Practical-2026-Lab-Mastery-Guide
• Pair with microscopy skills so drawings and measurements are exam-ready: https://eclatinstitute.sg/blog/h2-biology-experiments/H2-Biology-Optical-Microscope-Mastery-Guide
• Drill enzyme kinetics next to complete the Paper 4 core set: https://eclatinstitute.sg/blog/h2-biology-experiments/H2-Biology-Enzyme-Kinetics-Catalase-Practical-Guide
• Add a rate-based investigation with photosynthesis or respiration to deepen your Core Idea 3 coverage: https://eclatinstitute.sg/blog/h2-biology-experiments/H2-Biology-Photosynthesis-and-Respiration-Rate-Practical-Guide
• Drill the Paper 4 planning template (aim, variables, procedure, risk, evaluation) with H2 Biology Planning and Evaluation for Paper 4.

Assessment anchors (MMO/PDO/ACE)

• MMO (Manipulation, Measurement, Observation)
  • Potato cores cut to uniform dimensions; consistent blotting; fixed soak time and temperature recorded.
  • Agar blocks cut cleanly to target sizes; consistent endpoint criterion for decolourisation timing.
  • Single independent variable per run; duplicates at the isotonic range and for at least one block size.
• PDO (Presentation of Data and Observations)
  • Osmosis: % mass change table with initial/final masses; graph of %Δm vs concentration; isotonic point interpreted from intercept.
  • Diffusion: table of size, SA/V (or size proxy), and time; graph of time vs size or vs 1/(SA/V) with axes/units.
  • Significant figures consistent between raw and processed values; captions note ambient temperature.
• ACE (Analysis, Conclusions, Evaluation)
  • Osmosis: link isotonic point to water potential; discuss variability from tissue heterogeneity and blotting.
  • Diffusion: explain why larger blocks take longer; discuss endpoint subjectivity and improve with photos/video timestamps.
  • Suggest improvements (more repeats near isotonic, controlled temperature baths, thinner slabs for diffusion depth‑time studies).

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References

• SEAB. (2024). Biology (Syllabus 9477) GCE A-Level 2026. Singapore Examinations and Assessment Board. (Paper 4 MMO/PDO/ACE weighting and practical expectations.)
• Campbell, N. A. et al. (2020). Campbell Biology (12th ed.). Pearson - transport across membranes; diffusion and osmosis labs.

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