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Q: What does H2 Biology Practical: Osmosis and Diffusion (Potato Cores & Agar Blocks) cover?
A: Two cornerstone investigations for Paper 4: determine isotonic concentration via potato core mass change (osmosis) and model surface-area-to-volume effects with phenolphthalein agar blocks (diffusion). Includes serial dilutions, controlled variables, initial measurements, graphing, and evaluation pointers aligned to MMO/PDO/ACE.TL;DR Δ m ≈ 0 \Delta m \approx 0 Δ m ≈ 0 1 Aims & assessment focus Osmosis: quantify how external solute concentration influences net water movement across plant tissue; estimate isotonic concentration from your graph.
Diffusion: show how block size (and surface-area-to-volume ratio) affects time for indicator to change; discuss limits to cell size.
Assessment hooks: MMO (accurate cutting/volumes/timing), PDO (clean tables/graphs/units), ACE (uncertainties, trends, biological explanations).
2 Apparatus & reagents Fresh potatoes (firm; similar cultivar), cork borer or chip cutter, scalpel, ruler, balance (0.01 g).
Sucrose solutions for a serial series (e.g.,
0.00 , 0.10 , 0.20 , 0.30 , 0.40 , 0.50 m o l ⋅ d m − 3 \pu{0.00, 0.10, 0.20, 0.30, 0.40, 0.50 mol.dm-3} 0.00 , 0.10 , 0.20 , 0.30 , 0.40 , 0.50 mol ⋅ d m − 3
) or % w/v equivalents.
Phenolphthalein agar (pink in alkaline) cut into 3 cube sizes (e.g.,
1.0 c m , 1.5 c m , 2.0 c m \pu{1.0 cm, 1.5 cm, 2.0 cm} 1.0 cm , 1.5 cm , 2.0 cm ); 0.1 mol dm⁻³ NaOH and neutral water bath.
Beakers/test tubes, labels, forceps, paper towels, timer, thermometer, cutting board.
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.
Cut potato cores to a uniform length (e.g.,
30.0 m m \pu{30.0 mm} 30.0 mm ) and diameter; trim ends square. Blot gently.
Record initial mass for each core (
m i m_i m i ). Assign one core per concentration.
Immerse cores for a fixed period (e.g., 20-30 min) without stirring; keep temperature constant.
Remove, blot consistently, and record final mass (
m f m_f m f ).
Quick calculation: % , Δ m = m f − m i m i × 100 % \%,\Delta m = \frac{m_f - m_i}{m_i} \times 100\% % , Δ m = m i m f − m i × 100%
Graph: plot % , Δ m \%,\Delta m % , Δ m % , Δ m = 0 \%,\Delta m = 0 % , Δ m = 0
4 Diffusion: agar block decolourisation Prepare phenolphthalein agar blocks in alkaline solution (pink). Cut three sizes (e.g.,
1.0 , 1.5 , 2.0 c m \pu{1.0, 1.5, 2.0 cm} 1.0 , 1.5 , 2.0 cm cubes) with clean edges.
Rinse briefly, then immerse in neutral water (or mild acid) and start timing.
At intervals, bisect a block to inspect the colourless penetration depth, or end each run when fully decolourised and note the time.
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) 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.
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 Minute Task 0-10 Label tubes, prep sucrose series; cut potato cores; weigh \(m_i\) 10-40 Start osmosis soak; in parallel, cut agar blocks and run diffusion trials 40-50 Weigh \(m_f\); compute \(\% \space \Delta m\); compile diffusion times 50-60 Plot graphs; write conclusion + limitations/improvements
8 Links & next steps
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).
References SEAB. (2024). Singapore-Cambridge GCE A-Level Biology (9729) Syllabus (for examination in 2026). Singapore Examinations and Assessment Board.
Campbell, N. A. et al. (2020). Campbell Biology (12th ed.). Pearson - transport across membranes; diffusion and osmosis labs.
H2 Biology Practical: Osmosis and Diffusion (Potato Cores & Agar Blocks)
