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

Study guide12 Oct 2025, 00:00 ZUpdated 17 Jul 2026

Reviewed by

Ezekiel Tan·Academic Advisor (Biology)

Q: What does this H2 Biology osmosis and diffusion practical guide cover?
A: It shows you how to run the revised 9477 Paper 4 potato-core osmosis practical and agar-block diffusion experiment, process the graphs, and evaluate the usual sources of error.

TL;DR
If you are searching for the H2 Biology osmosis practical or agar block diffusion experiment, this is the Paper 4 version to rehearse: cut uniform potato cores, run a sucrose series, and plot percentage mass change vs concentration to estimate the isotonic point.
For diffusion, time decolourisation in phenolphthalein agar blocks of different sizes and relate time to the surface area-to-volume ratio so conclusions are quantitative.

Concrete example: If a potato core gains mass in dilute sucrose and loses mass in concentrated sucrose, the isotonic point is where the graph crosses 0% mass change.

For the wider Paper 4 sequence around this experiment, use the H2 Biology practical guide before or after this page.

For JC1-JC2 students who need marked feedback on transport practicals, use H2 Biology tuition Singapore as the programme route after this self-check.

Status: SEAB's 2026 A-Level syllabus index lists revised H2 Biology 9477 for school candidates and marks 9744 as last year of exam in 2026. For the revised 9477 Paper 4, Paper 4 is 2 h 30 min, 50 marks, and 20% of H2 Biology, with Planning, MMO, PDO, and ACE assessed in practical contexts.

1 Aims, Paper 4 fit, and 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.

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Practise this in a lab

Compare current lesson timings, pricing, and venues for the practical programme that matches this guide.

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Practical course record and certificate note

Eclat keeps centre-held attendance records and may issue an internal attendance or completion document based on participation and internal assessment.

For SEAB private-candidate declarations, the centre record is the key evidence. Any Eclat completion document is not an MOE or SEAB qualification or accreditation.

Search phrase students use	What the answer must prove	Paper 4 move
`how to find isotonic point on graph`	The isotonic point is the x-value where percentage mass change is 0.	Interpolate from the best-fit line and state that there is no net water movement at that concentration.
`agar block experiment`	Larger blocks have a lower surface area-to-volume ratio and usually take longer for indicator change.	Compare timing with block size, then link the trend to diffusion distance and surface area-to-volume ratio.
`diffusion practical`	The measured endpoint must be defined before timing starts.	State the colour endpoint, keep lighting constant, and use the same inspection method for every block.

Paper 4 demand	What to write	Common weak answer
Movement across membranes	State whether the evidence shows osmosis, simple diffusion, or a surface area-to-volume effect.	Naming osmosis without linking it to the concentration gradient.
PDO graphing	Plot the processed value, not only raw mass or raw time.	A table of readings with no percentage change, rate, or intercept.
ACE evaluation	Identify the main uncertainty and its likely effect on the trend.	"Repeat for reliability" without naming blotting, block cutting, endpoint judgement, or temperature.

What went wrong	Likely Paper 4 strand	Best next action
Potato cores were different sizes or blotting changed between tubes	MMO	Repeat the setup with one cutting and blotting rule, then record the rule in the method.
The graph used raw mass instead of percentage mass change	PDO	Rebuild the table with initial mass, final mass, percentage change, and concentration.
The isotonic point was guessed from one point	ACE	Interpolate from the best-fit line and state the concentration interval.
The plan did not explain how concentration range was chosen	Planning	Use the H2 Biology Planning and Evaluation guide to add range, repeats, controls, and data treatment.
The same error appears after self-marking	repeated feedback gap	Use H2 Biology tuition Singapore only after identifying which strand keeps failing.

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

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

1 Aims, Paper 4 fit, and assessment focus

Practise this in a lab

2 Apparatus & reagents

3 Osmosis: serial dilution and setup

4 Diffusion: agar block decolourisation

5 Data treatment (PDO)

Isotonic point and agar block search checkpoint

9477 transport practical checkpoint

Transport-to-Paper-4 weakness diagnosis

6 Evaluation (ACE)

7 60-minute pacing guide

8 Links & next steps

Assessment anchors (MMO/PDO/ACE)

References