DSE Biology — Osmosis, Diffusion and Cell Transport Lesson-Prep Guide (for tutors) — water potential and surface area to volume ratio

2026-11-03

Tutor lesson-prep guides · Secondary Biology

Students say water moves from high to low concentration but reverse the direction when two sugar solutions are compared. This lesson fixes direction with water potential and drills percentage change until the marks are safe.

Objectives — Students can: (1) separate diffusion, osmosis and active transport by membrane, energy and direction; (2) compute percentage change in mass and locate the isotonic concentration from data; (3) use surface area to volume ratio to explain why cells stay small.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Membrane structure	What sits inside the phospholipid bilayer	Proteins, as channels and carriers
Concentration	What 0.4 M means	0.4 mol of solute per litre
Percentage change	From 50 to 60 is what change	Plus 20 percent

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-10 min	Three transports	Sort with three questions	Classify six examples orally
10-24 min	Diffusion and osmosis	Water potential versus solute concentration	Predict water movement three times
24-38 min	Potato experiment	Start at 2.00 g, compute percentage change	Get plus 12 and minus 7 percent
38-48 min	Isotonic point	Find the zero crossing	Answer about 0.4 M
48-56 min	Surface area to volume	Compare 20 micrometre and 10 micrometre cubes	0.30 and 0.60 per micrometre
56-60 min	Exit check	Three questions collected and marked	Independent

03 Teaching script (read it out)

Scene 1: Three questions sort diffusion, osmosis and active transport

Ask whether a membrane is involved, whether ATP is used, and what actually moves.
Diffusion moves any particle down its gradient with no membrane and no energy, as oxygen does into alveoli. Osmosis moves only water through a partially permeable membrane, from high water potential to low. Active transport moves substances against the gradient using carrier proteins and ATP, as root hairs absorb mineral ions.
Water potential is solute potential plus pressure potential; pure water has zero solute potential, so adding solute makes it negative and water flows towards the lower value.

Ask the student to repeat: osmosis is about water only, and direction follows water potential.

Scene 2: The potato experiment: how percentage change is marked

Each cylinder starts at 2.00 g. In distilled water it rises to 2.24 g, which is (2.24 – 2.00) divided by 2.00, plus 12.0 percent, because cells take up water and turgor rises. In strong sugar solution it falls to 1.86 g, minus 7.0 percent, because cells lose water and plasmolysis begins.
Plot concentration on the x axis and percentage change on the y axis; the crossing point is the isotonic concentration, about 0.4 M here.
Controls must match species, size, time and temperature, otherwise the difference cannot be attributed to osmosis.

Ask the student to repeat: the denominator is always the starting mass, never an average.

Scene 3: Surface area to volume ratio: why cells stay small

A cube of side 20 micrometres has surface area 2400 and volume 8000, a ratio of 0.30 per micrometre. Halving the side to 10 gives a ratio of 0.60, doubling the exchange surface per unit volume.
The same principle explains villi and microvilli in the small intestine, elongated root hair cells and the rounded alveoli.
Diffusion time scales roughly with the square of distance, so doubling the distance multiplies time by about four, which is why large organisms need circulation.

Ask the student to repeat: smaller cells have a larger ratio, and the ratio must be named, not just “faster”.

04 Question types and worked steps

Type	Example	Steps	Answer
Classify	Diffusion, osmosis or active transport	Membrane, energy, what moves	Water uses osmosis, ions may use active transport
Calculate	Percentage change in mass	(after – before) / before x 100	2.00 to 2.24 gives plus 12 percent
Explain	Why cells are small	Surface area to volume ratio	0.30 against 0.60 per micrometre
Experiment	Find the isotonic concentration	Plot and read the zero crossing	About 0.4 M

Sanity check (teach this): The denominator in percentage change is always the starting mass of 2.00 g, so using an average or final value gives the wrong sign or size. Gaining mass always means the tissue was in the higher water potential solution; a gain in 1.0 M sugar would mean mislabelled tubes. Surface area to volume ratio has units of inverse length and always falls as a cell grows.

05 Independent practice (3 questions)

Question	Answer	What to watch
Percentage change from 2.00 g to 1.86 g	Minus 7.0 percent	Was the starting mass used?
Surface area to volume ratio of a 10 micrometre cube	0.60 per micrometre	Were all six faces counted?
Water moving from 0.2 M to 0.8 M solution	Osmosis, high to low water potential	Are direction and name both correct?

06 Common mistakes and what to say

Mistake	Student writes	Say this
Using the average mass as denominator	Gets minus 6.9 or minus 7.5 percent	“The denominator is always the starting 2.00 g.”
Treating diffusion and osmosis as the same	Writes that sugar osmoses into the cell	“Osmosis is water only; sugar diffuses, and only if the membrane lets it through.”
Reversing the water direction	Says water moves from 1.0 M to 0.0 M	“Water moves from high water potential, meaning less solute, to low.”
Animal cells in distilled water	Says nothing happens	“Animal cells have no wall, so they swell and burst; plant cells develop turgor instead.”

07 Homework (paper, section and question numbers)

DSE Biology paper walkthrough (Paper 1) — Osmosis, transport and data-handling questions
Secondary biology chapter index — All chapters

08 Exit check (last 5 minutes)

Question	Answer	Decision
Percentage change from 2.00 g to 1.86 g	Minus 7.0 percent	Three correct means move on to enzymes
What decides the direction of osmosis	Water potential, high to low	One wrong means reteach water potential
Why large organisms need circulation	Diffusion distance is too long, and time grows with distance squared	Two wrong means restart act three

09 Tutor FAQ

How do water potential and solute concentration relate?

More solute makes solute potential more negative, so water potential falls, and water moves towards it.

Do plant cells burst in pure water?

No. The cell wall creates pressure potential that offsets the water potential difference, giving turgor.

Why take all cylinders out at the same time?

Longer soaking moves each sample closer to equilibrium, so unequal times make groups incomparable.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.