SPEC 4.1.3.2 • RP3 • TRANSPORT IN CELLS
Osmosis & RP3
Diffusion's specialised cousin — water across a partially permeable membrane. Plus the Required Practical that comes up regularly on Paper 1.
The Phrase That Loses The Mark
Osmosis is the most language-strict question on Paper 1. The wrong preposition or missing word can lose you the direction mark on every osmosis question.
The Trap
The most common student answer: "water moves from high concentration to low concentration." Mark schemes for osmosis questions explicitly reject this phrasing — specifically, they reject "high (solute) concentration to low (solute) concentration" because that's the wrong direction. And without saying which (water or solute), the answer is ambiguous. Either way, no mark.
Why This Lesson
You'll learn the three accepted phrasings mark schemes credit, and the one phrasing they reject. Plus Required Practical 3 — its method, its calculation, and the two examiner traps that catch most students.
By The End Of This Lesson
Three skills — every one tested directly on Paper 1.
1. Define
Define osmosis using mark-scheme accepted wording — including the words "water", "partially permeable membrane", and the correct direction phrasing.
2. Apply
Explain what happens to plant and animal cells placed in dilute or concentrated solutions — using the correct terms (turgid, plasmolysed, lysed).
3. Investigate
Describe the RP3 method — IV, DV, controls, and the % change in mass calculation. Plus the two examiner traps to avoid.
Spec Anchor
This is spec 4.1.3.2 + Required Practical 3, Paper 1, Higher Tier. AT skills 1, 3, and 5. Also tests maths skill MS 1a + 1c (using percentages).
What Is Osmosis?
The exact spec definition. Memorise the wording verbatim — every word is a marking point.
Osmosis — Spec 4.1.3.2
Osmosis is the diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane.
Four Components Of The Definition
Diffusion — osmosis is a special type of diffusion (passive — no energy needed).
Of water — only water moves by osmosis. Solutes (salt, sugar) do NOT.
From dilute to concentrated — the direction. (Same as: high water → low water concentration.)
Through a partially permeable membrane — the membrane allows water through but blocks larger solute molecules.
Only Water Moves By Osmosis
A common error: writing "solutes move by osmosis." Wrong. Osmosis is specifically water across a partially permeable membrane. Solutes can't pass through the membrane — that's why water does the moving instead.
How Osmosis Works
Three things to understand: where water moves, why, and what the membrane does.
The Direction
Water moves from where there's MORE water (a dilute solution — lots of water, little solute) to where there's LESS water (a concentrated solution — little water, lots of solute). Both phrasings describe the same direction.
The Partially Permeable Membrane
The cell membrane is partially permeable (also called "selectively permeable" or "semi-permeable"). Small molecules like water can pass through freely. Larger molecules like sucrose, glucose, salt cannot. This is what makes osmosis happen.
Mark-scheme phrase
Mark schemes accept "partially permeable", "selectively permeable", or "semi-permeable" membrane. All three score.
From The Cell's Perspective
If the outside is more concentrated than inside → water moves OUT of the cell. If the outside is less concentrated than inside → water moves IN. If concentrations are equal → no net movement.
Accepted vs Rejected Phrasings
The mark scheme is precise about what counts. The same direction can be described four ways — three score, one doesn't.
✓ Accepted (3 ways)
1
"From a dilute solution to a concentrated solution" (spec wording)
2
"From less concentrated to more concentrated solution"
3
"From high water concentration to low water concentration" (or "down a water concentration gradient")
✗ Rejected (1 way)
"From high concentration to low concentration" — without specifying water or solute.
Why? "High concentration" could mean high water OR high solute. They're opposites — without saying which, the answer is ambiguous. Mark scheme: "do not accept."
The Critical Distinction
If you say "high concentration" you must specify what's at high concentration. "Water concentration" goes high → low. "Solute concentration" goes low → high. They're describing the same movement from opposite ends. The safe choice: use "dilute → concentrated solution" — unambiguous.
Effects On Plant Cells
Plant cells have a cell wall — this changes everything. Two scenarios, two named outcomes.
In Pure Water → Turgid
Water moves INTO the cell by osmosis. The cell swells. The cell wall (rigid, made of cellulose) prevents the cell from bursting. The cell becomes firm and full — turgid.
Turgid plant cells make stems and leaves stiff. Wilting = cells losing turgidity.
In Concentrated Solution → Plasmolysed
Water moves OUT of the cell by osmosis. The vacuole shrinks. The cell membrane pulls away from the cell wall. The cell is plasmolysed.
The cell wall stays the same shape — it's the membrane that pulls inward.
Mark-Scheme Words
Use the named terms: turgid (in dilute solution), plasmolysed (in concentrated solution). Vague phrases like "the cell gets bigger" or "the cell shrinks" score less than the specific terms.
Effects On Animal Cells
Animal cells have NO cell wall. So when water moves in, there's nothing to stop bursting. Two scenarios, two very different outcomes.
In Pure Water → Burst (Lyse)
Water moves INTO the cell by osmosis. The cell swells. Without a cell wall, the membrane can't withstand the pressure — the cell bursts (lyses).
This is why blood transfusions must use solutions of the right concentration — pure water would burst red blood cells.
In Concentrated Solution → Shrink (Crenate)
Water moves OUT of the cell. The cell shrinks and shrivels (this is called crenation).
The membrane develops crinkly folds as the cell loses volume.
Plant vs Animal Cell — Crucial Difference
Plant cells in pure water → turgid (don't burst — cell wall protects). Animal cells in pure water → burst (no cell wall = no protection). The cell wall is the deciding factor. Mark schemes test this distinction directly.
Required Practical 3 — Setup
RP3 investigates the effect of solute concentration on plant tissue (usually potato). The standard setup:
VariableWhat it is
Independent (IV)Concentration of sugar or salt solution (e.g. 0.0, 0.2, 0.4, 0.6, 0.8, 1.0 mol/dm³)
Dependent (DV)Percentage change in mass of the potato tissue
ControlsSame mass / surface area of potato; same volume of solution; same temperature ; same time submerged (specific time, e.g. 30 minutes)
The Method
Cut potato pieces of equal mass (or equal dimensions if using SA control). Measure each initial mass.
Place each piece in a different concentration of sugar solution (use at least 5 different concentrations).
Leave for the same fixed time (30 minutes is standard).
Blot dry each piece (removes excess surface water — important for accurate mass).
Measure each final mass. Calculate percentage change in mass.
Repeat at least 3 times for each concentration. Calculate the mean.
RP3 — Calculation & Traps
The maths skill: percentage change in mass. The traps: vague timings and the wrong control variable.
% change in mass = final mass − initial massinitial mass × 100
Why Percentage Change?
Pieces of potato can never be cut to exactly the same starting mass. Using percentage change (rather than just absolute change in mass) makes the results comparable — it accounts for slight differences in initial mass between samples.
Two RP3 Traps
Trap 1 — Vague timings. Mark schemes do NOT credit "leave overnight" or "leave for a day." Use a specific time like 30 minutes. Examiner reports flag this directly.
Trap 2 — Length as control. Some students write "control: same length of potato." Mark schemes typically credit same mass or same surface area — these are the relevant variables. Length alone is insufficient.
Interpreting Results
Pieces in concentrated solution lose mass (water out — % change negative). Pieces in dilute solution gain mass (water in — % change positive). The concentration where mass doesn't change = the concentration of the cell sap inside the potato.
Worked Example — 3-Mark Explain
The classic Higher Tier RP3 question: explain a result using cause-effect logic. Three marks, three discrete points — connected by at least one "because" or "so" linker.
A student placed pieces of potato in a 0.5 mol/dm³ sugar solution. After 30 minutes, the pieces had decreased in mass.
Explain why the potato pieces decreased in mass.3 marks
📋 Pattern: 3-mark explain — what + why + how (with cause-effect linkers)
Strategy
Three marks means three points. Plan a causal chain: (1) what happened (water left), (2) why (concentration difference), (3) how (osmosis through partially permeable membrane). Use "because" or "so" between points to score the explanation.
Worked Example — Three-Point Solution
Each MP uses a cause-effect linker. The wording is mark-scheme-strict.
MP1 — What Happened
"The potato pieces lost mass because water moved out of the cells..."
Direct cause-effect: lost mass → because water left.
MP2 — Why The Water Moved
"...the solution in the cells is less concentrated than the 0.5 mol/dm³ sugar solution outside..."
The concentration comparison. Note: "less concentrated" — accepted phrasing.
MP3 — The Mechanism
"...so water moved out of the cells by osmosis (down the water concentration gradient through the partially permeable membrane)."
Names the process: osmosis. Cause-effect linker: "so".
Full mark answer
"The potato pieces lost mass because water moved out of the cells. The solution in the cells is less concentrated than the sugar solution outside, so water moved out by osmosis."
Worked Example — Mark Scheme Tick-Off
Three marking points. Each maps to one tick.
✓
MP1: (pieces) lost mass because water left cells / potato
✓
MP2: the solution in the cells / potato is less concentrated than outside (or equivalent — see acceptable alternatives)
✓
MP3: water left cells by osmosis (allow: through a partially / selectively / semi permeable membrane)
Acceptable Alternatives For MP2
Mark schemes typically accept any of: "solution in cells less concentrated than outside" / "solution in cells more dilute than outside" / "solution outside cells more concentrated than inside" / "solution outside cells less dilute than inside" / correct references to water concentration / potential.
The Killer Phrase
Mark schemes for this question type explicitly do NOT accept: "water moves from area of high (solute) concentration to low (solute) concentration." This is the wrong direction for solute concentration — and ambiguous if you don't specify "solute" or "water."
Practice Question
A plant cell is placed in pure water. The cell becomes turgid but does not burst.
Explain why the cell becomes turgid but does not burst.2 marks
MP1
Becomes turgid: Water moves into the cell by osmosis (the cell sap is more concentrated than the pure water outside, so water enters and the cell swells).
MP2
Does not burst: The cell wall is strong (made of cellulose) and prevents the cell from bursting. (Compare animal cells — no cell wall, so they burst in pure water.)
⚠️
Lose-the-mark traps: writing "the cell becomes full of water" without naming osmosis; writing "the membrane stops it bursting" — it's the cell wall, not the membrane.
Three Examiner Alerts
The three traps that cost the most marks on osmosis questions.
Alert 1 — "High To Low Concentration"
Mark schemes explicitly state "do not accept water moves from high (solute) concentration to low (solute) concentration." If you say "high to low concentration," you must specify water or solute — they're opposites. The safe choice: use "dilute → concentrated solution" or "less → more concentrated" — never ambiguous.
Alert 2 — "Salt Drying Out The Potato"
A common student answer: "the salt sucks water out of the potato" or "the salt dries it out." Examiner reports flag this as not credit-worthy. The mechanism is water moving out by osmosis — driven by the concentration difference, not by salt "drying" anything. Name the process.
Alert 3 — RP3 Vague Timings
Mark schemes for RP3 method questions do NOT credit "overnight," "for a day," or "for several hours." Examiner reports explicitly say specify a time, e.g. "30 minutes". Same applies to controls — say "same mass" not "same potato."
Common Errors That Cost Marks
Four wrong-vs-right pairs lifted directly from mark-scheme guidance.
✗ Wrong
"Water moves from high to low concentration"Ambiguous — water or solute? Mark schemes reject.
✓ Right
"Water moves from less concentrated to more concentrated solution"Or: "from dilute to concentrated solution" / "down water concentration gradient".
✗ Wrong
"Salt dries out the potato"Wrong mechanism — salt doesn't dry. Doesn't score.
✓ Right
"Water moves out of the cells by osmosis"Names the actual process.
✗ Wrong
"Plant cell bursts in water"No — the cell wall prevents this.
✓ Right
"Plant cell becomes turgid (cell wall prevents bursting)"Animal cells without cell walls DO burst.
✗ Wrong
"Control: same length of potato"Length alone is insufficient.
✓ Right
"Control: same mass or surface area of potato"Mark-scheme accepted variables.
Three Things To Remember
The whole lesson, distilled to three takeaways.
1. The Definition
Osmosis = diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane. Use spec wording. Avoid ambiguous "high → low concentration."
2. The Cell Effects
Plant cells in dilute → turgid (cell wall prevents bursting). Plant cells in concentrated → plasmolysed.
Animal cells in dilute → burst (lyse, no cell wall). Animal cells in concentrated → shrink (crenate).
3. RP3 Essentials
IV: solute concentration. DV: % change in mass. Controls: same mass (NOT length), same volume, specific time (NOT "overnight"). Take 3+ repeats; calculate mean.
Final Word
Osmosis questions reward precise wording: name the process (osmosis), name the membrane (partially permeable), use unambiguous direction phrasing (dilute → concentrated). The marks live in the named terms.
Next: Lesson 1.08 — Active Transport
The third transport mechanism. The only one that needs energy. Plus the mark-scheme verb that catches most students out: "energy is released" — never "produced", "made", or "created".
Coming Up
The spec definition of active transport — and why "against the gradient" is the key phrase.
How active transport differs from diffusion and osmosis — a direct comparison table.
Two spec-named examples: root hair cells absorbing minerals, and small intestine absorbing glucose.
Before You Move On
Practise writing the osmosis definition from memory, including all four components: diffusion / of water / dilute → concentrated / partially permeable membrane. Then take the practice questions and EOL test. The 3-mark explain question is the highest-yield to master.
Next: The Phrase That Loses The Mark