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Detailed notes on Movement into and out of Cells for Cambridge IGCSE Biology, covering key concepts, explanations, examples, and exam-focused revision points.
Diffusion of WATER through a partially permeable membrane. Cambridge Extended wants 'water potential' wording — Core-level 'dilute to concentrated' won't get full marks.
Mapped to the Cambridge IGCSE 0610 syllabus (2026-2028).
Core: dilute → concentrated. Extended: HIGH water potential → LOW water potential.
Core definition.
"The net movement of water molecules from a dilute solution to a more concentrated solution, through a partially permeable membrane."
Extended definition (memorise this for full marks):
"The NET movement of water molecules from a region of HIGHER WATER POTENTIAL to a region of LOWER WATER POTENTIAL, through a PARTIALLY PERMEABLE MEMBRANE."
Why two definitions? "Dilute to concentrated" is intuitive but loses precision (e.g. comparing two solutions with different solutes is complicated). "Water potential" handles all cases.
Water potential (Ψ).
Worked qualitative. Two solutions: A = 1% sugar; B = 5% sugar. Which has higher water potential?
Cambridge tip. At Extended level, use "water potential" — Cambridge mark schemes accept "from high water potential to low water potential" but not "from dilute to concentrated" for the higher-tier mark.
Pure water → turgid. Concentrated solution → plasmolysed.
In pure water (or solution of higher water potential than the cell sap).
In a solution of equal water potential.
In a solution of LOWER water potential than the cell sap (concentrated).
Why plants wilt. Without enough water uptake, vacuoles shrink → less turgor pressure → cells become flaccid → leaves and stems droop. Watering replaces water → cells swell → plant stiffens.
Worked qualitative. A wilted lettuce, soaked in water, becomes crisp again. Why? Water enters the plant cells by osmosis; cells become turgid; cell walls support the plant.
Pure water → can burst. Concentrated solution → crenated.
Animal cells have NO CELL WALL. They're at the mercy of osmosis.
| Surrounding solution | Plant cell | Animal cell |
|---|---|---|
| Higher water potential (e.g. pure water) | Water in → turgid; cell wall prevents bursting | Water in → swells → bursts (haemolysis) |
| Equal water potential | No net movement → unchanged | No net movement → normal shape |
| Lower water potential (concentrated) | Water out → plasmolysed; cytoplasm pulls away from wall | Water out → crenated (shrivelled) |
In pure water (or HIGHER water potential than cell contents).
In concentrated solution (LOWER water potential).
Why animal cells need to be in balanced fluid. Tissue fluid around cells must have water potential close to cell contents. The body controls this carefully (homeostasis):
Worked qualitative. Why do red blood cells stored in saline solution last longer than in pure water?
Root water uptake, kidney function, food preservation by salting.
1. Water uptake by roots.
2. Kidney function.
3. Food preservation by salting / sugaring.
4. Why drinking sea water is dangerous.
Worked qualitative. Why are seedlings often watered with weak fertiliser instead of strong fertiliser? Strong fertiliser solution has very low water potential → roots could lose water by osmosis → seedlings dehydrate. Weak fertiliser is safer.
Verbatim phrases and definitions Cambridge mark schemes credit.
Osmosis is examined every Paper 4 (5-8 marks): define using water potential, predict cell behaviour in different solutions, explain plant wilting/recovery. Examiner reports flag using Core wording at Extended level (loses the higher-tier mark).
Sources: Cambridge IGCSE Biology 0610 syllabus 2026-2028 (3.2); 0610/42 Oct/Nov 2024 — Q6 (osmosis, plant cells); 0610 Examiner Reports 2022-2024. Last reviewed 2026-05-07.
Step-by-step solutions to past-paper-style questions on osmosis, written exactly the way a tutor would explain them at the board.
Question
Give the Cambridge Extended definition of osmosis.
Step-by-step solution
Step 1
Net movement of WATER molecules from a region of HIGHER water potential to a region of LOWER water potential, through a PARTIALLY PERMEABLE MEMBRANE.
Answer
The net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane.
Examiner tip
Core students can say 'from dilute to concentrated solution'. Extended students MUST use 'water potential' for full marks.
Question
A cell whose cytoplasm has a low water potential is placed in pure water. Which way does water move, and why?
Step-by-step solution
Step 1
Pure water has the highest possible water potential; the cytoplasm has a lower water potential because it contains dissolved solutes.
Step 2
Water always moves by osmosis from higher to lower water potential.
Step 3
So water moves INTO the cell, from the pure water (higher potential) to the cytoplasm (lower potential).
Answer
Water moves into the cell, because the pure water has a higher water potential than the cell contents and water moves down the water potential gradient.
Examiner tip
Water moves DOWN the water potential gradient: high → low. State the comparison explicitly.
Question
A plant cell is placed in PURE WATER. Describe what happens.
Step-by-step solution
Step 1
Water has HIGHER water potential outside than inside the cell (cytoplasm contains dissolved solutes).
Step 2
Water moves IN by osmosis, through the partially permeable cell membrane.
Step 3
The vacuole swells; cytoplasm pushes against the cell wall.
Step 4
Cell becomes TURGID (firm). The cell wall prevents bursting.
Answer
Water moves into the cell by osmosis; the cell becomes TURGID. The cell wall stops it bursting.
Question
Compare what happens to (a) an animal cell and (b) a plant cell placed in a CONCENTRATED salt solution.
Step-by-step solution
Step 1
Salt solution has LOWER water potential than the cell contents.
Step 2
Water moves OUT of both cells by osmosis.
Step 3
(a) Animal cell: shrinks, becomes wrinkled — CRENATED. No cell wall to keep shape.
Step 4
(b) Plant cell: cytoplasm shrinks, pulls away from the cell wall — PLASMOLYSED. The cell wall keeps the outline.
Answer
Both lose water. Animal: crenated (shrivelled). Plant: plasmolysed (cytoplasm pulls away from the wall).
Question
Identical potato cylinders are weighed, left for an hour in different sugar solutions, then reweighed. In distilled water a cylinder rose from 5.0g to 5.6g; in concentrated sugar solution another fell from 5.0g to 4.4g. Calculate the percentage change in mass for each and explain the results.
Step-by-step solution
Step 1
Percentage change =starting masschange in mass×100.
Step 2
Distilled water: change =+0.6g, so 5.00.6×100=+12%.
Step 3
Concentrated sugar: change =−0.6g, so 5.0−0.6×100=−12%.
Step 4
In distilled water (higher water potential than the cells) water moved IN by osmosis, so the cylinder gained mass. In concentrated sugar (lower water potential than the cells) water moved OUT, so it lost mass.
Answer
Distilled water: +12% (water entered by osmosis). Concentrated sugar: −12% (water left by osmosis). Using percentage change allows fair comparison even if starting masses differ slightly.
Examiner tip
Cambridge wants PERCENTAGE change (not just mass change) so different starting masses can be compared fairly. Link the gain/loss to the water potential difference.
Question
Explain, in terms of osmosis and water potential, why a plant wilts when the soil is dry and recovers after watering.
Step-by-step solution
Step 1
Normally the cells take in water by osmosis and become turgid; the contents press outwards on the cell walls, and this turgor pressure supports the soft (non-woody) parts of the plant.
Step 2
When the soil is dry, there is little water of high water potential available, so cells lose more water than they gain by osmosis.
Step 3
The cells become flaccid — they lose turgor, so the tissues are no longer firm and the plant wilts.
Step 4
After watering, the soil water has a higher water potential than the cell contents, so water moves back into the cells by osmosis, they become turgid again, and the plant recovers its support.
Answer
Turgid cells provide support. Dry soil → cells lose water by osmosis → become flaccid → plant wilts. Watering raises the external water potential → water re-enters by osmosis → cells turgid again → plant recovers.
Examiner tip
Link support to turgor pressure and the cell wall, and link wilting/recovery to the direction of osmosis driven by the water potential difference.
High-scoring sample answers for osmosis on the Cambridge IGCSE 0610 paper, with examiner-style notes mapping each response to the mark scheme and assessment objectives.
Through what type of membrane does osmosis occur? (1 mark)
Model answer
Through a partially (selectively) permeable membrane.
Why this scores
One mark for 'partially permeable' (or 'selectively permeable').
A red blood cell is placed in pure water. Describe and explain what happens to it. (2 marks)
Model answer
Water moves into the cell by osmosis, because the pure water has a higher water potential than the cell contents. As an animal cell has no cell wall to resist the pressure, it swells and bursts (lysis).
Why this scores
Two marks: water enters by osmosis (higher water potential outside); cell swells and bursts because there is no cell wall.
Define osmosis. (3 marks)
Model answer
Osmosis is the net movement of water molecules from a region of higher water potential (a dilute solution) to a region of lower water potential (a more concentrated solution), through a partially permeable membrane.
Why this scores
Three marks: (1) net movement of water; (2) from higher to lower water potential; (3) through a partially permeable membrane. At Extended, 'water potential' is required for full credit.
Explain what happens to a plant cell when it is placed in a concentrated sugar solution. (4 marks)
Model answer
The concentrated sugar solution has a lower water potential than the cell contents, so water moves out of the cell by osmosis through the partially permeable membrane. As water leaves, the vacuole shrinks and the cytoplasm shrinks, so the cell loses turgor and becomes flaccid. If a lot of water is lost, the cytoplasm and membrane pull away from the cell wall — the cell becomes plasmolysed.
Why this scores
Four marks: lower water potential outside; water leaves by osmosis; cell loses turgor/becomes flaccid; cytoplasm pulls from wall = plasmolysis.
Describe how you could use potato cylinders to investigate the effect of sugar solution concentration on osmosis. (5 marks)
Model answer
Cut several potato cylinders of the same length and diameter and measure the mass of each at the start with a balance. Place each cylinder in a different known concentration of sugar solution (for example 0.0, 0.2, 0.4, 0.6 and 0.8 mol/dm³), keeping the volume of solution, temperature and time the same for each. After a set time, remove each cylinder, blot it dry and reweigh it. Calculate the percentage change in mass for each concentration using starting masschange in mass×100, and plot percentage change against concentration. Cylinders in dilute solutions will gain mass (water enters by osmosis) and those in concentrated solutions will lose mass (water leaves), with the cross-over point giving the concentration equal to the cell contents.
Why this scores
Marks for: same-sized cylinders; measure mass before and after; range of known concentrations; controlled variables (volume, temperature, time, blot dry); percentage change calculation. The 'blot dry' and 'percentage change' details are commonly rewarded.
Explain the importance of water potential and osmosis in the uptake of water by a plant and in the support of its non-woody parts. (6 marks)
Model answer
Water uptake begins at the roots. The soil water has a higher water potential than the cytoplasm and cell sap of the root hair cells, so water moves into the root hair cells by osmosis through their partially permeable membranes. Water then passes from cell to cell across the root, always moving down a water potential gradient, towards the xylem. Inside the plant, when cells take in water they become turgid: the contents press outwards on the cell wall, creating turgor pressure. This turgor pressure is what supports the soft, non-woody parts of the plant, such as leaves and young stems, keeping them firm and held up to the light. If the plant loses too much water, the cells lose turgor and become flaccid, so the plant wilts, showing how essential the maintenance of turgor by osmosis is for support.
Why this scores
Up to 3 marks for uptake (soil higher water potential → water into root hair cells by osmosis → cell to cell down the gradient) and up to 3 for support (turgid cells press on cell wall → turgor pressure supports soft parts; loss of turgor → flaccid → wilting).
Definitions to memorise and the exact keywords mark schemes credit for osmosis answers — sharpened from recent examiner reports for the 2026 0610 sitting.
The net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane.
A measure of the tendency of water to move out of a solution. PURE WATER has the highest water potential; adding solutes LOWERS water potential.
A membrane that allows SMALL molecules (e.g. water) through but blocks LARGER molecules (e.g. dissolved sugars and salts).
A plant cell that has taken up water by osmosis; cytoplasm pressing firmly on the cell wall. Plants need turgor for support.
A plant cell that has lost turgor; soft and limp. The plant wilts.
Severe water loss from a plant cell — cytoplasm shrinks AND PULLS AWAY from the cell wall.
Shrinkage of an animal cell after losing water by osmosis (no cell wall to hold shape).
Bursting of an animal cell (e.g. red blood cell) after taking in too much water by osmosis (no cell wall to resist).
The traps other students keep falling into on osmosis questions — taken from recent Cambridge IGCSE 0610 examiner reports and mark schemes — and how to avoid them.
Why it happens
It's the Core-level definition.
How to avoid it
Extended candidates must use 'water potential'. Cambridge mark schemes give the higher mark only for the water-potential wording.
Why it happens
Confused with active transport.
How to avoid it
Osmosis is a special case of DIFFUSION (of water). It's PASSIVE.
Why it happens
Animal cells (e.g. red blood cells) DO burst.
How to avoid it
Plant cells have a strong CELL WALL that prevents bursting. The cell becomes turgid but doesn't burst.
Why it happens
Higher water potential sounds like 'more pull on water', but it's the opposite.
How to avoid it
Water moves DOWN the water potential gradient (high → low). Pure water is the highest possible water potential (Ψ=0).
The things students keep getting wrong in this sub-topic, answered.