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Water moves in and out of cells constantly, and getting the water balance right is a matter of life and death for a cell. The special name for the diffusion of water across a cell membrane is osmosis. This lesson, part of Topic B2 of your OCR Gateway Combined Science course, defines osmosis, explains what happens to animal and plant cells in solutions of different strengths, and works carefully through the required practical that investigates osmosis in potato (plant) tissue — including the percentage change in mass calculation that examiners love to set.
By the end of this lesson you should be able to define osmosis, describe its effect on animal and plant cells, calculate a percentage change in mass, and describe the required practical investigating osmosis in plant tissue.
This lesson builds AO1 (understanding of osmosis and its effect on cells), AO2 (applying it in the percentage-change-in-mass calculation and the required practical) and AO3 (interpreting the practical results to work out the concentration of a solution).
Osmosis is the net movement of water molecules across a partially permeable membrane, from a region of higher water concentration (a dilute solution) to a region of lower water concentration (a concentrated solution).
Two phrases need unpacking:
So osmosis is really just diffusion of water — the net movement of water down its own concentration gradient, through a membrane that holds the solute back. Like diffusion, it is passive and needs no energy.
In the diagram, the large brown circles are solute molecules, too big to cross the membrane; the small blue circles are water molecules, which do cross. The net flow of water is from the dilute side towards the concentrated side.
Exam Tip: A full definition of osmosis must include all three ideas: water molecules, a partially permeable membrane, and movement from high water concentration (dilute) to low water concentration (concentrated). Saying only "water moves across a membrane" loses marks.
Animal cells have no cell wall, so they are vulnerable to gaining or losing too much water:
| Surrounding solution | Water movement | Effect on an animal cell (e.g. red blood cell) |
|---|---|---|
| More dilute than the cell (lots of water outside) | Water moves in by osmosis | The cell swells and may burst (lyse) |
| More concentrated than the cell (little water outside) | Water moves out by osmosis | The cell shrinks / shrivels (crenates) |
| Same concentration as the cell (isotonic) | No net movement | The cell stays the same |
This is why body fluids have to be kept at a steady concentration — a red blood cell placed in pure water would take in water and burst.
Plant cells do have a strong cell wall, and this changes the picture completely — the wall stops the cell bursting and gives plants their support.
| Surrounding solution | Water movement | Effect on a plant cell |
|---|---|---|
| More dilute than the cell (e.g. pure water) | Water moves in | The cell becomes turgid (firm); the wall stops it bursting |
| More concentrated than the cell | Water moves out | The cell becomes flaccid (floppy), then plasmolysed (membrane pulls away from the wall) |
| Same concentration (isotonic) | No net movement | The cell stays the same |
Exam Tip: Know the three plant-cell states and the order they happen in as a cell loses water: turgid → flaccid → plasmolysed. A frequent misconception is that a plant cell in pure water will burst like an animal cell — it does not, because the cell wall stops it bursting and it becomes turgid instead.
It is worth understanding why turgor matters so much to a plant, because it explains a sight you have seen many times: a plant that has been forgotten and gone floppy, then perked up again after watering. A non-woody plant, such as a bedding plant or a lettuce, has no wood to hold it rigid. Instead it relies on its cells being turgid. When each cell is full of water, the water pushes the cell contents firmly against the cell wall, and the wall pushes back; the cell becomes stiff, rather like a football pumped hard with air. Millions of stiff, turgid cells packed together give the stem and leaves enough support to stand upright and hold the leaves out flat to catch the light.
If the plant loses more water than it takes up — on a hot day, or when the soil dries out — its cells lose water by osmosis and become flaccid. Flaccid cells are floppy, so the whole plant droops, or wilts. Crucially, wilting is often reversible: water the plant, and water moves back into the cells by osmosis, the cells become turgid again, and the plant stands back up. This everyday recovery is a neat demonstration that osmosis works in both directions depending on which way the water gradient points — water leaves the cells when the soil is dry and re-enters them when the soil is watered.
Exam Tip: If a question asks why a non-woody plant wilts and then recovers after watering, the marks are for: cells lose water by osmosis and become flaccid (so the plant wilts); watering makes water move back into the cells by osmosis so they become turgid again (so the plant recovers). Always tie support to turgid cells pushing against the cell wall.
Because osmosis is simply the movement of water down its concentration gradient, you can spot it at work outside the biology lab, and being able to explain these examples shows real understanding.
Each of these examples follows exactly the same rule you met in the definition: water moves from where it is more concentrated (more water) to where it is less concentrated (less water) across cell membranes. If you can state which side is more concentrated, you can always predict which way the water — and therefore the mass — will move.
Exam Tip: When you meet an unfamiliar osmosis example (a wilting plant, a shrivelled slug, a preserved food), do not memorise it as a separate fact. Work it out from first principles: decide which side is more concentrated, and water will move out of the cells towards it. This one habit handles any application question.
A core practical of Topic B2 investigates osmosis using pieces of potato (plant tissue) placed in sugar or salt solutions of different concentrations. By measuring how the mass of each piece changes, you can tell which way water has moved.
flowchart TD
A["Cut several potato cylinders<br/>of equal size using a cork borer"] --> B["Blot dry and measure the<br/>starting mass of each on a balance"]
B --> C["Place each piece in a different<br/>concentration of sugar solution"]
C --> D["Leave for a set time<br/>(e.g. 30 minutes)"]
D --> E["Remove, blot dry the same way,<br/>and measure the final mass"]
E --> F["Calculate the percentage change<br/>in mass for each concentration"]
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