IB Biology HLTopic 1 — Life’s Building Blocks & VarietyPaper 1 & 2Core idea~11 min read
Osmosis in Cells
Put a red blood cell in pure water and it bursts. Put a plant cell in the same beaker and it goes stiff and healthy. Same physics, completely different outcome — and the reason is one structure the animal cell does not have.
📚 What you need to know
In a hypertonic solution water leaves the cell; in a hypotonic solution water enters.
In an isotonic solution water still crosses both ways, but there is no net movement — the cell is in dynamic equilibrium.
Animal cells have no cell wall, so the effects are severe: they become crenated (shrivelled) in a hypertonic solution and burst in a hypotonic one.
Multicellular organisms keep their tissue fluid isotonic to prevent this.
Amoeba lives in hypotonic freshwater and uses a contractile vacuole to pump excess water back out.
Plant cells gain water until the protoplast presses on the inelastic cell wall, creating turgor pressure. A full plant cell is turgid.
In a concentrated solution a plant cell loses water, the protoplast pulls away from the wall, and the cell becomes flaccid and plasmolysed.
Medical fluids such as IV drips must be isotonic with blood plasma — usually 0.9% sterile saline.
Which way does the water go?
Everything on this page comes back to one comparison: is the outside solution more dilute or more concentrated than the cytoplasm?
In a hypertonic solution, the cytoplasm is the more dilute of the two, so there is a net movement of water out of the cell. In a hypotonic solution the outside is more dilute, so water moves in. In an isotonic solution the movement in is balanced exactly by the movement out, so the cell is in dynamic equilibrium and its volume does not change.
Why animal cells care so much. An animal cell has to keep its osmotic concentration steady, because any drift away from equilibrium makes it either shrink or burst. There is no wall to save it.
Cells without a cell wall
Animal cells gain and lose water by osmosis just like any other cell, but the consequences are worse because there is no supporting cellulose cell wall.
In a hypertonic solution
The cell loses water and shrivels up. A shrivelled red blood cell is described as crenated. That matters clinically: crenated red blood cells can get stuck as they travel through narrow capillaries, which raises the risk of blood clots forming.
In a hypotonic solution
The cell gains water and swells. Because there is no cell wall to build up turgor pressure and stop the inflow, water keeps entering until the cell surface membrane is stretched too far and the cell bursts. This is why multicellular organisms bathe their cells in isotonic tissue fluid.
The dashed outline on the right is a membrane stretched past the point it can hold. Once it fails, the cell contents spill out.
How Amoeba survives fresh water
Some unicellular organisms, such as the protozoan Amoeba, live in freshwater habitats that are permanently hypotonic to their cytoplasm. There is a constant net influx of water, which steadily raises the internal pressure.
To avoid bursting, these organisms contain contractile vacuoles. Excess water is collected in the vacuole and then pumped back out of the organism, keeping the osmotic concentration of the cytoplasm stable.
Cells with a cell wall
Put a plant cell in a hypotonic solution and water enters through the partially permeable cell surface membrane, because the outside has the lower solute concentration. Water gathers in the vacuole and the volume of the cell increases.
Now the difference. The expanding protoplast — the living part of the cell inside the wall — pushes outwards against the cell wall, and pressure builds up. That pressure is turgor pressure. The wall is inelastic, so it pushes back and stops the cell bursting, and that same back-pressure limits how much more water can enter. A fully inflated, rigid cell is described as turgid.
Turgidity is not just about survival. When all the cells in a plant are firm, they support the whole structure, holding the stem upright and the leaves out to catch sunlight. When a plant does not get enough water the cells cannot stay turgid and the plant wilts.
Losing water: plasmolysis
In a more concentrated solution the flow reverses. Water leaves through the cell surface membrane, the vacuole shrinks, and the volume of the cell decreases. The protoplast gradually shrinks and stops exerting pressure on the wall. Shrink it further and the protoplast begins to pull away from the cell wall. This is plasmolysis: the cell is now flaccid and is said to be plasmolysed.
The gap on the right is not empty. The external solution fills the space between the wall and the protoplast, because the wall is freely permeable.
A detail examiners like: the cell wall is made of cellulose and is freely permeable, while the cell surface membrane is a phospholipid bilayer and is partially permeable. That is why the external solution can reach right up to the shrunken protoplast.
Isotonic solutions in medicine
All of this has a direct medical use. Patients being treated for dehydration, or given a drug straight into the bloodstream, receive an intravenous (IV) drip. The fluid in that drip has to be isotonic with blood plasma, and is usually a 0.9% sterile saline solution.
Think through what happens if it is not. A hypotonic drip would send water into the red blood cells until they burst, cutting the oxygen-carrying capacity of the blood. A hypertonic drip would pull water out of them, leaving them crenated and more likely to lodge in capillaries and trigger clots.
The same reasoning applies to transplant surgery. Donated organs are stored in isotonic saline so that no net osmosis damages the cells before the organ is used.
Worked examples
WORKED EXAMPLE
A red blood cell and a plant cell are both placed in distilled water. Explain why only one of them bursts. [3]
Step 1 — what both cells do
Distilled water is hypotonic to both cells, so water enters both by osmosis and their volume increases.
Step 2 — the animal cell
The red blood cell has no cell wall, so nothing resists the inflow; the membrane stretches until it bursts.
Step 3 — the plant cell
In the plant cell the protoplast presses on the inelastic cell wall, building turgor pressure that resists further water entry, so the cell becomes turgid instead of bursting.
3 marksThe mark is for the wall creating pressure, not just for the wall existing.
WORKED EXAMPLE
Explain why the fluid used in an intravenous drip must be isotonic with blood plasma. [3]
Step 1 — the safe case
An isotonic solution means no net movement of water into or out of the red blood cells, so their volume stays normal.
Step 2 — too dilute
A hypotonic fluid would cause a net movement of water into the red blood cells, making them burst and reducing the oxygen-carrying capacity of the blood.
Step 3 — too concentrated
A hypertonic fluid would draw water out, leaving the cells crenated and more likely to cause blood clots in capillaries.
3 marksNaming 0.9% saline is a nice detail but the marks are for the two failure cases.
💡 Exam tip
Use the right vocabulary for the right cell: animal cells are crenated or burst; plant cells are turgid, flaccid or plasmolysed.
Always name turgor pressure when explaining why a plant cell survives pure water.
Say protoplast, not “cytoplasm”, when describing plasmolysis — the whole living contents pull away, not just the cytoplasm.
In isotonic questions, write dynamic equilibrium: movement continues, net movement is zero.
For medical applications, always link the change back to a consequence for the patient, such as reduced oxygen transport.
If a question gives you a solution and a cell, decide the tonicity first. Everything else follows from that one decision.
⚠ Common mix-up
Plant cells do not burst. The wall prevents it — they become turgid.
Flaccid and plasmolysed are not identical. Flaccid means the cell has lost turgor; plasmolysed means the protoplast has actually pulled away from the wall.
The gap in a plasmolysed cell is not a vacuum. External solution fills it through the freely permeable wall.
Turgor pressure is not osmosis. It is the hydrostatic pressure that results from osmosis.
A contractile vacuole uses energy. Pumping water out is not osmosis and should not be described as such.
Crenated cells have not died of dehydration. They have lost water by osmosis to a hypertonic solution.
Up next: Osmosis (Skills) — designing the potato practical properly, calculating percentage change in mass, and reading the point where the line crosses zero.
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