IB Biology SLTopic 2 — Osmosis & Water PotentialPaper 1 & 2Core idea~10 min read
Osmosis in Cells
Put a red blood cell in pure water and it bursts. Put a plant cell in the same water and it gets firmer and healthier. Same physics, completely different outcome — and the whole difference is one cell wall.
📘 What you need to know
Hypotonic solution: more dilute than the cytoplasm, so water moves into the cell.
Hypertonic solution: more concentrated than the cytoplasm, so water moves out of the cell.
Isotonic solution: the same concentration, so there is no net movement — a dynamic equilibrium, not a standstill.
Animal cells have no wall: they become crenated (shrivelled) in a hypertonic solution and burst in a hypotonic one.
Plant cells have an inelastic wall: they become turgid in a hypotonic solution and plasmolysed in a hypertonic one.
Fluids given to patients, such as an IV drip, must be isotonic with blood plasma — usually 0.9% saline.
The three words
Everything on this page comes back to comparing the solution outside the cell with the cytoplasm inside it.
Outside solution
Compared with the cytoplasm
Net movement of water
Hypotonic
More dilute (lower solute concentration)
Into the cell
Hypertonic
More concentrated (higher solute concentration)
Out of the cell
Isotonic
The same concentration
None overall — movement in balances movement out
The isotonic row catches people out. Water molecules have kinetic energy and are crossing the membrane constantly in both directions. Write “no net movement”, never “no movement” — the cell is in dynamic equilibrium, not frozen.
Water crosses the membrane partly through the phospholipid bilayer and partly through protein channels called aquaporins, which make the membrane far more permeable to water than it would otherwise be.
Cells without a wall
Animal cells gain and lose water by osmosis just like any other cell, but with no supporting cellulose wall the consequences are more severe.
The middle cell is what your red blood cells should look like. The other two are what happens when the tissue fluid gets it wrong.
In a hypertonic solution the cell loses water and becomes crenated — shrivelled. Crenated red blood cells can get stuck moving through capillaries, which may lead to blood clots.
In a hypotonic solution the cell gains water. With no cell wall there is no turgor pressure to oppose it, so water keeps entering until the membrane is stretched too far and the cell bursts.
Multicellular organisms therefore have to maintain isotonic tissue fluid around their cells.
What single-celled organisms do instead
Some unicellular organisms, such as the protozoan Amoeba, live in freshwater that is permanently hypotonic to their cytoplasm. There is a constant net influx of water, which raises the internal pressure.
They survive using a contractile vacuole: excess water is collected inside it and then pumped out of the organism, keeping the osmotic concentration of the cytoplasm steady. Without it they would burst.
Cells with a wall
The cell wall is freely permeable, so solution passes straight through it. Only the cell surface membrane is partially permeable.
In a hypotonic solution: turgid
Water enters the cell by osmosis through the partially permeable cell surface membrane, because the outside solution has a lower solute concentration.
Water enters the vacuole and the volume of the cell increases.
The expanding protoplast (the living part inside the wall) pushes outwards against the wall, and pressure builds up. This is turgor pressure.
The inelastic cell wall prevents the cell from bursting, and the pressure it creates also limits how much more water can enter.
A fully inflated, rigid, firm cell is described as fully turgid. All the cells being firm together is what gives a plant support and strength, holding it upright with its leaves out to catch sunlight.
In a hypertonic solution: plasmolysed
Water leaves the cell by osmosis, the vacuole shrinks and the volume of the cell decreases.
The protoplast gradually shrinks and stops exerting pressure on the wall.
As it shrinks further it pulls away from the cell wall. This is plasmolysis, and the cell is flaccid and described as plasmolysed.
If plants do not get enough water, the cells cannot stay turgid and the plant wilts.
A detail examiners like: the cell wall is made of cellulose and is freely permeable, so the external solution flows straight through it. The gap in a plasmolysed cell is full of that solution, not empty space, and it presses on the protoplast.
Isotonic solutions in medicine
Patients sometimes need an intravenous (IV) drip to treat dehydration or to deliver medicine straight into the bloodstream. The solution used must be isotonic with blood plasma — normally a 0.9% sterile saline solution.
If the drip were hypotonic, water would move into the red blood cells and they would burst, decreasing the oxygen-carrying capacity of the blood.
If it were hypertonic, water would move out and the cells would shrivel and become crenated, increasing the risk of clots because those cells cannot move freely through capillaries.
The same reasoning applies to donated organs for transplant surgery. They are kept in isotonic saline so that no net movement of water damages the cells before the operation.
Worked examples
WORKED EXAMPLE
Predict the effect on two cells
A red blood cell and a plant cell are both placed in distilled water. Describe what happens to each and explain the difference.
Step 1: Identify the solution
Distilled water is hypotonic to both cells, so water enters both by osmosis.
Step 2: The red blood cell
It has no cell wall, so it keeps swelling until the membrane is stretched too far and it bursts.
Step 3: The plant cell
The inelastic cell wall resists the expanding protoplast, so turgor pressure builds and the cell becomes turgid rather than bursting.
One bursts, one becomes turgidBoth cells do the same thing; only the wall changes the outcome.
WORKED EXAMPLE
Explain a medical requirement
Explain why the saline solution used in an IV drip must be isotonic with blood plasma.
If it were hypotonic
Water would enter the red blood cells by osmosis and they would burst, reducing the oxygen-carrying capacity of the blood.
If it were hypertonic
Water would leave the cells, which would become crenated and more likely to block capillaries and cause clots.
So the requirement
An isotonic solution gives no net movement of water, so the cells keep their normal shape and function.
Isotonic means no net water movementGive both failure cases. A question worth three or four marks is asking for exactly this structure.
WORKED EXAMPLE
Interpret an observation
A student looks at onion cells that have been left in strong sucrose solution and sees that the cell contents have pulled away from the cell wall. Name this and explain how it happened.
Step 1: Name itPlasmolysis — the cells are plasmolysed and flaccidStep 2: The cause
The sucrose solution is hypertonic to the cytoplasm, so water leaves the cells by osmosis through the partially permeable membrane.
Step 3: The consequence
The vacuole shrinks, the protoplast loses volume and stops pressing on the wall, then pulls away from it.
Plasmolysis caused by water lossRemember the gap is filled with the external solution, since the wall is freely permeable.
💡 Exam tip
Learn the three words as a set and always say what they are compared with — the cytoplasm of the cell.
Write no net movement for isotonic, and mention dynamic equilibrium if there is room.
Use the right vocabulary for each cell type: crenated and burst for animal cells, turgid and plasmolysed for plant cells.
Name turgor pressure and the inelastic cell wall when explaining why plant cells do not burst.
The IV drip example is a favourite. Learn 0.9% saline and both consequences of getting it wrong.
Say water moves by osmosis through the partially permeable membrane rather than just “water moves in”.
⚠ Common mix-up
Saying there is no movement of water in an isotonic solution. There is no net movement.
Swapping hypotonic and hypertonic. Hypo means below, so a hypotonic solution has less solute than the cell.
Saying plant cells burst. The wall prevents it — they become turgid instead.
Thinking the cell wall is partially permeable. It is freely permeable; the membrane does the selecting.
Describing the gap in a plasmolysed cell as empty. It contains the external solution.
Calling a wilting plant plasmolysed. Wilting is what you see in the whole plant; plasmolysis is what has happened inside individual cells.
Up next: Osmosis (Skills) — the potato practical, calculating percentage change in mass, and using a graph to work out what is inside the cells.
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