The big idea: Water moves into or out of a cell by osmosis — from a higher water potential (more dilute) to a lower water potential (more concentrated), across the partially permeable membrane.
A plant cell has something an animal cell does not: a strong cell wall around the outside.
The wall does not stop water moving — it changes the outcome. When water rushes in, the wall pushes back and stops the cell bursting, so the cell becomes firm, or turgid, instead.
The osmosis principle behind it all: water moves by osmosis from a higher to a lower water potential. In a plant cell the wall changes the OUTCOME — instead of bursting, a cell in a dilute (hypotonic) solution becomes firm and turgid.
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- Osmosis
- The net movement of water across a partially permeable membrane, from a higher water potential (dilute) to a lower water potential (concentrated).
- Cell wall
- A strong, fully permeable layer (mainly cellulose) outside the plant cell membrane. It lets water through freely but resists the cell swelling and bursting.
- Turgor pressure
- The outward pressure of the cell contents pushing against the cell wall when a plant cell has taken in water.
- Turgid
- Describes a plant cell that is full of water and firm, with the contents pressing hard against the wall (high turgor pressure).
- Flaccid
- Describes a plant cell that has lost water and is limp, with little or no turgor pressure pushing on the wall.
- Plasmolysis
- When a plant cell loses so much water that the cytoplasm and membrane pull away from the cell wall.
Wall vs membrane — don't mix them up: The membrane is partially permeable: it controls what gets in and out, and osmosis happens across it.
The wall is fully permeable: water passes straight through it. Its job is purely mechanical — to resist pressure so the cell does not burst.
It is the wall that gives a plant cell a different fate from an animal cell in the same solution.
The direction water moves depends only on how the solution outside compares with the cytoplasm inside.
There are three cases, and each leaves the plant cell in a different state — turgid, flaccid or plasmolysed.
1. Hypotonic solution → turgid: If the solution outside is more dilute (hypotonic), water enters the cell by osmosis.
The cell swells, but the strong wall pushes back, so it does not burst. The contents press hard against the wall and turgor pressure builds up.
The cell is now turgid — full and firm. Turgid cells are what keep soft plant tissues, such as leaves and young stems, upright.
2. Isotonic solution → flaccid: If the solution outside is the same concentration (isotonic) as the cytoplasm, there is no net movement of water.
Turgor pressure is low, so the cell is flaccid — soft and limp, neither swollen nor shrunken. Tissue made of flaccid cells starts to droop.
3. Hypertonic solution → plasmolysed: If the solution outside is more concentrated (hypertonic), water leaves the cell by osmosis.
The cell loses turgor and becomes flaccid; if it keeps losing water, the cytoplasm and membrane shrink and pull away from the cell wall. The cell is now plasmolysed and the whole tissue wilts.
The gap that opens up between the shrunken contents and the wall fills with the external (bathing) solution, because the wall is fully permeable.
Same rule every time: dilute (hypotonic) outside → water enters; concentrated (hypertonic) outside → water leaves; equal (isotonic) → no net movement. For a WALLED plant cell these give turgid, plasmolysed and flaccid in turn.
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Explore the labelled diagram, charts and maps for this topic in full study mode.
| External solution | Net water movement | What the plant cell becomes |
|---|---|---|
| Hypotonic (more dilute outside) | Water enters the cell by osmosis | Turgid — the cell swells, the contents push hard against the wall, and turgor pressure builds. The strong wall stops it bursting. |
| Isotonic (equal concentration) | No net movement of water | Flaccid — the cell is limp with little turgor pressure; it neither gains nor loses water overall. |
| Hypertonic (more concentrated outside) | Water leaves the cell by osmosis | Plasmolysed — so much water is lost that the cytoplasm and membrane pull away from the wall; the cell becomes flaccid and the tissue wilts. |
Hypotonic → turgid
- Outside solution is more dilute
- Water enters by osmosis
- Cell swells but the wall stops it bursting
- Turgor pressure builds → cell is turgid (firm)
Hypertonic → plasmolysed
- Outside solution is more concentrated
- Water leaves by osmosis
- Membrane and cytoplasm pull away from the wall
- Cell is plasmolysed → tissue wilts
The wall is the whole difference: Place a plant cell and an animal cell in pure water and the same osmosis happens in both — water enters.
But the animal cell has no wall, so it keeps swelling and bursts (cytolysis). The plant cell's wall resists the pressure, so it just becomes turgid and does not burst.
That single structural difference — having a wall — is the classic exam comparison.
| In PURE water (very hypotonic) | Animal cell (no wall) | Plant cell (with a wall) |
|---|---|---|
| Net water movement | Water enters by osmosis | Water enters by osmosis |
| What happens | Keeps swelling and bursts (lysis / cytolysis) | Swells until the wall pushes back, becoming firm and turgid |
| Does it burst? | Yes — there is nothing to resist the pressure | No — the strong cell wall resists the pressure and stops it bursting |
| Useful or harmful? | Harmful — the cell is destroyed | Useful — turgid cells keep soft plant tissues firm and upright |
A memory hook: Turgid = Tip-top full (water in). Flaccid = flat and limp (no net change). Plasmolysed = membrane peels away from the wall (water out).
Water always moves towards the more concentrated solution — so to lose water (and plasmolyse a cell), the solution outside must be the more concentrated one.
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How this is tested: On Paper 2 a high-mark Explain question can ask you to describe osmosis and its effects on plant cells in hypotonic, hypertonic and isotonic solutions — you must link the direction of water movement to turgid / flaccid / plasmolysed for each case.
On Paper 3 a short data question often shows plasmolysed plant cells and asks you to identify what fills the gap between the shrunken contents and the wall (the external solution), or to explain the effect of a hypertonic salt solution.
An Outline question may pair a plant cell with an animal cell in pure water — the marks are for the contrast the wall creates: the animal cell bursts, the plant cell becomes turgid.
IB-style question — explain the effect of a hypertonic solution
Plant tissue is placed in a concentrated (hypertonic) salt solution. Explain the effect this has on the cells. [2]
How to score both marks
- Direction of water movement. The solution outside has a lower water potential than the cytoplasm, so water leaves the cells by osmosis across the partially permeable membrane.
- Resulting state. The cells lose turgor and become flaccid; with enough water loss the membrane and cytoplasm pull away from the cell wall — the cells become plasmolysed and the tissue wilts. (Mark 1: water leaves by osmosis / down the water-potential gradient. Mark 2: cell becomes plasmolysed / membrane pulls away from the wall / tissue wilts.)
Final answer
Water leaves the cells by osmosis (the outside solution has a lower water potential), so the cells lose turgor and become plasmolysed — the membrane and cytoplasm pull away from the cell wall and the tissue wilts.
✓ Why this scores full marks: It names the cause (water leaves by osmosis, down the water-potential gradient) and the effect (plasmolysis — the membrane pulls away from the wall).
Saying only 'the cells shrink' would not be enough: the marks are for osmosis out plus the specific term plasmolysis.
| External solution | Net water movement | What the plant cell becomes |
|---|---|---|
| Hypotonic (more dilute outside) | Water enters the cell by osmosis | Turgid — the cell swells, the contents push hard against the wall, and turgor pressure builds. The strong wall stops it bursting. |
| Isotonic (equal concentration) | No net movement of water | Flaccid — the cell is limp with little turgor pressure; it neither gains nor loses water overall. |
| Hypertonic (more concentrated outside) | Water leaves the cell by osmosis | Plasmolysed — so much water is lost that the cytoplasm and membrane pull away from the wall; the cell becomes flaccid and the tissue wilts. |