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.
What the wall changes: 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.
Interactive diagram
Explore the labelled diagram, charts and maps for this topic in full study mode.
- 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: Don't mix up the membrane and the wall. It is the wall that gives a plant cell a different fate from an animal cell in the same solution.
Membrane
- Partially permeable
- Controls what gets in and out
- Osmosis happens across it
Wall
- Fully permeable — water passes straight through it
- Its job is purely mechanical
- Resists pressure so the cell does not burst
Free preview
This is the free notes preview
You're reading the free notes. Aimnova Pro unlocks the full study experience — and you can try it with your first topic free to keep:
- FlashcardsLock in vocabulary and key terms with spaced repetition.
- Practice questionsAnswer exam-style questions and get instant AI marking.
- Mock exams & past-paper vaultSit full mocks and see exactly how examiners award marks.
- Personalised study planA daily plan built around your exam date and weak areas.
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.
Turgor pressure
- 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.
How plasmolysis happens
Turgor is lost
The cell loses turgor and becomes flaccid.
Contents pull away
If it keeps losing water, the cytoplasm and membrane shrink and pull away from the cell wall.
Plasmolysed
The cell is now plasmolysed and the whole tissue wilts.
The gap fills
The gap that opens up between the shrunken contents and the wall fills with the external (bathing) solution, because the wall is fully permeable.
Water out → flaccid → plasmolysed → wilting
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.
Interactive diagram
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. That single structural difference — having a wall — is the classic exam comparison.
Animal cell (no wall)
- Keeps swelling
- Bursts (cytolysis)
Plant cell (with a wall)
- The wall resists the pressure
- Becomes turgid and does not burst
| 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.
Never wonder what to study next
Get a personalized daily plan based on your exam date, progress, and weak areas. We'll tell you exactly what to review each day.
How this is tested: Three question types test this micro. Each one wants you to link the direction of water movement to what happens to the cell.
| Question | What it asks | Where the marks are |
|---|---|---|
| Paper 2, high-mark Explain | Describe osmosis and its effects on plant cells in hypotonic, hypertonic and isotonic solutions | Link the direction of water movement to turgid / flaccid / plasmolysed for each case |
| Paper 3, short data question | Shows plasmolysed plant cells: identify what fills the gap between the shrunken contents and the wall, or explain the effect of a hypertonic salt solution | The gap holds the external solution |
| Outline | A plant cell paired with an animal cell in pure water | 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. |