Key Idea: This whole topic rests on a single idea: water moves by osmosis from where it is more dilute to where it is more concentrated — and everything else is just that rule applied to different cells. You will meet it five times: what osmosis and water potential actually are (4.6.1), the language of tonicity and osmolarity for comparing solutions (4.6.2), how solute and pressure potential combine in a walled cell (4.6.3), what happens to a cell without a wall — it bursts or shrivels (4.6.4), and what happens to a cell with a wall — it goes turgid or plasmolyses (4.6.5). D2.3 is a guaranteed earner: quick Paper 1A MCQs on the direction of flow, Paper 1B / Paper 3 data questions (read a graph or table of tissue mass and deduce the tonicity), and Paper 2 extended answers (explain the effect of each solution on a plant cell).
Keep the two cell types apart from the start, because the direction of water movement is the same for both — only the outcome differs: No wall (animal cell): swells and bursts (lysis) in a hypotonic solution; shrivels (crenation) in a hypertonic one. With a wall (plant cell): becomes firm (turgid) in a hypotonic solution — it does not burst; plasmolyses in a hypertonic one. Almost every slip in this topic is either swapping hypo-/hyper- or forgetting that the wall changes the result.
💧 Osmosis & water potential (4.6.1)
When a solute (such as salt or sugar) dissolves, water molecules gather around each particle and hold it in solution — this is solvation. So a concentrated solution has fewer free water molecules than a dilute one. Osmosis is the net movement of water across a partially permeable membrane, and water potential sets its direction: water moves from a higher water potential (dilute) to a lower water potential (concentrated).
The one rule behind the whole topic: water moves by osmosis from a higher water potential (a dilute solution) to a lower water potential (a concentrated one). A cell in a hypotonic solution gains water; in an isotonic solution there is no net movement; in a hypertonic solution it loses water.
🔒 Interactive diagram
Explore the labelled diagram, charts and maps for this topic in study mode.
Osmosis needs two things: a partially permeable membrane (lets water through, holds back the solute) and a difference in water potential (a solute-concentration gradient). Add solute → water potential drops. Pure water is the highest; the more concentrated the solution, the lower its water potential. And osmosis is passive — it needs no energy (no ATP); water simply moves down its own gradient until the two sides are equal.
Wherever the solute is more concentrated, that side has the lower water potential, so water moves toward it. Say the membrane is partially permeable — just 'a membrane' usually loses the mark.
🧂 Tonicity & osmolarity (4.6.2)
Osmolarity is the total concentration of solute particles in a solution — the more solute, the higher the osmolarity. Water always moves by osmosis toward the higher osmolarity (the more concentrated side). Tonicity is how the outside solution compares with the cell: hypotonic (lower osmolarity), isotonic (equal) or hypertonic (higher osmolarity).
| Outside solution | Osmolarity vs the cell | Net water movement | Effect on the cell |
|---|---|---|---|
| Hypotonic | LOWER (more dilute than the cell) | water moves INTO the cell | cell gains water and swells |
| Isotonic | EQUAL (same as the cell) | no NET movement | cell stays the same |
| Hypertonic | HIGHER (more concentrated than the cell) | water moves OUT of the cell | cell loses water and shrinks |
Membrane topics are a Paper 1B / Paper 3 data favourite: you are given a graph or table of tissue mass (or length) after soaking, and asked to explain it. Map it straight to the rule: Mass gained = water entered = HYPOTONIC. No change = no net movement = ISOTONIC. Mass lost = water left = HYPERTONIC.
A solution is never 'hypertonic' on its own — it is hypertonic to something. hypo- = less, iso- = equal, hyper- = more. Always ask: more or less concentrated than what?
💪 Solute & pressure potential in walled cells (4.6.3)
In a cell with a wall, the water potential is set by two contributions added together: water potential = solute potential + pressure potential (Ψ = Ψs + Ψp). They pull in opposite directions — solutes lower the water potential (more negative), while the wall's pressure raises it (less negative). The balance decides which way water moves.
Solute potential (Ψs): Caused by **dissolved solutes**. **Lowers** the water potential (more negative). More solute → **stronger pull** on water. Always **zero or negative**.
Pressure potential (Ψp): Caused by the **wall pushing back**. **Raises** the water potential (less negative). High in a **turgid** cell, near zero when **flaccid**. The term a **walled cell** has and an animal cell lacks.
Key Idea: An animal cell has no wall, so when water rushes in it just keeps swelling and can burst. A plant cell's wall resists: water entry builds a pressure potential instead, so the cell becomes firm (turgid) rather than bursting. That extra pressure term is exactly why a walled cell needs two potentials to describe it, not one — and given two water-potential values, water still moves from the higher (less negative) to the lower (more negative).
🩸 Osmosis in cells WITHOUT a wall (4.6.4)
An animal cell has only a thin, flexible plasma membrane — no wall to resist water. So its fate is decided entirely by the solution outside, and the size change is visible under the microscope.
| Solution outside | Net water movement | Effect on the animal cell |
|---|---|---|
| Hypotonic (dilute) | water moves INTO the cell | swells and may burst — this bursting is lysis (haemolysis in red blood cells) |
| Isotonic (equal) | no NET movement | stays the same — keeps its normal shape and size |
| Hypertonic (concentrated) | water moves OUT of the cell | shrinks and wrinkles — this shrivelling is crenation |
Key Idea: A single-celled organism such as Paramecium lives in fresh water, which is hypotonic to it, so water constantly enters by osmosis. With no wall it should burst. It survives using a contractile vacuole, which collects the excess water and pumps it back out. This control of water balance is osmoregulation — the contractile vacuole's 'function of life' is excretion / homeostasis.
Bursting = lysis; shrivelling = crenation. 'Gets bigger / smaller' alone loses marks. In data questions, deduce backwards: a burst cell means the solution was hypotonic; a shrunken cell means it was hypertonic. No cells visible after distilled water = they lysed.
🌱 Osmosis in cells WITH a wall (4.6.5)
A plant cell undergoes the same osmosis, but its strong cell wall changes the outcome. The wall is fully permeable (water passes straight through) — its job is purely mechanical: to resist pressure so the cell does not burst. The result is one of three states — turgid, flaccid or plasmolysed.
| External solution | Net water movement | What the plant cell becomes |
|---|---|---|
| Hypotonic (dilute) | water enters by osmosis | turgid — swells, turgor pressure builds, the wall stops it bursting (firm) |
| Isotonic (equal) | no net movement | flaccid — limp, with little turgor pressure |
| Hypertonic (concentrated) | water leaves by osmosis | plasmolysed — membrane and cytoplasm pull away from the wall; tissue wilts |
In a hypertonic solution water leaves the cell; with enough loss the membrane and cytoplasm pull away from the cell wall — the cell is plasmolysed. The gap that opens between the shrunken contents and the wall fills with the external (bathing) solution, because the wall is fully permeable — not air and not a vacuum. This is a classic 1-mark Identify point.
Same rule, two outcomes. An animal cell (no wall) bursts in a hypotonic solution and crenates in a hypertonic one; a plant cell (with a wall) only becomes turgid in a hypotonic solution and plasmolyses in a hypertonic one. The direction of water movement is identical — the cell wall changes the result.
🔒 Interactive diagram
Explore the labelled diagram, charts and maps for this topic in study mode.
Turgid = Tip-top full (water in). Flaccid = flat and limp (no net change). Plasmolysed = membrane peels off the wall (water out). In pure water, an animal cell bursts but a plant cell only becomes turgid — the wall is the whole difference.
🧠 The big contrast — animal vs plant cell
If you remember one table from this topic, make it this one. The direction of water movement is identical for both cells; only the outcome differs, because of the wall:
| Solution outside | Animal cell (no wall) | Plant cell (with a wall) |
|---|---|---|
| Hypotonic (dilute) | swells and bursts — lysis | swells but does NOT burst — turgid (firm) |
| Isotonic (equal) | unchanged | flaccid (limp) |
| Hypertonic (concentrated) | shrinks and wrinkles — crenation | membrane pulls from the wall — plasmolysed |
| The reason | only a flexible membrane holds it in | a strong wall resists the pressure |
✍️ Worked examples
IB-style question — outline the conditions for osmosis
A sugar solution is separated from pure water by a membrane in a glass tube, and the level rises on the sugar side. Outline the conditions required for osmosis to take place across the membrane. [2]
How to score both marks:
The membrane condition. There must be a partially permeable membrane — one that lets water molecules through but holds back (most of) the dissolved solute.
The gradient condition. There must be a difference in water potential (a difference in solute concentration) between the two sides, giving water a gradient to move down. (Mark 1: partially permeable membrane. Mark 2: water-potential / concentration difference.)
Osmosis needs a partially permeable membrane AND a difference in water potential (solute concentration) between the two sides.
IB-style question — deduce tonicity from the data
Equal cubes of beetroot were left in three solutions and reweighed. Cubes in solution 1 gained mass, cubes in solution 2 did not change in mass, and cubes in solution 3 lost mass. Deduce the tonicity of each solution relative to the tissue, and justify your answer using osmosis. [3]
How to score all three marks:
Solution 1 — gained mass. Water moved into the cells by osmosis, so solution 1 had a lower osmolarity (higher water potential) than the cells — it is hypotonic.
Solution 2 — no change. There was no net movement of water, so solution 2 has the same osmolarity as the cell contents — it is isotonic (this estimates the cells' own internal concentration).
Solution 3 — lost mass. Water moved out of the cells by osmosis, so solution 3 had a higher osmolarity (lower water potential) — it is hypertonic. (1 mark each: hypotonic, isotonic, hypertonic, justified by osmosis.)
Solution 1 = hypotonic (gained water), solution 2 = isotonic (no net change), solution 3 = hypertonic (lost water). Water always moves toward the higher osmolarity / lower water potential.
IB-style question — animal cell in a hypertonic solution
A red blood cell is placed in a concentrated salt solution. Describe what happens to the cell and explain why. [3]
How to score all three marks:
Direction of water movement. The salt solution is hypertonic (more concentrated than the cell), so water moves out of the cell by osmosis.
Named effect. The cell shrinks and its surface becomes wrinkled — this is crenation.
Reason. There is more free water inside the cell than in the concentrated solution outside, so water moves down its gradient out of the cell — and with no wall, the shape change is unresisted. (Mark 1: water out by osmosis. Mark 2: shrinks / crenates. Mark 3: because the outside is hypertonic.)
Water leaves by osmosis because the solution is hypertonic; the cell shrinks and wrinkles (crenation), and with no wall nothing resists the change.
IB-style question — plant cell in a hypertonic solution
Plant tissue is mounted on a slide and a concentrated salt solution is added. The cell contents shrink and a clear region appears between the shrunken contents and each cell wall. Identify what fills the clear region, and explain why the cells changed in this way. [3]
How to score all three marks:
Identify the gap. The clear region is filled by the external (bathing) salt solution — the cell wall is fully permeable, so the surrounding solution flows in (not air, not a vacuum).
Direction of water movement. The salt solution is hypertonic (a lower water potential than the cytoplasm), so water leaves the cells by osmosis across the partially permeable membrane.
Named effect. As the cells lose water, the membrane and cytoplasm pull away from the cell wall — the cells are plasmolysed. (Mark 1: external solution fills the gap. Mark 2: water leaves by osmosis / solution is hypertonic. Mark 3: plasmolysis / membrane pulls from the wall.)
The gap is filled by the external solution (the wall is permeable); water leaves the cells by osmosis because the solution is hypertonic, so the membrane and cytoplasm pull away from the wall — the cells are plasmolysed.
✅ Quick self-check
Tap each card to check yourself.
What is osmosis, and which two conditions does it need? The net movement of water across a partially permeable membrane, from a higher water potential (dilute) to a lower water potential (concentrated). It needs a partially permeable membrane AND a difference in water potential — and it is passive (no ATP).
What is osmolarity, and which way does water move? The total concentration of solute particles in a solution. Water moves by osmosis toward the higher osmolarity (the more concentrated side) — i.e. from higher to lower water potential, until the two are equal.
How do solute and pressure potential combine in a walled cell? Water potential = solute potential + pressure potential (Ψ = Ψs + Ψp). Solutes lower it (more negative); the wall's pressure potential raises it (less negative). Water still moves from the higher to the lower water potential.
What happens to an animal cell in each solution? Hypotonic → water in → swells and bursts (lysis). Isotonic → no net movement → unchanged. Hypertonic → water out → shrinks and wrinkles (crenation). It has no wall, so the changes are unresisted.
What happens to a plant cell in each solution? Hypotonic → water in → turgid (firm, does NOT burst — the wall resists). Isotonic → no net movement → flaccid (limp). Hypertonic → water out → plasmolysed (membrane pulls from the wall; tissue wilts).
How do you deduce tonicity from a data table of tissue mass? Mass gained = water entered = hypotonic; no change = isotonic (estimates the cell's own concentration); mass lost = water left = hypertonic. Always justify with osmosis toward the higher osmolarity.
Exam Tips
- Every answer traces to one rule: water moves from a HIGHER to a LOWER water potential (dilute → concentrated, toward the higher osmolarity). Adding solute always lowers water potential.
- Say the membrane is PARTIALLY permeable — just 'a membrane' usually loses the mark. Osmosis needs TWO conditions: the partially permeable membrane AND a water-potential difference.
- Osmosis is PASSIVE — it needs no ATP. Don't say energy is required.
- Define osmolarity as the TOTAL concentration of solute particles — not just 'how salty'. Tonicity is relative: a solution is hypertonic TO another, never on its own.
- Data rule: mass gained = hypotonic, no change = isotonic, mass lost = hypertonic — and explain each via osmosis, don't just label.
- Walled cell: water potential = solute potential + pressure potential. Solutes LOWER it; the wall's pressure RAISES it. Keep the two directions straight.
- Name the effects precisely: animal cell → lysis (bursting) and crenation (shrivelling); plant cell → turgid and plasmolysed. 'Gets bigger/smaller' loses marks.
- Plasmolysis = the MEMBRANE pulls away from the wall; the gap fills with the EXTERNAL solution (not air, not a vacuum).
- In pure water an animal cell BURSTS but a plant cell only becomes TURGID — the cell wall is the whole reason. A Paramecium survives fresh water using a contractile vacuole (osmoregulation).
Key Idea: Osmosis (4.6.1) is the net movement of water across a partially permeable membrane, from a higher water potential (dilute) to a lower one (concentrated); it needs a partially permeable membrane AND a water-potential difference, and it is passive. Osmolarity (4.6.2) is the total solute concentration, and tonicity compares the outside with the cell — water moves toward the higher osmolarity, so a data table reads mass up = hypotonic, no change = isotonic, mass down = hypertonic. In a walled cell (4.6.3) the water potential is solute potential + pressure potential (Ψ = Ψs + Ψp): solutes lower it, the wall's pressure raises it. A cell without a wall (4.6.4) swells and bursts (lysis) in a hypotonic solution and shrivels (crenation) in a hypertonic one — and a contractile vacuole keeps a freshwater Paramecium from bursting (osmoregulation). A cell with a wall (4.6.5) instead becomes turgid (hypotonic), flaccid (isotonic) or plasmolysed (hypertonic) — the gap in a plasmolysed cell filling with the external solution. Same osmosis every time; the wall is what decides whether a cell bursts or just goes turgid.