4.5.2Additional lessons
Osmosis in Cells — IB Biology SL Revision Notes & Topical Questions
5 min read 5 parts Lesson 114 of 142
Part 1 of 5Animal Cells
Red blood cells show the effects clearly because they are surrounded by plasma whose concentration must stay steady.
No cell wall
- Animal cells have only a flexible cell surface membrane, so they cannot resist large changes in volume caused by water movement.
In hypotonic fluid
- Water enters by net osmosis, the cell swells, and the membrane can stretch too far so that the cell bursts, which is called lysis.
In hypertonic fluid
- Water leaves by net osmosis, the cell shrinks and its surface becomes shrivelled and spiky, which is called crenation.
In isotonic fluid
- Water movement in each direction is balanced, so the cell keeps its normal volume and shape in dynamic equilibrium.
Part 2 of 5Plant Cells
Plant Cells
Hypotonic outside
- Water enters the vacuole, the protoplast expands and presses on the cell wall, building up turgor pressure.
Turgid cell
- A fully inflated, firm cell;
- the inelastic cellulose wall resists further swelling so the cell does not burst.
Hypertonic outside
- Water leaves the vacuole, the protoplast shrinks and eventually pulls away from the wall, which is plasmolysis.
Flaccid and wilting
- When cells lose turgor the plant loses support, and leaves and stems droop; watering restores turgor.
Role of turgor
- Turgid cells hold leaves out to the light and keep stems upright, so turgor works as support in non-woody tissue.
| Cell type | Hypotonic solution | Hypertonic solution |
|---|---|---|
| Animal | Swells and may burst (lysis) | Shrinks, crenated |
| Plant | Becomes turgid, wall prevents bursting | Protoplast shrinks, plasmolysis |
Part 3 of 5Living Things in Dilute Water
1
Freshwater protozoa such as Amoeba live in water that is hypotonic to their cytoplasm, so water constantly enters by osmosis.
2
Without a control mechanism this inflow would raise internal pressure until the cell membrane ruptured.
3
Excess water collects in a contractile vacuole inside the cytoplasm.
4
The vacuole moves to the cell surface and expels its contents, using energy from ATP.
5
This cycle repeats continuously, keeping the volume and solute concentration of the cell steady.
Part 4 of 5Medical Applications
Intravenous drips
- Fluids given into the blood must be isotonic with plasma;
- 0.9% sterile saline is commonly used for this purpose.
Hypotonic drip risk
- Red blood cells would take in water and burst, reducing the oxygen-carrying capacity of the blood.
Hypertonic drip risk
- Red blood cells would shrink and become crenated, and they may then block capillaries.
Organs for transplant
- Donated organs are bathed in an isotonic solution so that their cells neither swell nor shrink while stored.
Part 5 of 5Exam Checklist & Tips
4.5.2 – Key Points for the Exam
- Lysis Bursting of an animal cell after excess water uptake.
- Crenation Shrinking and shrivelling of an animal cell after water loss.
- Turgor pressure Pressure of the protoplast pushing against the cell wall of a plant cell.
- Plasmolysis Shrinking of the protoplast away from the cell wall when a plant cell loses water.
- Contractile vacuole Organelle that collects and expels excess water in some freshwater unicellular organisms.
Common Mistakes
- Saying plant cells burst in pure water; the inelastic wall resists and the cell becomes turgid instead.
- Confusing the cell wall with the membrane; the wall is freely permeable, the membrane is partially permeable.
- Writing that a plasmolysed cell has an empty gap that is air;
- the space between wall and membrane is filled with external solution.
- Calling the 0.9% saline drip hypotonic; it is chosen because it matches the concentration of blood plasma.
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Cambridge 9700 AS Biology
Core Formula
Magnification Triangle & Unit Conversion
Formula: M = I / A ⇔ A = I / M ⇔ I = M × A
Units: 1 mm = 1,000 μm = 1,000,000 nm.
Rule: Convert Image size (ruler mm) to μm by ×1,000 before dividing by Actual size.