4.2.124 · Cell Membranes and Transport

Comparing Osmosis in Plants and Animals — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 9 parts Lesson 20 of 20 1 quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision

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Lesson summary

4.2.12 Comparing Osmosis in Plants and Animals — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Compare the effects of osmosis on plant and animal cells and explain the differences using cell structure.

Plant cell
Tolerance of dilute surroundings
Tolerance of salty surroundings
Plants' strategy
Animals' strategy
Plant tissue

Part 1 of 7Same process, different structures

1Osmosis

Osmosis in all cells it is the net movement of water from higher to lower water potential across a partially permeable membrane.

2Plant cell

Plant cell it has a strong, slightly elastic cellulose wall and a large permanent vacuole holding most of the water,
so it can build a positive pressure potential.

3Animal cell

Animal cell it cannot generate pressure potential,
- so its water potential equals its solute potential and its response to dilute solutions is more extreme.

Part 2 of 7Membrane and wall

Feature	Plant cell	Animal cell
Cell surface membrane	Present; phospholipid bilayer, selectively permeable	Present; phospholipid bilayer, selectively permeable
Cell wall	Present; cellulose, fully permeable	Absent
Osmosis across the membrane	Can occur	Can occur

Part 3 of 7Effects in solutions of different water potential

Feature	Plant cell	Animal cell
Solution of higher ψ	Water enters, protoplast expands and pushes on the wall, the wall resists, and the cell becomes turgid, firm and fully inflated	Water enters, no wall resists the pressure, the membrane overstretches and the cell bursts (lysis)
Solution of lower ψ	Water leaves, the volume decreases and the protoplast pulls away from the wall (plasmolysis)	Water leaves, the cell shrinks and the membrane crinkles (crenation)
Pressure potential	Positive in turgid cells	Effectively zero
Vacuole	Large; stores most water and solutes	Small or absent

Part 4 of 7Features shared and features that differ

Features shared and features that differ

Shared

Shared a partially permeable cell surface membrane, movement down a water potential gradient, no ATP for osmosis itself, aquaporins that increase the rate,
- and a dynamic equilibrium.

Tolerance of dilute surroundings

Tolerance of dilute surroundings plant cells tolerate soil or pond water because of the wall, whereas animal cells would lyse.

Tolerance of salty surroundings

Tolerance of salty surroundings plants suffer in salty soils where water leaves roots and plasmolysis occurs,
- and animal cells are damaged by both extremes.

Plants' strategy

Plants' strategy they use turgor for support and to drive growth.

Animals' strategy

Animals' strategy they live in a controlled internal environment and spend energy to keep body fluids isotonic with cells.
- (active transport, ADH regulation)

Part 5 of 7Evidence from experiments

Plant tissue

Plant tissue potato and onion are assessed by mass change or under the microscope.

Onion epidermis

Onion epidermis in a dilute sucrose solution it looks normal, in a concentrated solution many cells are plasmolysed,
- and in pure water the cells are turgid with cytoplasm pressed against the wall.

Red blood cells

Red blood cells in 0.9% sodium chloride they keep their shape, in water they burst, and in strong salt solution they shrink.

Detection

Detection lysis is shown by a clear suspension or under the microscope.

Buffering by the wall

Buffering by the wall the wall limits how far a plant cell can swell, so it buffers the cell against changes in external water potential.

Whole organisms

Whole organisms a freshwater protoctist without a wall must pump out water with a contractile vacuole using ATP.

Part 6 of 7Links to wider topics

Plants

Plants osmosis in guard cells, root uptake and transpiration.

Animals

Animals osmoregulation, kidney function and tissue fluid.

Both

Both membrane structure and transport proteins.

Part 7 of 7More from the specification

1Simple Diffusion vs Facilitated Diffusion

Simple diffusion involves passive movement of small, non-polar molecules directly across the phospholipid bilayer;
- facilitated diffusion enables large or charged polar molecules to cross down their concentration gradient using specific transmembrane channel or carrier proteins.

2Investigating Osmosis in Plant Tissue Cylinders

In laboratory investigations, uniform plant tissue cylinders are placed in serial dilutions of sucrose solutions; measuring changes in cylinder mass or length identifies the isotonic point where water potential inside cells equals external solution water potential.
- (such as potato cylinders)

3Bulk Transport Mechanisms

Endocytosis and Exocytosis — Bulk transport of large macromolecules into cells occurs via endocytosis,
where the cell surface membrane invaginates and pinches off to form intracellular vesicles;
- this energy-requiring active process allows uptake of substances that cannot cross through channel or carrier proteins.

4Diffusion

The net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient,
- as a result of random movement.

5Carrier

A female with one recessive sex-linked allele who is unaffected.

6Indicator

A chemical that changes colour depending on pH.

7Endocytosis

Bulk transport into a cell using a membrane vesicle.

8Facilitated diffusion

Passive movement through transport proteins.

9Channel protein

Hydrophilic pore for specific ions or water.

10Carrier protein

A membrane protein that binds specific molecules or ions and changes shape to move them across.

Key terms

Term	Meaning
Turgor	Pressure within a plant cell that presses the protoplast against the wall.
Lysis	Bursting of an animal cell in hypotonic surroundings.
Crenation	Shrinkage and crinkling of an animal cell in hypertonic surroundings.
Plasmolysis	Shrinking of the protoplast away from the cell wall.
Cell wall	Rigid cellulose layer outside the plant cell membrane.
Pressure potential	Pressure of the wall on the cell contents.
Isotonic	Equal in water potential.

Avoid these mistakes

Claiming osmosis does not happen in plant cells in dilute solutions because of the wall — water still enters and the wall only limits swelling.

Using shrivel for plant cells without saying the protoplast pulls away from the wall.

Treating turgor as the same as water potential — turgor is pressure, while water potential combines pressure and solute effects.

Comparing only outcomes without explaining them using structure.

Last updated: 15 October 2026