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Learn4BI1Paper 2B · Paper 2 (all content)Structure and functions in living organismsDiffusion and osmosis
Structure and functions in living organisms · Subtopic 2 of 21

Diffusion and Osmosis

Diffusion is the net movement of particles from a higher to a lower concentration, down a concentration gradient, because of random movement. It's faster with a steeper gradient, larger surface area, shorter distance and higher temperature. Osmosis is the net movement of water molecules from a dilute solution (high water potential) to a more concentrated solution (low water potential) through a partially permeable membrane. Plant cells in water become turgid; in strong sugar solution they become flaccid then plasmolysed. Animal cells in water may burst; in strong solutions they shrink.

Cambridge 5090 Paper 1Cambridge 5090 Paper 2Cambridge 0610 Papers 2, 4Cambridge 0610 Papers 1, 3Edexcel 4BI1 Paper 1BEdexcel 4BI1 Paper 2B≈ 40 minRevision 1 · updated 2026-10-30
Section 1 of 14

Learning objectives

  • Define diffusion and osmosis.
  • Describe factors affecting the rate of diffusion.
  • Explain the effects of osmosis on plant and animal cells.
  • Interpret results of osmosis experiments.
Pause and think

Why do supermarkets spray water on lettuces?

Section 2 of 14

Core idea

Start hereWilting and recovering

A wilted plant perks up within an hour of watering. Water rushes into its cells by osmosis, pushing against the cell walls until the stems stand up again.

The big idea

Particles spread from high to low concentration by random movement. Water does the same across partially permeable membranes, moving towards the more concentrated solution.

1. Diffusion

  • Steeper concentration gradient → faster.
  • Larger surface area → faster.
  • Shorter distance → faster.
  • Higher temperature → faster.

2. Osmosis

Cells in different solutions

Plant cells have walls; animal cells don't.

Worked examplePotato practical

Potato cylinders: 0.0 M: +12%; 0.2 M: +4%; 0.4 M: −5%; 0.6 M: −13%.

Goal
Estimate the concentration of the cell sap.
Strategy
Find where % change = 0.
  1. between 0.2 and 0.4 M\text{between }0.2\text{ and }0.4\ \mathrm{M}
    The sign changes here.
  2. 0.2+0.2×44+5≈0.29 M0.2+0.2\times\frac{4}{4+5}\approx0.29\ \mathrm{M}
    Interpolate (or read from a graph).
AnswerAbout 0.30.3 mol/dm³.
✓ CheckNo net water movement where solution = sap concentration ✓.
Exam tipKey words "Water moves from a dilute solution to a more concentrated solution through a partially permeable membrane" (or "from high to low water potential").
Pause and think

Why do we calculate percentage change instead of using mass change directly?

Summary

  • Diffusion: high → low.
  • Osmosis: water, partially permeable membrane.
  • Turgid, flaccid, plasmolysed.
Key words
Concentration gradient
Difference in concentration between two regions.
Water potential
Tendency of water to move; highest in pure water.
Turgid
Swollen and firm with water.
HistoryPfeffer. In 1877 Wilhelm Pfeffer measured osmotic pressure using a porous pot lined with a membrane, the first quantitative study of osmosis.
Fun factSalting slugs. Salt kills slugs because water rushes out of their moist skin by osmosis into the concentrated salt.
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