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Biology · 2. Membranes and transport

Diffusion, osmosis and active transport

Predict transport direction from gradients and test it using percentage mass change.

40 minutesContent version: 2.1
Start the interactive pathwayOpen the virtual experiment

Driving question

What determines the direction in which water or solute crosses a membrane?

Curriculum coverage

Vietnam Biology 10; Cambridge IGCSE/AS membrane transport.

Model scope and limitations

Osmosis is net water movement through a selectively permeable membrane; in living tissue, wall pressure also affects equilibrium.

Learning objectives

  • Explain diffusion and osmosis using particle motion.
  • Calculate percentage mass change.
  • Estimate the isotonic point from a graph.

Prerequisite knowledge

  • Concentration, percentages and membrane structure.

Core knowledge

Diffusion

Particles move randomly, with net movement from higher to lower concentration.

Osmosis

Water moves net from higher to lower water potential through a selectively permeable membrane.

Active transport

Membrane proteins use energy to move substances against an electrochemical gradient.

Worked example

A potato cylinder rises from 2.50 g to 2.75 g: % change = (2.75 − 2.50)/2.50 × 100 = +10%.

Misconceptions to avoid

  • Water still moves both ways at equilibrium; only net movement is zero.
  • In plasmolysis the membrane pulls from the wall; the wall itself does not shrink.

Virtual experiment procedure: Osmosis and plasmolysis

  1. Cut equal tissue cylinders, blot consistently and record initial mass.
  2. Immerse for equal times across concentrations, controlling temperature and volume.
  3. Calculate % change, plot against concentration and interpolate the 0% crossing.

Safety and cautions

  • Use cutting tools on a board away from fingers; never taste solutions.

Evidence to collect

  • Initial/final mass table, percentage changes, graph and isotonic estimate.
Open the virtual experiment

Diagnostic check

1. Which way does water move net?

  1. Lower water potential
  2. Higher water potential
  3. Always out of cells
Show answer and explanation

Correct answer: Lower water potential

Net water movement is from higher to lower water potential.

2. Why use percentage rather than raw mass change?

  1. Compare different starting masses
  2. Make values always positive
  3. Remove all error
Show answer and explanation

Correct answer: Compare different starting masses

Normalising to starting mass makes comparisons fair.

Application practice

1. A sample falls from 4.0 g to 3.6 g. Percentage change?

  1. −10%
  2. +10%
  3. −0.4%
Show answer and explanation

Correct answer: −10%

(3.6 − 4.0)/4.0 × 100 = −10%.

2. A plant cell in a hypertonic solution will?

  1. Lose water and may plasmolyse
  2. Burst from water uptake
  3. Make another wall
Show answer and explanation

Correct answer: Lose water and may plasmolyse

The external solution has lower water potential, so water leaves.

3. The graph point at 0% mass change estimates what?

  1. Isotonic point
  2. Maximum diffusion rate
  3. Zero concentration
Show answer and explanation

Correct answer: Isotonic point

No mass change suggests no appreciable net water movement.

Academic references

  1. OpenStax Biology 2e — peer-reviewed foundational biology

This lesson is maintained against the cited sources below. Simulations are learning models and do not replace supervised physical-laboratory safety procedures.

Content publisher
WebLab STEM
Technical maintainer
Đức Tiến — Control & Automation Engineer