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Activity Number
Overview and Learning Objectives
Classroom Practice
Central Concepts
Textbook References
Benchmarks and Standards
Extensions and Connections
Additional Info
Macro Micro Link
Activity Credits

Diffusion, Osmosis, and Dialysis (a 5pp activity)

Interactive, scaffolded model

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  • Java 1.5+ - Java 1.5+ is available for Windows, Linux, and Mac OS X 10.4 and greater. If you are using Mac OS X 10.3, you can download MW Version 1.3 and explore within it instead.

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Overview and Learning Objectives

Materials such as water, nutrients, dissolved gases, ions and waste are constantly moving across a cell's membrane. In this activity, students interact with models of diffusion and osmosis and observe the net flow of molecules in air, in cells, and across a cell's semi-permeable membrane.

Students will be able to:

  • determine that diffusion results from random motion and/or collisions of particles;
  • learn that particles diffuse from high concentration to low concentration;
  • explore simple diffusion across a semi-permeable membrane.
  • connect the process to dialysis.

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Download a pre- and post-assessment from:


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Classroom Practice

In order to understand diffusion and osmosis, students need to be clear that all atoms and molecules move about randomly and exchange energy and direction of movement during collisions. This will enable them to begin to grasp the mechanism by which a substance moves from areas of high concentration to lower concentration.

Students might like to consider the strategies of cells in dealing with excess water (see, for instance, http://www.accessexcellence.org/RC/VL/GG/ecb/osmotic_swelling.html).

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Central Concepts

Key Concept:

Diffusion, the movement of particles from regions of high concentration to areas of lower concentration results from random collisions of particles. Osmosis is a chemical process in which the solvent molecules move from a region of higher solvent concentration through a semi-permeable membrane to a region of lower solvent concentration.

Additional Related Concepts

Concept Map Available


  • Cellular fluid
  • Tugor

Molecular Biology

  • Osmosis


  • Diffusion


  • Equilibrium

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Textbook References

  • Biology (Miller and Levine) Prentice Hall 5th Edition - Unit 2: Chapter 5 - Cell Structure and Function
  • Biology (Miller and Levine) Prentice Hall 5th Edition - Unit 9 (Human Biology) Chapter 40 - Repiratory System
  • Biology (Prentice-Hall) New York Edition - Chapter Seven: Cell Structures and Functions
  • Biology: Concepts and Connections (Pearson) 5th Edition - Chapter 22: Respiration: The Exchange of Gases
  • Biology: Concepts and Connections (Pearson) 5th Edition - Chapter 25: Control of the Internal Environment
  • Biology: Concepts and Connections (Pearson) 5th Edition - Chapter 4: A Tour of the Cell
  • Biology: Concepts and Connections (Pearson) 5th Edition - Chapter 5: The Working Cell
  • Biology: Exploring Life - Chapter 29: Nutrition and Digestion
  • Biology: Exploring Life - Chapter 30: The Circulatory and Respiratory Systems
  • Biology: Exploring Life - Chapter 32: Regulation of the Internal Environment
  • Biology: Exploring Life - Chapter 7: The Working Cell: Energy from Food
  • Biology: The Dynamics of Life - Chapter 37: Respiration, Circulation, and Excretion
  • Biology: The Dynamics of Life - Chapter 8: Cellular Transport and the Cell Cycle
  • BSCS Blue (8th Edition) - Chapter 3: Exchanging Materials with the Environment
  • BSCS Blue (8th Edition) - Chapter 7: Transport Systems
  • BSCS Human - Chapter 8: The Cellular Basis of Activity
  • Cell Biology (Pollard and Earnshaw) Saunders 2002 - Chapter Five: Research Strategies
  • Web of Life - Chapter 3: Cells and Their Environment

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Benchmarks and Standards


  • THE LIVING ENVIRONMENT: CELLS - Every cell is covered by a membrane that controls what can enter and leave the cell (Full Text of Standard)


  • Life Science: The Cell - 1 Cells have particular structures that underlie their functions (Full Text of Standard)

  • Life-Science: Matter, energy, and organization - 1 All matter tends toward more disorganized states (Full Text of Standard)

  • Physical-Science: Energy Conservation / Entropy - 3 Heat consists of random motion and the vibrations of atoms, molecules, and ions (Full Text of Standard)

  • Physical-Science: Motions and Forces - 1 Objects change their motion only when a net force is applied (Full Text of Standard)

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Extensions and Connections

This activity works well if followed by the activity Gradients and Barriers (Osmosis) -- Activity #72 (http://molo.concord.org/database/activities/72.htm) in this database.

Gradients and Barriers models the movement of water and particles between a cell and its surrounding environment. It looks at the motion of particles, diffusion, the formation of water shells, and has students experiment with a red blood cell in different solutions.

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Additional Info

Additional Questions

One teacher provided us with the following question that she used to engage her students in a relevant familiar case that relates to studying diffusion and osmosis. You may want to engage your class in a discussion using this question.

Your friend's fingers shrivel up when she swims in the ocean (salt water). Explain to her why this happens.

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Macro Micro Link

The phenomenon of diffusion is related to osmosis and the medical procedure of dialysis. Osmosis plays a critically important role in biology. Without osmosis, water would not have been able to reach cells and there would not have been life.

It may be helpful to discuss in class the way diffusion of gases and nutrients was the rule in the living world until hemoglobin.

How Stuff Works writes: "The movement of air through the tracheal system of most insects relies solely on diffusion. Because most insects rely on diffusion, which occurs best over small distances, they cannot get very large. You will not see huge ants, like in the movie 'Them,' because enough air could not diffuse that far into their bodies to keep their cells alive. However, some larger insects can use their abdominal muscles to force air in and out of the tracheal system in a limited way.

So, with this system in mind, it would be difficult to strangle a bug. However, if the tracheal system fills with water, it takes much longer for air to diffuse through the system. Therefore, an insect can drown fairly easily."

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Activity Credits

Created by CC Project: Molecular Logic using Molecular Workbench

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  • Java 1.5+ - Java 1.5+ is available for Windows, Linux, and Mac OS X 10.4 and greater. If you are using Mac OS X 10.3, you can download MW Version 1.3 and explore within it instead.

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These materials are based upon work supported
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