Activity Number
Overview and Learning Objectives
Classroom Practice
Central Concepts
Benchmarks and Standards
Extensions and Connections
Additional Info
Activity Credits

Diffusion, Osmosis, and Active Transport (SAM) (a 2-day activity)

Interactive, scaffolded model

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Overview and Learning Objectives

Students investigate diffusion, osmosis and study the role of surface area in facilitating diffusion. They apply what they have learned to a blood cell traveling through different concentrations of oxygen. Then, they explore a 3D aquapore embedded in a membrane. Using the Molecular Rover, they fly through the aquapore, and consider how it facilitated their travel from one side of the membrane to another.

Learning Objectives:

Students will be able to:

• Compare diffusion and osmosis.

• Explain how concentration differences affect the overall flow of molecules.

• Contrast molecular movement of materials in and out of equilibrium and describe the dynamic nature of equilibrium.

• Apply the principles of diffusion to red blood cells in a real biological system.

• Explain how increased surface area increases the rate of diffusion.

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Classroom Practice

Possible student pre/misconceptions

• Molecules move with a purpose. They “know” to move from areas of high concentration to areas of low concentration.

• Molecular motion stops when equilibrium is reached

• Diffusion happens at the same speed and is not affected by concentration difference.

• The process of diffusion is dependent on the type of solute.

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Central Concepts

Key Concept:

Additional Related Concepts


  • Membrane
  • Transportation

Molecular Biology

  • Active transport
  • Membrane pore
  • Osmosis


  • Diffusion

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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: Matter, energy, and organization - 4 The complexity and organization of organisms accommodates the need for obtaining, transforming, transporting, releasing, and eliminating the matter and energy used to sustain the organism (Full Text of Standard)

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Extensions and Connections

Other SAM activities:

The goal of this activity is for students to explore the concepts of passive diffusion, osmosis (and osmotic pressure), and the pumping of materials across a membrane against the natural equilibrium, a process known as active transport.

From Atoms and Energy students learn about conservation of energy and in Electrostatics they learn about ion transfer, which is important when studying chemical and electrical potential in active transport. A background in Atomic Structure is necessary for students to understand that atoms are made of protons, neutrons and electrons. Ions exist as a result of atoms that have gained or lost electrons. Newton’s Laws at the Atomic Scale focuses on the concept that atoms are in constant motion, moving in a straight line until they collide. This underscores the randomness of the motion involved in diffusion.

Phase Change serves as a reference to the states of matter (particularly solids and liquids) that are addressed in this unit. Gas Laws highlights the random motion of gas particles, which behave much like particles dissolved in water or other solvents seen in this unit. Understanding osmotic pressure is also quite similar to the underlying principles behind gas pressure. The Solubility unit is helps students understand why some things can or cannot cross membranes while the Chemical Reactions and Energy activity allows them to explore the chemical energy used to push ions across a membrane.

ATP‐Biological Energy highlights how ATP is used to move materials across the cell membrane. Four Levels of Protein Structure gives students background in how proteins are organized so they can appreciate the details about aquapores provided in this unit. The Structure and Function of Proteins allows students to recognize that one function of proteins is acting as transmembrane molecules.

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Additional Info

Additional Questions

Directions: After completing the unit, answer the following questions to review.

  1. During diffusion molecules move from areas of high concentration to areas of low concentration. Is this process spontaneous or directed by the cell? Explain.
  2. Red blood cells (with the help of hemoglobin) pick up oxygen in the lungs and drop it off to the rest of the body tissues. Explain how this is an example of diffusion in action.
  3. Give an example of a molecule that can move easily through a cell membrane. Give an example of a molecule that requires assistance to cross the cell membrane. Why are there differences in how molecules can cross the cell membrane?
  4. Diffusion and osmosis refer to the movement of particles until they reach equilibrium. What is active transport? Why is this process sometimes necessary for cells?

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Activity Credits

Created by CC Project: SAM using Molecular Workbench

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NSF Logo
These materials are based upon work supported
by the National Science Foundation under grant numbers
9980620, ESI-0242701, EIA-0219345, DUE-0402553, and 0628181.

Any opinions, findings, and conclusions or recommendations expressed in this
material are those of the author(s) and do not necessarily reflect
the views of the National Science Foundation.