Activity Number
72
Not Editable
Overview and Learning Objectives
Assessment
Classroom Practice
Central Concepts
Textbook References
Benchmarks and Standards
Extensions and Connections
Macro Micro Link
Activity Credits
Requirements

Cell Membranes: Diffusion and Osmosis (1-p exploration; interactive, Flash)

Interactive model, with minimal support

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This Activity Requires:

  • 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.
  • QuickTime for Java - Important Note: Due to a recent QuickTime update, you may not be able to see some models with embedded Flash components. We are currently trying to develop a workaround.

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Overview and Learning Objectives

Using dynamic models, students compare free diffusion with movements of molecules when a solution is separated into two parts by a barrier, a semi-permeable membrane that allows water molecules to pass through it, but only some or none of the other molecules in solution (osmosis). They explore osmosis and osmotic pressure on the membrane and discuss how the changes in osmotic pressure may affect living cells.

Students will be able to:

  • consult a computer model to reason about diffusion of dissolved materials moving within a container;
  • vary concentrations of dissolved materials on either side of the membrane, and describe the role of particle concentration and particle size for the diffusion of the materials.

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Assessment

If you removed the wall, after a long wait how would you expect the ions and water to be distributed? Why?

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Classroom Practice

  1. ) Set the concentration levels. 2.) Update the concentrations. 3.) Run the model. 4. Notice what happens to the membrane. What happens to the shape of the cell? Does it become more rounded? More dried out? Why?

See: http://www.concord.org/~barbara/workbench_web/unitIII_mini/act_sol_gradient.html

Consider other pieces of the unit.

Follow with a study of cystic fibrosis and cholera:

http://www.concord.org/~barbara/workbench_web/unitIII_mini/act_sol_summary.html

Students might be interested in cell strategies to deal with excess fluids: http://www.accessexcellence.org/RC/VL/GG/osmoSwell.html

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Central Concepts

Key Concept:

Water moves, carrying particles of different sizes in solution. Dissolved particles are always moving about randomly in the water; the flow always goes from a region of high concentration to a region of low concentration.

Additional Related Concepts

Concept Map Available

Biology

  • Membrane
  • Waste Disposal

Molecular Biology

  • Osmosis

Physics/Chemistry

  • Diffusion

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Textbook References

  • Biology (Prentice-Hall) New York Edition - Chapter Seven: Cell Structures and Functions
  • Biology: Exploring Life - Chapter 6: A Tour of the Cell
  • BSCS Blue (8th Edition) - Chapter 1: The Chemistry of Life
  • BSCS Blue (8th Edition) - Chapter 3: Exchanging Materials with the Environment
  • BSCS Human - Chapter 4: The Internal Environment of Organisms
  • Web of Life - Chapter 2: Chemical Basis of Life
  • Web of Life - Chapter 3: Cells and Their Environment

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Benchmarks and Standards

AAAS

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

NSES

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

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Extensions and Connections

Research: Extremophiles: Extremophile bacteria live in places that are too challenging for most living beings. How do halophile (salt-loving) bacteria maintain their osmotic pressure while living in very salty environments? Read about halophiles, bacteria that can survive in extremely salty environments. Salts hold onto water. Lots of salt outside a cell can dry it out by changing osmotic pressure in the less desirable direction. So survival strategies include either making small molecules to hold on to water, or increasing salts inside cell, the strategy of archaea.

Research: Desalinization: Research some good ways to remove salt from water, so people living in areas with low rainfall can have more water. [Search word: desalinization, reverse osmosis]

Research: Read about plant turgor.

Research: Ion gradients and cell death: Different types of cells have their own "normal" concentration of ions. A normal neuron cell has, for example, 10 times as many potassium (K ) pores as sodium (Na) pores. (http://www.biology.washington.edu/bsa/IonTransport/cellpotentialsmanyion.html) When a person gets older and less healthy, cell gradients can weaken and the cells become less efficient in maintaining normal concentrations.

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Macro Micro Link

Changing the concentrations of particles dissolved in water results in changes in the osmotic pressure on the cellular membrane and turgor* in plant cells.

(* turgor: when the vacuoles within the cell are filled with water to the point that they press.)

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Activity Credits

Created by CC Project: Molecular Workbench using Molecular Workbench + Pedagogica

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Requirements

  • 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.
  • QuickTime for Java - Important Note: Due to a recent QuickTime update, you may not be able to see some models with embedded Flash components. We are currently trying to develop a workaround.

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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.