Cell Membranes: Diffusion and Osmosis

Interactive, scaffolded model

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 osmostic 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;
• work with barriers, changing pore sizes and predicting changes in the movement of dissolved materials and the effect on the pressure developed on the cellular membrane;
• 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

Suppose you have a container in which you place 1000 water molecules on the left side of a wall and on the right side 900 water molecules and 100 NaCl molecules (200 ions). Then you remove the wall. How many molecules of water would end up on each side of the container? How many ions would end up on each side of the container?

Now start over again with a water-permeable wall (it lets water flow through it, but not salt) in the container. You place 1000 water molecules on the left side of the wall and on the right side 900 water molecules and 100 NaCl molecules (200 ions). How many molecules of water would end up on each side of the container? How many ions would end up on each side of the container?

If you could take a movie of the molecules and ions in the container with the water-permeable wall, what would you see if you watched the movie? Be specific as to the forces driving any changes that occur.

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Classroom Practice

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 osmosic 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: Plant Turgor. 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.

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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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