Interactive model, with minimal support
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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:
If you removed the wall, after a long wait how would you expect the ions and water to be distributed? Why?
Consider other pieces of the unit.
Follow with a study of cystic fibrosis and cholera:
Students might be interested in cell strategies to deal with excess fluids: http://www.accessexcellence.org/RC/VL/GG/osmoSwell.html
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
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.
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.)