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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.
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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).
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
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.
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.
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."
Last Update: 11/25/2008
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