Osmosis

Osmosis:

Osmosis is the diffusion of water down its concentration gradient. Normally one thinks of water as the solvent, and focuses on the concentration of the solutes, but water itself has a concentration in any solution. Pure water has a molecular weight of 18 grams/mole, so its concentration is approximately 55 Molar!

Solutes take up space that would otherwise have been occupied by water in a solution, and they also associate with a number of the water molecules, further lowering its activity (effective concentration). The following demonstration simulates the process of dissolving a solute in water. Watch what happens to the concentration of free water as you use the chooser menu to increase the number of solute particles.

Simulation of Water and Solute in an Aqueous Solution

This simulation shows the behavior of water and solute in an idealized, simple solution. Initially only 150 water molecules are present, and they are exhibiting Brownian motion. Use the pull-down menu at the lower right to increase (or decrease) the number of solute molecules in solution. Clicking on the large compartment itself resets the simulation to its starting condtion.

Each additional solute associates with several water molecules: in this simulation, 3. When no solute is present, 150 water molecules are free. When 10 solute atoms are added, and the initial volume maintained at 150 particles, only 140 water molecules are present and 110 of these are free. What do you think would happen to the solution, if the solute amount were increased to 30? Try it and see!

You are missing a great java simulation of solute dissolving in water

For a more extensive review of Diffusion, Osmosis, and Nernst Potentials, see my separate chapter on that subject.

The other critical component for osmosis is a barrier that permits water to cross, but holds back some or all of the solutes. Under these conditions, a gradient in solute concentration means that there is also a gradient in the free water concentration (the other way!). The following demo illustrates this process.

Simulation of Osmotic Flow Between Two Flexible Compartments
You're missing a java applet demonstrating osmosis	Fluctuating volumes and effective Molar concentrations for each compartment are depicted numerically across the top of the simulation. Use the mouse and the pull-down bar at the right to change the solute number in the right-hand compartment from 10 to 5 particles and describe what happens. (This experiment may take as much as 30 minutes to reach equilibrium!) Does the membrane ever return to its original location? How about a change from 10 to 20? And then from 20 to 5? Repeat these experiments several times to obtain a statistical sample.

The demonstration above shows the membrane moving (and the volume changing) as water moves. This is just what would occur between two flexible compartments that always had equal hydrostatic pressure. Cells behave this way, as will be discussed below

As we have also seen, hydrostatic pressure can also cause water to move. When pressure is applied against the direction of osmotic movement, then the osmotic flow will be slowed or even reversed. When the pressure is just enough to stop the osmotic flow an equilibrium is reached. This pressure, by definition, is called the "Osmotic Pressure" of a solution.

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Copyright 1999, Joe Patlak, Department of Physiology, University of Vermont.
For comments, problems or questions regarding this web contact Joe Patlak.
Last updated: November 01, 2000.