Tuesday, November 11, 2008


One of the most common misconceptions I usually encounter in biology textbooks is about osmosis. The following is a direct quotation from one of these textbooks. “Osmosis is the diffusion of water through a selectively permeable membrane from a greater concentration of water molecules to a lesser concentration of water molecules.”

Do you see anything wrong with that statement? I hope you do. It’s the phrase “concentration of water molecules.” Water is not concentrated, it’s the solution that is concentrated. A solution may contain more water molecules or less water molecules. The former is a diluted solution while the latter is concentrated. So it’s the solution that is either concentrated or diluted, not the water.

During osmosis, water moves through a differentially permeable membrane towards an area that has more solute molecules. Wherever the solutes are, that is where water will move to. Thus, for solutions that are separated by a differentially permeable membrane, water will always move from where there is more water molecules (the diluted solution) to where there is less water molecules (the concentrated solution). Please remember, water is not concentrated!

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