Osmosis (AQA GCSE Biology): Revision Note

Principles of osmosis

The process of osmosis

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• The movement of water into and out of cells occurs by osmosis

• Osmosis in biology is the diffusion of water molecules from a dilute solution to a concentrated solution through a partially permeable membrane

• Osmosis is just a special type of diffusion, so water molecules move about randomly just like any other particle that can diffuse

• Water will always move from a more dilute solution to a more (solute) concentrated solution to even up the concentration of water molecules on each side of the membrane

Effect of solution concentration on water potential

• A dilute solution has a high concentration of water molecules, a high water potential, and a low concentration of solute molecules (lots of water, not a lot of solute)

• A concentrated solution has a low concentration of water molecules. a low water potential, and a high concentration of solute molecules (lots of solute, not a lot of water)

 Water potential gradient and osmosis

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The effect of osmosis on plant tissue

• The movement of water by osmosis can easily be observed by placing two similar pieces of plant tissue into solutions with differing concentrations

Method to observe the effects of osmosis

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• If the plant tissue gains mass:

  • water must have moved into the plant tissue from the solution surrounding it by osmosis

  • the solution surrounding the tissue is more dilute than the plant tissue (which is more concentrated)

• If plant tissue loses mass:

  • water must have moved out of the plant tissue into the solution surrounding it by osmosis

  • the solution surrounding the tissue is more concentrated than the plant tissue (which is more dilute)

• If there is no overall change in mass:

  • there has been no net movement of water as the concentration in both the plant tissue and the solution surrounding it must be equal

  • remember that water will still be moving into and out of the plant tissue, but there wouldn’t be any net movement in this case