Osmosis (AQA GCSE Biology): Revision Note

Principles of osmosis

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

• Osmosis is:

the diffusion of water molecules from a dilute solution to a concentrated solution through a partially permeable membrane

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

• There is a net movement of water from a more dilute solution to a more (solute) concentrated solution; this continues until the concentrations on each side of the membrane are equal

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The effect of osmosis on plant tissue

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

  • 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

  • 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 has a higher solute concentration than the plant tissue

  • If there is no overall change in mass:

    • there has been no net movement of water

    • the concentration in the plant tissue and the solution surrounding it must be equal

    • water is moving into and out of the plant tissue at equal rates

• Percentage change in mass = (final mass - starting mass) / starting mass x 100. Always divide by the starting mass. A loss of mass gives a negative percentage change

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