Osmosis (AQA GCSE Biology: Combined Science)

Osmosis in biology

The Process of Osmosis

Osmosis occurs when two solutions are separated by a partially permeable membrane

• 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 a 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 Diffusion Table

 Water Potential Gradients

Always identify which solution is more concentrated, and which is more dilute, to determine the movement of water by osmosis

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

Potatoes are usually used in osmosis experiments to show how the concentration of a solution affects the movement of water, but radishes can be used too

• 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