Osmosis

• Osmosis


  The diffusion of water molecules, from a region where the water molecules are in higher concentration, to a region where they are in lower concentration, through a partially permeable membrane
• Partially permeable membrane


  Allows water and other small molecules to pass through, but not larger molecules such as starch
• Dilute solution


  Contains a high concentration of water molecules and has a high water potential
• Concentrated solution


  Contains a low concentration of water molecules and has a low water potential
• Osmosis refers to the movement of water molecules only
• Osmosis


  1. Water molecules move from solution 1 to solution 2

  2. When the concentration of water is the same on both sides of the membrane, there will be no net movement of water molecules
• Similar observations will be made with solutions containing different solutes, for instance, salt instead of sugar
• Dilute solution


  A solution with a low concentration of solute
• Substances in cells


  • Ions
  • Sugars
  • Amino acids
• Cell membrane


  A selectively permeable membrane surrounding the cell and controlling the entry and exit of materials
• Plant cells


  • Isolated plant cells placed in a dilute solution or water will take in water by osmosis
  • Root hair cells will take up water by osmosis
  • Leaf cells of land plants will have a tendency to lose water
• Cell wall


  Outer structure which provides support and prevents the cell from bursting from the uptake of water by osmosis
• Effect of increasing solute concentration on plant cells

  1. Pure water: Cell contents push against cell wall, cell becomes turgid

  2. Concentrated solution: Cell contents lose water, cell becomes flaccid and plasmolysed

  3. Highly concentrated solution: Cell undergoes full plasmolysis as it loses more water
• Plants would be exposed to higher concentrations of solutes if there was less water in the soil, or in drought conditions
• Aquatic, freshwater plants placed in the sea, or a seaweed in a rock pool where the water evaporated in the Sun, would also lose water by osmosis
• Animal cells


  • They do not have a cell wall, so will change size and shape when put into solutions that are at a different concentration to the cell contents
  • The concentration of body fluids must be kept within strict limits, if cells lose or gain too much water by osmosis, they do not function efficiently
• Red blood cells lose water and shrink in a concentrated solution, they swell and burst in a solution that is too dilute