Osmosis

• Permeable


  A substance can pass through a membrane
• Impermeable


  A substance cannot pass through a membrane
• Selectively permeable membrane


  Allows some but not all molecules to pass through
• Selectively permeable membranes allow water, oxygen and carbon dioxide to pass through freely but do not allow sugars, proteins and salts to pass through easily
• Cells have specialised mechanisms in their membranes to allow substances to pass through when necessary, e.g. glucose has to pass in through the plasma or cell membrane to allow for respiration
• Diffusion


  The spreading out of molecules from a region of high concentration to a region of low concentration along a concentration gradient
• Everyday examples of diffusion


  • Smell of perfume spreading
  • Smell of bread baking
  • Unpleasant effects of a stink bomb
• Cause of diffusion


  Kinetic energy of the molecules moving randomly and tending to spread out
• Diffusion is passive and does not require external energy
• Diffusion in the leaf

  • Carbon dioxide diffuses from the atmosphere into the leaf
  • Oxygen diffuses from the leaf into the atmosphere
• Solvent


  A liquid that dissolves other substances (e.g. water)
• Solute


  A substance that has been dissolved (e.g. salt in water)
• Solution


  A mixture of a solute and solvent (e.g. salt water)
• Osmosis


  The movement of water molecules across a semi-permeable membrane from a region of high water concentration to a region of low water concentration
• Osmosis is a special type of diffusion and is also passive (requires no energy)
• Animal cells


  Enclosed by a cell (or plasma) membrane, do not have cell walls (unlike plant cells)
• Animal cells in a solution that is the same concentration as their cytoplasm


  Water moves in and out at the same rate, volume of the cell remains the same
• Sea water has the same concentration as the cytoplasm of many animals that live in the sea, and the cells of most land animals are surrounded by tissue fluid that has the same concentration as the cells
• Animal cells in a solution that is less concentrated than their cytoplasm


  Water moves into the cell, causing it to enlarge and possibly burst and die
• Amoeba


  • A single celled organism that lives in freshwater pools and ponds, has a contractile vacuole to expel water and prevent the cell from bursting
• Animal cells in a solution that is more concentrated than their cytoplasm


  Water moves out of the cell, causing it to shrivel (crenation) and die
• Plant cells


  Enclosed by a cell membrane and a strong cell wall, cell walls are fully permeable to water, gases and many solutes
• Plant cells in a solution less concentrated than their cytoplasm


  Water moves into the cytoplasm and vacuole, causing turgor pressure that gives the plant strength and support
• Turgor pressure


  The outward pressure of the cytoplasm and vacuole against the cell wall of a plant
• Plant cells in a solution more concentrated than their cytoplasm


  Water moves out of the cell, causing the vacuole and cytoplasm to shrivel, the cell membrane to move away from the cell wall, and the cell to become flaccid (limp)
• Plasmolysis


  The loss of water from the cytoplasm and the movement of the cell membrane away from the cell wall
• Deplasmolysis


  The restoration of plasmolysed cells to normal by placing them in a less concentrated solution
• Bacteria and fungi are similar to plants in that they are also enclosed by walls
• Food preservation techniques based on osmosis


  • Soaking fish and meat in salty solution
  • Using high sugar concentration in jams and marmalades