CHAP 2

• Diffusion


  The net movement of particles from a region of higher to lower concentration, down a concentration gradient
• Diffusion


  1. Particles move from higher to lower concentration

  2. Concentration becomes equal on both sides of membrane
• Factors affecting rate of diffusion

  • Concentration gradient
  • Diffusion distance
• Shorter diffusion distance


  Higher rate of diffusion
• Greater surface area to volume ratio


  Higher rate of diffusion
• Surface area to volume ratio

  SA = length x width x 6 faces

  Volume = length x width x height
• Cells obtain food and oxygen from all sides
• As a cell grows larger, the rate of food and oxygen intake slows down
• Some cells have long and narrow protrusions to increase surface area to volume ratio
• As a cell continues to grow in size


  The growth rate slows down and stops once it reaches its optimum size
• It is not beneficial for a cell to grow too big
• Osmosis


  The net movement of water molecules from a region of higher to lower water potential through a partially permeable membrane
• Factors affecting rate of osmosis


  • Water potential gradient
  • Distance over which molecules move
  • Surface area to volume ratio
• Plant cell


  Cell sap has a lower water potential compared to solution

  Water molecules enter the cell through a partially permeable membrane

  Water in the vacuole creates turgor pressure to maintain the plant's shape

  When water leaves the cell, the cell becomes plasmolysed and flaccid
• Animal cell


  Cytoplasm has a lower water potential than the solution

  Water molecules move out of the cell by osmosis

  The cell may burst if placed in a solution with a lower water potential than the cytoplasm
• When a cell is placed in a solution with the same water potential, there is no net movement of water molecules in or out of the cell
• Turgor pressure


  Maintains the shape of soft tissues in plants

  Allows non-woody plants to remain firm and erect
• High rate of water loss from cells


  Plant wilts and loses its turgidity
• Plasmolysis


  Causes cells to be flaccid and die if it continues for too long
• Concentrated soil solution


  Unless sufficient water is added, the plant will eventually die
• Water molecules move out of roots by osmosis


  Causes the plant to wilt due to water loss and evaporation from leaves