Chapter 2

• What is Diffusion

  The net movement of particles (atoms, molecules or ions) from a region where they are of higher concentration to a region where they are of lower concentration; that is, down a concentration gradient (until equilibrium is reached)

  
• What is Net movement

  Overall movement of particles when the movement of particles in one direction is greater than the movement of particles in the opposite direction

  
• Concentration gradient


  Difference in concentration of particles in two regions
• Equilibrium


  When both regions being compared have the same concentration
• Diffusion does not require a partially permeable membrane to be present
• Both the solvent and solutes can pass through a permeable membrane
• Factors that affect the rate of diffusion

  • Concentration gradient
  • Diffusion distance
  • Surface area-to-volume ratio
• The steeper the concentration gradient


  The faster the rate of diffusion
• The shorter the diffusion distance


  The faster the rate of diffusion
• The greater the surface-area-to-volume ratio


  The faster the rate of diffusion
• The rate of food and oxygen intake is slower as the cell grows larger
• It is not beneficial for the cell to grow too big
• Osmosis


  The net movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane

  
• Water potential

  A measure of the tendency of water to move from one place to another
• A dilute solution has a higher water potential
• A concentrated solution has a lower water potential
• A water potential gradient is established when a partially permeable membrane separates two solutions of different water potentials

  
• Concentrated solution


  High concentration of solute, low water potential of solvent
• Diluted solution


  Low concentration of solute, high water potential of solvent
• Water potential


  A water potential gradient is established when a partially permeable membrane separates two solutions of different water potentials

  
• Partially permeable membrane


  Allows some substances to pass through it but not others
• Factors affecting rate of osmosis

  • Water potential gradient
  • Distance that water molecules need to move
  • Surface area-to-volume ratio
• Factors affecting rate of diffusion

  • Concentration gradient
  • Diffusion distance (travelled by solute molecules)
  • Surface area-to-volume ratio

  
• Cells as living osmotic systems

  • A living cell is enclosed by a partially permeable membrane
  • A plant cell behaves differently from an animal cell when placed in solutions of differing water potentials due to its cell wall
• What happens to a plant cell in a solution with higher water potential

  1. Cell sap has lower water potential than surrounding solution

  2. Water enters by osmosis

  3. Cell expands and becomes turgid

  4. Cell wall prevents cell from bursting
• What happens to a plant cell in a solution with lower water potential

  1. Cell sap has higher water potential than surrounding solution

  2. Water leaves by osmosis

  3. Cell becomes flaccid

  4. Cytoplasm shrinks away from cell wall and cell becomes plasmolysed
• Turgor


  • Plays an important role in maintaining the shape of soft tissues in plants
  • Keeps herbaceous (non-woody) plants firm and upright
  • Loss of turgidity causes the plant to wilt
• Plasmolysis causes tissues to become limp or flaccid
• When cells of a plant become flaccid, the plant wilts
• What happens to a cell in a solution with same water potential

  A cell immersed in a solution with the same water potential as its cytoplasm will not change its size or shape
• Dead cells no longer have an intact partially permeable cell membrane, and therefore osmosis can no longer occur
• Reverse osmosis also uses pressure to move water molecules against the water potential gradient
• Active transport


  The process in which cellular energy is used to move the particles of a substance across a membrane against its concentration gradient, i.e. from a region where the particles are of lower concentration to a region where they are of higher concentration
• Differences between diffusion and active transport

  • Diffusion: Down a concentration gradient, energy from respiration not required, cell membrane not required

  Active transport: Against a concentration gradient, energy from respiration required, cell membrane required
• Active transport requires energy obtained through respiration, and thus occurs only in living cells
• Dissolved mineral salts are taken up by root hair cells via active transport