Chapter 2 Movement of Substances

• Diffusion


  Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration down a concentration gradient until equilibrium is reached
• Concentration gradient is the difference in concentration between two regions. The steeper the concentration gradient, the faster the rate of diffusion.
• Factors That Affect the Rate of Diffusion


  
  

  1. Conc. Gradient : The steeper the concentration gradient, the faster the rate of diffusion.
  2. Diffusion Distance : The shorter the diffusion distance, the faster the rate of diffusion.
  3. Surface area-to-volume ratio : Larger SA:V ratio, faster rate of diffusion
• Osmosis


  Osmosis is 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 is a measure of the tendency of water to move from one place to another.
• 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.
• What happens to an animal cell in a solution with higher water potential?


  1. Cytoplasm has lower water potential than outside solution
  2. Water molecules enter cell by osmosis
  3. Cell expands and burst
• What happens to an animal cell in a solution with lower water potential?


  1. Cytoplasm has higher water potential than outside solution
  2. Water molecules leave cell by osmosis
  3. Cell shrinks and becomes crenated
• Turgor


  • Turgor maintains the shape of soft tissues in plants.
  • It 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.
• Active Transport


  Active transport is the process in which

  • cellular energy is used (obtained through respiration)
  • to move the particles of a substance across a membrane
  • against its concentration gradient