Chapter 2: Movement of Substances

• 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 only takes place in the absence of a partially permeable membrane
• Diffusion refers to the movement of particles down a concentration gradient
• At equilibrium (i.e. no concentration gradient), movement of particles will stop
• Diffusion in everyday life


  • The fragrance of food given out during cooking spreads throughout the kitchen
  • Digested food particles move through the wall of the small intestine into the bloodstream
  • Carbon dioxide in the air moves into the leaves through the stomata
• Osmosis in everyday life


  • Pieces of potato placed in water swell
  • Water from the soil moves through the root hair cell
• Factors affecting rate of diffusion

  • Concentration gradient
  • Diffusion distance
  • Surface area-to-volume ratio
• The shorter the diffusion distance, the faster the rate of diffusion
• In living organisms, diffusion distance is kept short by having a one-cell thick area for exchange of substances
• One-cell thick areas for diffusion


  • Gaseous exchange in the lungs
  • Gaseous exchange in plants
  • Absorption of nutrients in small intestine
• Surface area-to-volume ratio


  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.
• The epithelial cells that line the small intestine have folds in their membranes which increase surface area-to-volume ratio for faster rate of absorption of substances from the lumen of the small intestine
• Do you know how Kimchi is made?
• Marination


  Changes the environment surrounding the cabbage
• The cabbage pieces shrink after a while
• The cabbage pieces shrink

  Because of osmosis
• Chapter 2: Movement of Substances
• Osmosis


  A special form of diffusion, 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
• Partially permeable membrane


  • Allows some substances to pass through it but not others
• Examples of partially permeable membranes


  • Cell membranes
  • Dialysis tubing
• Demonstration of osmosis


  1. Set up experiment with dialysis tubing

  2. Observe level of solution rising in dialysis tubing
• Water potential


  A measure of the tendency of water to move from one place to another
• Dilute solution


  Has higher water potential
• Concentrated solution


  Has lower water potential
• Comparison of concentrated and dilute solutions


  • Concentration of solute: High
  • Low

  Water potential of solvent: Low

  • High
• Factors affecting rate of osmosis


  • Water potential gradient
  • Distance that water molecules need to move
  • Surface area-to-volume ratio
• Cells are living osmotic systems
• 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 an animal cell in a solution with higher 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 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 lower water potential

  Cell swells up and may burst
• A cell immersed in a solution with the same water potential as its cytoplasm will not change its size or shape
• Turgor


  Plays an important role in maintaining the shape of soft tissues in plants, keeps herbaceous plants firm and upright
• Loss of turgidity


  Causes the plant to wilt
• Changes in turgor


  Cause the movements of some plant parts, e.g. opening and closing of stomata
• Plasmolysis causes tissues to become limp or flaccid
• When cells of a plant become flaccid, the plant wilts
• Water may be added to the soil to dilute the soil solution, causing water molecules to enter the plant cells and keeping the plant firm and upright