3. Movement into & out of Cells

• Diffusion


  The movement of molecules from a region of its higher concentration to a region of its lower concentration
• Diffusion

  • Molecules move down a concentration gradient, as a result of their random movement
  • For living cells, the cell is surrounded by a cell membrane which can restrict the free movement of the molecules
  • The cell membrane is a partially permeable membrane - this means it allows some molecules to cross easily, but others with difficulty or not at all
  • The simplest sort of selection is based on the size of the molecules
• Diffusion in living organisms


  • Obtain many of their requirements
  • Get rid of many of their waste products
  • Carry out gas exchange for respiration
• Substances obtained by diffusion


  • Oxygen for respiration
  • Carbon dioxide for photosynthesis
• Energy for diffusion


  Comes from the kinetic energy of the random movement of molecules and ions (Brownian motion)
• Brownian motion


  All particles move randomly at all times
• Factors influencing diffusion

  • Surface area to volume ratio
  • Distance
  • Temperature
  • Concentration gradient
• The bigger a cell or structure is, the smaller its surface area to volume ratio is, slowing down the rate at which substances can move across its surface
• Cell adaptations for diffusion


  • Root hair cells in plants
  • Cells lining the ileum in animals
• The highly folded surface of the small intestine increases its surface area
• Distance


  The smaller the distance molecules have to travel the faster transport will occur
• Temperature


  The higher the temperature, the faster molecules move as they have more energy
• Concentration gradient


  The greater the difference in concentration on either side of the membrane, the faster movement across it will occur
• Water as a solvent

  • Many substances are able to dissolve in it
  • Dissolved substances can be easily transported around organisms
  • Digested food molecules can be moved to cells all over the body
  • Toxic substances and substances in excess can be removed from the body in urine
  • Plays a role in ensuring metabolic reactions can happen in cells
• Osmosis


  The diffusion of water molecules from a dilute solution (high concentration of water) to a more concentrated solution (low concentration of water) across a partially permeable membrane
• Water is moving down its concentration gradient during osmosis
• Partially permeable membrane


  Allows small molecules (like water) through but not larger molecules (like solute molecules)
• Osmosis experiments with plant cells

  1. Cut cylinders of root vegetables and place in distilled water and sucrose solutions of increasing concentration

  2. Weigh before and after to see if mass increases, decreases or stays the same

  3. If mass increases, water moved into the plant tissue by osmosis

  4. If mass decreases, water moved out of the plant tissue by osmosis

  5. If no change in mass, no net movement of water
• Dialysis tubing


  • A non-living partially permeable membrane made from cellulose
  • Pores are small enough to prevent the passage of large molecules (such as sucrose) but allow smaller molecules (such as glucose and water) to pass through by diffusion and osmosis
• Investigating osmosis using dialysis tubing

  1. Fill a section of dialysis tubing with concentrated sucrose solution

  2. Suspend the tubing in a boiling tube of water for a set period of time

  3. Note whether the water level outside the tubing decreases
• Osmosis


  The net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane
• If there is no overall change in mass

  There has been no net movement of water as the concentration in both the plant tissue and the solution surrounding it must be equal
• Investigating osmosis using dialysis tubing


  1. Filling a section of dialysis tubing with concentrated sucrose solution

  2. Suspending the tubing in a boiling tube of water for a set period of time

  3. Noting whether the water level outside the tubing decreases as water moves into the tubing via osmosis
• Dialysis tubing


  • A non-living partially permeable membrane made from cellulose
  • Pores in this membrane are small enough to prevent the passage of large molecules (such as sucrose) but allow smaller molecules (such as glucose and water) to pass through by diffusion and osmosis
• Water moves from a region of higher water potential (dilute solution)


  To a region of lower water potential (concentrated solution), through a partially permeable membrane
• Plant cells

  • When water moves into a plant cell, the vacuole gets bigger, pushing the cell membrane against the cell wall
  • Water entering the cell by osmosis makes the cell rigid and firm
  • The pressure created by the cell wall stops too much water entering and prevents the cell from bursting
  • If plants do not receive enough water the cells cannot remain rigid and firm (turgid) and the plant wilts
• Turgid


  Plant cells are rigid and firm due to water entering the cell by osmosis
• Flaccid


  Plant cells that have lost water by osmosis, making them soft and limp
• Plasmolysed


  Plant cells where the cell membrane has pulled away from the cell wall due to water loss by osmosis
• Animal cells


  • Animal cells also lose and gain water as a result of osmosis
  • As animal cells do not have a supporting cell wall, the results on the cell are more severe
  • If an animal cell is placed into a strong sugar solution (with a lower water potential than the cell), it will lose water by osmosis and become crenated (shrivelled up)
  • If an animal cell is placed into distilled water (with a higher water potential than the cell), it will gain water by osmosis and, as it has no cell wall to create turgor pressure, will continue to do so until the cell membrane is stretched too far and it bursts
• Crenated


  Animal cells that have shrivelled up due to water loss by osmosis
• Active transport


  The movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration using energy from respiration
• Protein carriers in active transport


  • Substance combines with carrier protein molecule in the cell membrane
  • Carrier transports substances across membrane using energy from respiration to give them the kinetic energy needed to change shape and move the substance through the cell membrane
  • Substance released into cell