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 principle of the movement down a concentration gradient is the same, but 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 that organisms obtain by diffusion

  • Oxygen for respiration at all times
  • Carbon dioxide for photosynthesis when conditions for photosynthesis are right (e.g. enough light and a suitable temperature)
• Energy for diffusion

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

  
• Diffusion table

  
• Brownian motion

  
• Surface area to volume ratio

  • 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
• Cells adapted for diffusion

  • Root hair cells in plants (which absorb water and mineral ions)
  • Cells lining the ileum in animals (which absorb the products of digestion)
• Distance

  The smaller the distance molecules have to travel the faster transport will occur
• Blood capillaries and alveoli have walls which are only one cell thick, to ensure the rate of diffusion across them is as fast as possible
• Temperature

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


  More collisions against the cell membrane and therefore a faster rate of movement across them
• Concentration Gradient

  The greater the difference in concentration on either side of the membrane, the faster movement across it will occur
• Higher concentration on one side


  More random collisions against the membrane will occur
• Cell adaptation for diffusion

  
• The highly folded surface of the small intestine increases its surface area

  
• Water


  A solvent that is important for all living organisms
• Many substances are able to dissolve in water
• Water is incredibly useful and essential for all life on Earth
• Importance of water as a solvent in organisms

  1. Dissolved substances can be easily transported around organisms - eg xylem and phloem of plants and dissolved food molecules in the blood

  2. Digested food molecules are in the alimentary canal but need to be moved to cells all over the body - without water as a solvent this would not be able to happen

  3. Toxic substances such as urea and substances in excess of requirements such as salts can dissolve in water which makes them easy to remove from the body in urine

  4. Water is also an important part of the cytoplasm and plays a role in ensuring metabolic reactions can happen as necessary in cells
• Water as a solvent

  
• Osmosis

  • All cells are surrounded by a cell membrane which is partially permeable
  • Water can move in and out of cells by osmosis
  • Osmosis is 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
  • In doing this, water is moving down its concentration gradient
  • The cell membrane is partially permeable which means it allows small molecules (like water) through but not larger molecules (like solute molecules)
• Osmosis and the partially permeable membrane

  
• Osmosis experiments

  1. Immersing plant cells in solutions of different concentrations

  2. Cutting cylinders of root vegetables such as potato or radish and placing them into distilled water and sucrose solutions of increasing concentration

  3. Weighing the cylinders before placing into the solutions

  4. Leaving the cylinders in the solutions for 20 - 30 minutes

  5. Removing, drying to remove excess liquid and reweighing the cylinders
• If the plant tissue gains mass


  Water must have moved into the plant tissue from the solution surrounding it by osmosis
• If the solution surrounding the tissue is more dilute than the plant tissue


  Water must have moved into the plant tissue from the solution surrounding it by osmosis
• If the plant tissue loses mass

  Water must have moved out of the plant tissue into the solution surrounding it by osmosis
• If the solution surrounding the tissue is more concentrated than the plant tissue

  Water must have moved out of the plant tissue into the solution surrounding it by osmosis
• 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
• Water will still be moving into and out of the plant tissue, but there wouldn't be any net movement in this case
• Dialysis tubing


  A non-living partially permeable membrane made from cellulose
• Dialysis tubing


  • 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
• 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
• 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
• Potatoes are usually used in osmosis experiments to show how the concentration of a solution affects the movement of water, but radishes can be used too

  
• An example setup of a dialysis tubing experiment

  
• Osmosis in plant tissues

  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