M2:S5 Biological Membranes

• Cell membranes


  Surround cells and organelles, controlling what substances enter and leave
• Functions of cell membranes

  • Act as a barrier between the cell and its environment, controlling what enters and leaves
  • Allow recognition by other cells
  • Allow cell communication
• Functions of membranes within cells

  • Divide the cell into different compartments, acting as a barrier
  • Form vesicles to transport substances
  • Control what enters and leaves organelles
  • Can be the site of chemical reactions
• Fluid mosaic structure


  Phospholipid bilayer with proteins and carbohydrates scattered through it, constantly moving
• Components of cell membranes

  • Phospholipids form a barrier to dissolved substances
  • Cholesterol gives the membrane stability
  • Proteins control what enters and leaves the cell
  • Glycolipids and glycoproteins act as receptors for messenger molecules
• Cell signalling


  1. One cell releases a messenger molecule

  2. Messenger molecule travels to another cell

  3. Messenger molecule binds to a receptor on the target cell's membrane
• Membrane-bound receptors


  Proteins in the cell membrane that act as receptors for messenger molecules
• Drugs binding to cell membrane receptors


  They either trigger a response in the cell or block the receptor and prevent it from working
• Investigating membrane permeability using beetroot

  Cut beetroot pieces, place in water at different temperatures, measure colour intensity of liquid to determine permeability
• Increasing temperature


  Increases membrane permeability
• Changing solvent type and concentration

  Affects membrane permeability
• Diffusion


  The passive movement of particles from an area of higher concentration to an area of lower concentration
• Factors affecting rate of diffusion

  • Concentration gradient
  • Thickness of exchange surface
  • Surface area
  • Temperature
• Investigating diffusion using agar jelly and phenolphthalein


  Make agar jelly with phenolphthalein and sodium hydroxide, cut into cubes and place in dilute hydrochloric acid, observe colour change as acid diffuses in
• Diffusion


  The movement of particles from an area of higher concentration to an area of lower concentration
• Factors affecting rate of diffusion

  • Distance particles have to travel (the shorter the distance, the faster the rate of diffusion)
  • Surface area (the larger the surface area, the faster the rate of diffusion)
  • Temperature (the warmer it is, the faster the rate of diffusion because the particles have more kinetic energy so they move faster)
• Investigating diffusion in model cells

  1. Make agar jelly with phenolphthalein and dilute sodium hydroxide

  2. Fill beaker with dilute hydrochloric acid

  3. Cut agar jelly cubes and put in acid

  4. Cubes turn colourless as acid diffuses in and neutralises sodium hydroxide
• Surface area to volume ratio of agar jelly cubes

  Cubes with larger surface area to volume ratio go colourless faster when placed in same concentration of acid
• Concentration of hydrochloric acid

  Cubes in higher concentration of acid go colourless faster
• Temperature of hydrochloric acid

  Cubes in higher temperature go colourless faster
• Passive transport


  Transport process that does not require energy
• Cell membranes


  Surround cells and organelles, controlling what substances enter and leave
• Functions of cell membranes


  • Act as a barrier between the cell and its environment, controlling what enters and leaves
  • Allow recognition by other cells
  • Allow cell communication
• Functions of membranes within cells


  • Divide the cell into different compartments, acting as a barrier
  • Form vesicles to transport substances
  • Control what enters and leaves organelles
  • Can be the site of chemical reactions
• Fluid mosaic structure


  Phospholipid bilayer with proteins and carbohydrates scattered through it, constantly moving
• Phospholipids


  Form a barrier to dissolved substances, with hydrophilic heads and hydrophobic tails
• Cholesterol


  Gives the membrane stability by binding to phospholipid tails and making the membrane less fluid
• Roles of membrane proteins


  • Form channels to allow small/charged particles through
  • Act as carrier proteins to transport molecules/ions by active transport and facilitated diffusion
  • Act as receptors for cell signalling
• Glycolipids and glycoproteins


  Stabilise the membrane and act as receptors for cell signalling and the immune response
• Cell signalling


  1. One cell releases a messenger molecule

  2. Messenger molecule travels to another cell

  3. Messenger molecule binds to a receptor on the target cell's membrane
• Membrane-bound receptors


  Proteins in the cell membrane that act as receptors for messenger molecules
• Example of cell signalling


  • Glucagon hormone binds to receptors on liver cells, causing them to break down glycogen to glucose
• Example of drug binding to receptors


  • Antihistamines bind to and block histamine receptors, preventing inflammation
• Investigating membrane permeability using beetroot


  Cut beetroot pieces, place in water at different temperatures, measure absorbance of coloured liquid to determine permeability
• Increasing temperature


  Increases membrane permeability
• Increasing solvent concentration


  Increases membrane permeability
• Diffusion


  The passive movement of particles from an area of higher concentration to an area of lower concentration
• Factors affecting rate of diffusion


  • Concentration gradient
  • Thickness of exchange surface
  • Surface area
  • Temperature
• Investigating diffusion using agar jelly and phenolphthalein

  Make agar jelly with phenolphthalein and NaOH, cut into cubes and place in dilute HCl, observe cubes turning colourless as acid diffuses in
• Diffusion


  The movement of particles from an area of higher concentration to an area of lower concentration