#2.5 Biological Membranes

• membranes are found around some organelles and at cell surface

  • seperating cell contents from outside environment
  • seperating cell contents from cytoplasm
  • cell recognition and signalling
  • holding the components of metabolic pathways in place
  • regulating the transport of materials into and out of the cell or organelle
• instrinsic / integral - goes from one side to another

  extrinsic / peripheral - only to one side
• `membranes fat composition varies

  • fat composition affects flexibility
  • membrane must be fluid + flexible
  • % unsaturated fatty acids in phospholipids

  -> keeps membrane less viscous

  -> cold-adapted organisms , like winter wheat

  (increased % in autumn)

  • cholesterol in membrane
• mitochondria

  • folded inner membranes , cristae
  • this gives a large surface area for aerobic respiration + localise some of the enzymes needed for aerobic respiration
• chloroplasts

  • inner membranes , thylakoids
  • house chlorophyll + photosynthesis occurs on thylakoids
• small intestine

  • there are some digestive enzymes on plasma membrane of the epithelial cells in the lining of small intestine
  • these enzymes catalyse some of the final stages in breakdown of certain types of sugars
• Fluid mosaic model

  In 1972, Singer and Nicolson proposed that fabric of membrane consisted of a phospholipid bilayer (double layer) with proteins floating in it, making up a mosaic pattern the lipids can change places with eachother and some of the proteins move, giving fluidity
• Fluid Mosaic Model 2

  • phospholipids form a bilayer, they can move freely, are not bonded together
  • proteins can move freely though lipid bilayer; they may be : free, bound to other components, extrinsic or intrinsic
  • the ease with which they do this is dependent on number of phospholipids with unsaturated fatty acids in phospholipid bilayer
• • the proteins are most likely anchored to cytoskeleton
  • some proteins have pores and act as channels to allow ions, which have an electrical charge and are surrounded by water molecules, to pass through
  • some are carriers and, by changing shape, carry specific molecules across the membrane
  • others may be attached to carrier proteins and function as enzymes, antigens or receptor sites for complementary-shaped signalling chemicals such as hormones
• • In neurones, the protein channels and carriers in the plasma membrane covering the long axon allow entry and exit of ions to bring about conduction of electrical impulses along their length

  →neurones have a myelin sheath formed by flattened cells wrapped around them several times giving several layers of cell membrane. The membrane forming the myelin sheath is about 20% protein and 76% lipid
• • the plasma membrane of white blood cells contain special protein receptors that enable to recognise the antigens on foreign cells, usually from invading pathogens but also from tissue or organ transplant
  • root hair cells in plants have many carrier protein to actively transport nitrate ions from soil into cells
  • the inner membranes of mitochondria are 76% protein and 24% lipid

  → due to their inner membranes contain many electron carriers that are made of protein, and hydrogen ion channels associated with ATP synthase enzymes
• Facilitated Diffusion

  • channel or carrier protein
  • passive - depends only on kinetic energy
  • net movement
  • high concentration to a low concentration
  • down a concentration gradient
  • until an equilibrium is reached
  • charged molecules or ions and larger molecules

  →ions need protein to pass through membrane
• Osmosis

  • pass through phospholipids
  • passive - depend only on kinetic energy
  • high concentration to a low concentration
  • down a concentration gradient
  • until an equilibrium is reached
  • special case of diffusion - just of water molecules
• Active Transport

  • specific carrier protein required
  • active - depends on energy from ATP
  • low concentration to a high concentration
  • against a concentration gradient
  • accumalates molecules on one side of membranes
  • charged molecules or ions and larger molecules
  • molecules moved at a faster rate
• Transport Across Membranes

  • pass though phospholipids
  • passive - depends only on kinetic energy
  • net movement
  • high concentration to a low concentration
  • down a concentration gradient
  • until an equilibrium is reached
  • lipophilic and very small molecules
• Bulk Transport

  • Endocytosis

  phagocytosis "cell eating”

  pinocytosis "cell drinking”

  cell engulfs particles/liquids by surrounded what was plasma membrane

  requires ATP

  • Exocytosis

  'opposite of endocytosis’

  vesicles fuse with plasma membrane releasing their contents

  what was vesicle membrane becomes plasma membrane

  requires ATP
• Factors affection diffusion

  • concentration gradient
  • surface area
  • diffusion distance
  • temperature
  • stirring / moving
  • size of molecules
• polar molecules are repelled by phospholipids due to polarity (can only go through channel protein (facilitated diffusion))

  bulk transport is for large molecules and particles to large to pass through plasma membrane
• Water Potential -

  a way of measuring the tendency of water molecules to move from one place to another

  • pure water has the highest water potential = 0
  • solute molecules lower the water potential, the more solute present the more negative the water potential
• the water potential of a solution surrounding an animal cell that:

  • has lysed (burst) will be higher than that of the cell ie. less negative and closer to 0
  • is normal will be the same as cell
  • has shrivelled will be more negative / lower than the cell
• the water potential of a solution surrounding a plant cell that:

  • is turgid will be higher / less negative / closer to 0
  • is flaccid will be same as cell
  • has plasmolysed will be more negative than the cell
• as temperature increases (protein)

  • atoms in protein vibrate more, breaking hydrogen and ionic bonds,thus unfold
  • 3° structure permanent changes = DENATURE
  • components of cytoskeleton become denatured
  • holes appear in membrane
• effects of solvent in phospholipids

  • organic solvent (e.g. acetone and ethanol) damage membranes as fatty acids are dissolved