1.3 - Membrane Structure

• Phospholipid molecules have a polar (charged) phosphate head and long non-polar lipid tails

  • The head is hydrophilic (attracted to water)
  • The tails are hydrophobic (repelled by water)
• When put into water, an emergent property is that phospholipids will self-organise to keep their heads ‘wet’ and their tails ‘dry’
• Phospholipid molecules can flow past each other laterally but can’t move vertically
• Integral proteins are permanently embedded, many go all the way through and are polytopic (poly = many, topic = surface), integral proteins penetrating just one surface are monotopic.
• Peripheral proteins usually have a temporary association with the membrane, they can be monotopic or attach to the surface
• Glycoproteins:

  • Are proteins with an oligosaccaride (oligo = few, saccharide = sugar) chain attached.
  • They are important for cell recognition by the immune system and as hormone receptors
• Membrane Protein Types:

  • Transport:  Protein channels (facilitated) and protein pumps (active)
  • Receptors:  Peptide-based hormones (insulin, glucagon, etc.)
  • Anchorage:  Cytoskeleton attachments and extracellular matrix
  • Cell recognition:  MHC proteins and antigens
  • Intercellular joinings:  Tight junctions and plasmodesmata
  • Enzymatic activity:  Metabolic pathways (e.g. electron transport chain)
• Cholesterol:

  • It makes the phospholipids pack more tightly and regulates the fluidity and flexibility of the membrane.
• Cholestrol:

  • Hydroxyl group makes the head polar and hydrophilic - attracted to the phosphate heads on the periphery of the membrane.
  • Carbon rings – it’s not classed as a fat or an oil, cholesterol is a steroid
  • Non-polar (hydrophobic) tail –attracted to the hydrophobic tails of phospholipids in the centre of the membrane
• It is important to regulate the degree of fluidity:

  • Membranes need to be fluid enough that the cell can move
  • Membranes need to be fluid enough that the required substances can move across the membrane
  • If too fluid however the membrane could not effectively restrict the movement of substances across itself
• Cholesterol role:

  • restricts the movement of phospholipids and other molecules
  • disrupts the regular packing of the of the hydrocarbon tails of phospholipid molecules - prevents the tails from crystallising and hence behaving like a solid.
  • Cholesterol also reduces the permeability to hydrophilic/water soluble molecules and ions such as sodium and hydrogen
• Singer-Nicholson fluid mosaic model

  • Phospholipid molecules form a bilayer - phospholipids are fluid and move laterally
  • Peripheral proteins are bound to either the inner or outer surface of the membrane
  • Integral proteins - permeate the surface of the membrane
  • The membrane is a fluid mosaic of phospholipids and proteins
  • Proteins can move laterally along membrane
• Biochemical techniques - evidence for Singer-Nicholson model:

  • Membrane proteins were found to be very varied in size and globular in shape
  • Such proteins would be unable to form continuous layers on the periphery of the membrane.
  • The membrane proteins had hydrophobic regions and therefore would embed in the membrane not layer the outside
• Fluorescent antibody tagging - evidence for Singer-Nicholson model:

  1. Red or green fluorescent markers attached to antibodies which would bind to membrane proteins
  2. The membrane proteins of some cells were tagged with red markers and other cells with green markers.
  3. The cells were fused together.
  4. Within 40 minutes the red and green markers were mixed throughout the membrane of the fused cell.
  5. This showed that membrane proteins are free to move within the membrane rather than being fixed in a peripheral layer.
• Davson-Danielli Model:

  • A protein-lipid sandwich
  • Lipid bilayer composed of phospholipids (hydrophobic tails inside, hydrophilic heads outside)
  • Proteins coat outer surface
  • Proteins do not permeate the lipid bilayer
• Evidence for Davson-Danielli model:

  • In high magnification electron micrographs membranes appeared as two dark parallel lines with a lighter coloured region in between.
  • Proteins appear dark in electron micrographs and phospholipids appear light - possibly indicating proteins layers either side of a phospholipid core.
• Falsification of Davson-Danielli Model - Freeze Fracturing:

  • The fracture occurs along lines of weakness, including the centre of membranes.
  • The fracture reveals an irregular rough surface inside the phospholipid bilayer
  • The globular structures were interpreted as trans-membrane proteins.
  • This is contrary to the Davson-Danielli model which only involves proteins coating the surface of the membrane.