cell membrane

• Fluid mosaic

  - ‘Fluid’ : membrane is a dynamic structure where both the phospholipids & membrane proteins are able to move laterally

  - ‘Mosaic’ because of the random arrangement of proteins which are embedded in the phospholipid bilayer
• Each phospholipid molecule is amphipathic. In an aqueous environment, they arrange to form a phospholipid bilayer where phosphate heads interact with aq medium and the hydrocarbon tails form a hydrophobic core in the interior of the bilayer .
• cholestrol: 4 ring structure, slightly amphipathic, a

  hydrophilic, polar, hydroxyl group + a

  hydrophobic 4 ring structure
• Cholesterol regulates membrane fluidity

  ➔ The membrane is prevented from being overly fluid at warmer temperatures as cholesterol restricts phospholipid movement

  ➔ The membrane is prevented from being overly firm at lower temperatures as cholesterol prevents the close packing of phospholipids and hence prevents solidification.
• 1. Channel Proteins

  ➔ have a hydrophilic channel

  2. Carrier Proteins (2 alternative conformations)

  ➔ bind the solute on one side of the membrane and as a result the protein undergoes a conformational change that allows access of the solute to the opposite side of the membrane
• embedded proteins are able to move laterally within indiv layers of bilayer
• 1. Regulate movement of substances as membranes are a selectively permeable barrier (to ions, polar & large mols)
  2. compartmentalisation which allow (i) unique environments to be formed for highly specialised activities (ii) spatial separation of biochemical processes & thus their sequential operation within a cell (iii) accumulation of ions to high concentrations
  3. act as a surface for chem rxn to occur in a sequential manner
  4. increase SA for chem rxn
  5. surface topography enable communication btwn cell & its surrounding
• memb prev mvmt of

  polar charged ions
• prev freezing


  decrease hydrophobic interactions
• unsat hydrocarbon tails with kinks increase membrane fluidity
• OH of cholesterol aligns w ohos head of PL, remaining tucked into HC of memb
• non-polar HC form HI with non-polar R grps of a.a

  charged phos head interacts w polar R grps of a.a
• membrane has transient pores
• non-polar substances move through memb faster