Structure of the cell surface membrane

• The plasma membrane consists of phospholipids arranged with their hydrophilic heads facing outwards towards the watery environment outside the cell and their hydrophobic tails facing inwards towards the cytoplasm.
• Functions of the cell surface membrane

  • separating cell contents from the outside environment
  • allows different conditions to be established inside and outside of the cell
  • regulating transport of materials - selectively permeable
  • cell signalling and recognition
• Cell surface membrane is made of a double layer of phospholipids with embedded molecules
• Molecules that can cross phospholipid bilayer

  • lipid-soluble molecules
  • water
  • gases - small and non-polar
• Molecules that can't cross phospholipid bilayer

  • anything charged
  • large molecules
  • polar molecules
• Intrinsic proteins span the whole width of the membrane and transport molecules that can't diffuse through the bilayer
• Extrinsic proteins are proteins that are not part of the cell membrane but are attached to it, act as receptors for signalling
• Glycolipids

  • act as recognition sites
  • help maintain membrane stability
  • helps cells attach to one another to form tissues
• Glycoproteins

  • act as recognition sites
  • helps cells attach to one another to form tissues
  • allows cells to recognise one another
• Cholesterol

  • type of lipid with formula C27H46O
  • restricts movement of molecules in membrane - controls membrane fluidity
  • keeps membrane stable at normal body temperature
• Plasma membrane can be described as fluid-mosaic
• Fluid

  • individual phosphate molecules can move relative to one another
  • gives membrane flexible shape that is constantly changing
• Mosaic

  • proteins embedded in bilayer vary in shape and size in ways that resemble a mosaic