Membrane structure

• Phospholipids, an integral component of all living cells, play a crucial role in biological systems, primarily constituting the bulk of cell membranes. In these comprehensive study notes, we delve into the structure and unique properties of phospholipids, including their amphipathic nature, the formation of a bilayer, and their role in creating a semi-permeable barrier.
• • It is made up of a phospholipid bilayer
  • Each molecule has a polar phoshate head and 2 non-polar long lipidtails
  • The head is hydrophilic (attracted to water)
  • The tails are hydrophobic (repelled by water)
  • This is an example of an amphipathic molecule (a molecule with both polar and non-polar molecules)
• Davidson-Danielli Model

  • The membrane is a protein-lipid sandwich.
  • Lipid bilayer composed of phospholipids.
  • Proteins coat outer surface.
  • Proteins don’t permeate the lipid bilayer.
  • Evidence: Light and dark layers were visible as distinct lines with an electron microscope.
• How the Davidson-Danielli Model was disproved

  1. Biochemical Technique (permeability of proteins)

  • Membrane proteins were found to be varied in size and globular in shape.
  • Such proteins would be unable to form continuous layers on the periphery of the membranes.
  • Proteins were also hydrophobic in certain places, so they would be attracted to tails in the centre of the layer.
• 2) Freeze Fracturing (permeability of proteins)

  • The cells were rapidly frozen then fractured
  • Lines of fracture occured on lines of weakness, including the centre of membranes
  • Fractures revealed a irregular rough surface and globuar structures inside the bilayer
  • These were interpreted as trans-membrane proteins
• 3) Fluorescent Antibody Tagging (mobility of proteins)

  • Red and Green fluorescent markers were attached to antibodies which would bind to membrane proteins
  • So the membrane proteins of some cells were tagged with red and other cells with green
  • The cells were fused
  • Within 40 minutes, the red and green markers were mixed throughout the fused cell

  This research gave rise to the Singer-Nicholson Model
• Glycoprotein: (type of integral protein)

  • Proteins with an oligosaccharide chain attached (8-10 sugar chain)
  • Important for recognition by immune system and as hormone receptors
• Singer-Nicholson Fluid Mosaic Model

  • Phospholipids form a bilayer
  • Phospholipids and proteins 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
• Cholestrol

  • Steroid lipid that makes the phospholipids pack more tightly and regulates the flexibility and fluidity of the membrane
  • Restricts molecular motion
• Cholestrol’s role in membrane fluidity

  • Resctricts the movement of phospholipids
  • so fluidity decreases
  • Cholestrol disrupts the regular packing of hydrocarbon tails of phospholipid molecules
  • increases flexibility as it prevents tails from crystallising and behaving like a solid
  • Reduces permeability to hydrophillic/water soluble molecules and ions such as sodium and hydrogen
• Functions of membrane proteins:

  • Hormone binding sites (hormone receptors) (Eg: insulin receptor)
  • Immobilized enzymes with active site on the outside (Eg: in small intestine)
  • Cell adhesion to form tight junctions between groups of cells in tissues and organs
  • Cell-to-cell communication (Eg: receptors for neurotransmitters at synapses)
  • Channels for passive transport to allow hydrophillic particles across by facilitated diffusion
  • Pumps for active transport which use ATP to move particles across membrane
• 2 groups of proteins:

  1. Integral Protein:

  • Hydrophobic on at least one part of their surface
  • Therefore embedded in the hydrocarbon chains in the centre of the membrane
  • Many are transmembrane (extend across membrane), with hydrophillic parts projecting through the regions of phosphate heads
• 1. Peripheral Protein:

  • Hydrophillic on their surface,
  • So not embedded in membrane
  • Most are attached to integral proteins - this attachment is often reversible.
  • Some have a single hydrocarbon chain attached that is inserted into membrane surface to anchor the protein
• Membrane structure

  • Membranes have inner and outer face, proteins must be oriented so they can perform function correctly-For example: pumps on root hair are oriented so they can pick up potassium ions from outside the cell
  • The fat and protein content in a membrane is variable, because funtions of membranes vary. More active membrane = more protein
  • Myelin Sheath is for insulation so low protein (18%) high fat
  • Chloroplasts and mitochondria are active in photosynthesis and respiration so high protein (75%) low fat
  • Plasma membranes surrounding cell have about 50% protein