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Cell Membrane Structure

Cell Membrane Structure

 

Phospholipid Bilayer:

  • CF: caused by a faulty transport protein in the surface membranes of epithelial cells
  • Basic structure of the bilayer is two layers of phospholipid

 

Phospholipid Molecule:

  • Only has two fatty acids; negatively charged phosphate group replaces the third fatty acid

 

 

 

  • Phosphate head is polar (one end slightly negative and the other positive) HYDROPHILIC
  • The fatty acid tails are non-polar HYDROPHOBIC
  • Phospholipids arrange themselves to avoid contact between the hydrophobic tails and the water
  • They may form a layer with the tails out of the water or form spherical clusters called micelles or form a bilayer

 

Phospholipid Bilayer:

  • Cells; filled with aqueous cytoplasm and surrounded by aqueous tissue fluid
  • Cell surface membrane; adopts most stable arrangement

 

Fluid Mosaic Model:

  • Phospholipid bilayer also contains proteins, glycoproteins, cholesterol, and glycolipids
  • Glycolipids; lipid molecules with polysaccharides attached
  • Glycoproteins; protein molecules with polysaccharides attached
  • Some proteins span the membrane whilst others are only on the outer/inner layer
  • Membrane proteins have hydrophobic areas (these are inside the bilayer)
  • Some proteins are fixed whilst other move around in the fluid (fluid mosaic model)

 

Evidence:

  • Old structure; three layer protein-lipid sandwich based on election micrographs (A)
  • However this structure did not allow the hydrophilic heads to be in contact with water and it does not allow hydrophobic tails to stay away from water
  • Phosphate heads are more dense

 

Experiments:

  • Showed two different types of protein; one removed from membrane easily by increasing ionic strength of solution, other could only be removed by adding detergents.
  • Supported fluid mosaic model; integral membrane proteins are fully embedded in phospholipids whilst those on the outside were loosely attached

 

 

  • Additional evidence; freeze fracture electron microscopy studies
  • Frozen membrane sections were fractured along the weak point between the lipid layers and the inner fractured surface coated in heavy metal
  • Scanning electron microscopy revealed a smooth mosaic-like surface (lipid tails) interspersed by much larger particles (integral proteins)

 

  • Experiments carried out using labelled molecules that only attach to other specific molecules
  • Showed membranes as asymmetric (the outside of the membrane is different to the inside)

 

Experiment; fusing mouse cells with human cells:

  • A specific membrane protein was labelled in each cell type
Light microscope used to follow where points moved.
  • Mouse; green fluorescent
  • Human; red fluorescent
  • Immediately after fusion the coloured labels remained in their respective halves
  • After 40 minutes at 370C there was a complete intermixing of proteins
  • Only possible by diffusion through membrane; proved fluidity of membrane

 

Unsaturated Phospholipids:

  • The more unsaturated; the more fluid the membrane
  • ‘kinks’ in hydrocarbon tail of unsaturated phospholipids prevent them packing tight together (making more movement)

 

Protein function:

  • Enzymes
  • Carrier and channel proteins
  • Glycoproteins and glycolipids have important roles in cell-to-cell recognition as receptors