Plasma Membrane

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Lipids

  • Glycerol and fatty acids combine by condensation to produce triglycerides.
  • Tryglycerides contain 3 fatty acids and glycerol.
  • All fatty acids have a -COOH group with a hydrocarbon chain
  • If there are no double carbon bonds in the chain, the fatty acid is saturated.
  • If there is a double bond the fatty acid is unsaturated.

Test for lipids: EMULSION TEST 

  • add ethanol to the solution 
  • shake the tube throughly 
  • add water and shake again 
  • a cloudy white colour indicates the presence of a lipid
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Phospholipids

  • Phospholipids are the same as lipids apart from that one fatty acid is replaced by a phosphate molecule
  • Fatty acid molecules repel water, phosphate molecules attract water 

Hydrophilic head: phosphate end which interacts with water 

Hydrophobic tail: fatty acid end which moves itself away from water 

  • Phospholipids form the bilayer
  • The inner layers hydrophilic heads point inwards to the cytoplasm 
  • The outer layers hydrophilic heads point inwards to the water surrounding the cell
  • The hydrophobic tails point to the centre of the membrane 

Phospholipids make the membrane flexible, allow lipid soluble substances to enter and leave the cell and prevent water soluble substances from entering or leaving the cell. 

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Proteins

Extrinsic proteins: 

  • occur on the surface of the bilayer or partially embedded in it
  • give mechanical support to the membrane 
  • act as receptors for molecules such as hormones 

Intrinsic proteins: 

  • span the whole phospholipid bilayer
  • act as carriers to transport water soluble material 

1. provide structural support  4. form recognition sites

2. act as carriers  5. help cells stick together 

3. allow active transport  6. act as receptors 

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Fluid Mosaic Model

  • the arrangement of various molecules in the cell surface membrane 

Fluid = individual phospholipid molecules can move relative to one               another, this gives the membrane a flexible structure that                       constantly changes shape 

Mosaic = the protiens that are embedded in the bilayer vary in shape,                   size and pattern

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