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Phospholipids forms bilayers because one end is soluble in water and other end is i soluble in water.

Triglycerides form droplets because of uneven distribution of charge within molecules.

Phospholipids are made up of one glycerol molecule attached to 2 fatty acid tails and 1 phosphate group. 

  • The fatty acid 'tails' are hydrophobic (repel water)
  • The phosphate 'head' is hydrophilic (attracts water)
  • Therefore, phospholipids are polar - there is an uneven distribution of charge.
  • Because one end of the phospholipid molecule is soluble in water and one end is not, when in water, the phospholipids position themselves so that their hydrophilic heads are close to the water and their hydrophobic tails are as far away from the water as possible --> resulting in the formation of a lipid bi-layer (two layers of phospholipids organised in 2 sheets). 

Triglycerides (one glycerol with 3 fatty acid tails) are not polar, meaning there is no uneven distribution of charge within the molecules. Therefore, they cannot form hydrogen bonds with water and therefore cannot dissolve in water:

  • They are hydrophobic i.e. they lack a water soluble part. 
  • Because they can't dissolve in water, they group together as fat droplets in the cytoplasm of the cell.
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