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Biological Molecules

Lipids

AQA A-level Biology

Triglycerides

  • A triglyceride is made from one glycerol and three fatty acids.
  • They join by condensation reactions with the removal of three molecules of water.
  • Each condensation forms an ester bond between glycerol and a fatty acid: three ester bonds in a triglyceride.
  • The water comes from the H of a glycerol OH group and the OH group of the fatty acid.
  • The R group of a fatty acid is everything other than the COOH.
HHCHCHCHOHHOCOROHHOCOROHHOCORHHCHCHCHOCOROCOROCOR+ 3H2Oglycerolthree fatty acidstriglycerideester bond
One glycerol and three fatty acids condense to a triglyceride with three ester bonds, releasing three molecules of water; the dashed outlines mark the atoms removed.

Saturated and unsaturated fatty acids

Fatty acidBonds between carbons in the hydrocarbon chain
SaturatedSingle bonds
UnsaturatedAt least one double bond between carbon atoms
  • On a diagram, an unsaturated fatty acid shows C=C.
  • Double bonds cause kinks in the fatty acid tail.
  • As the number of double bonds increases, melting point decreases.
  • The fatty acids in triglycerides and in phospholipids may be saturated or unsaturated.

Phospholipids

  • In a phospholipid, one fatty acid is replaced by a phosphate group.
  • Write phosphate. Never write phosphorus in place of a fatty acid.
  • The hydrophilic phosphate heads are attracted to water; the hydrophobic fatty acid tails are repelled from water.
Rphosphate groupglycerolhydrophobic tailsunsaturated fatty acidhydrophilic headsaturated fatty acid
A phospholipid has glycerol joined to two fatty acids and a phosphate group, giving a hydrophilic head and two hydrophobic tails.

Triglycerides and phospholipids compared

FeatureTriglyceridePhospholipid
Fatty acidsThreeTwo
Ester bondsThreeTwo
Phosphate groupNoneOne
GlycerolBoth contain glycerol
In waterBoth are insoluble in water
PolarityOnly hydrophobicHydrophilic and hydrophobic regions
Arrangement in waterDo not form a bilayerForm a bilayer

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