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Biological Molecules

Lipids

OxfordAQA International AS and A-level Biology

Triglycerides

  • A triglyceride is made from one glycerol and three fatty acids.
  • They join by condensation 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.
  • A fatty acid is RCOOH: R 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 only
UnsaturatedContains C to C double bonds
  • On a diagram, an unsaturated fatty acid shows C=C. Write double bond between C and C. Never write just double bond for unsaturated, because every fatty acid has a C=O double bond in its carboxyl group.
  • Double bonds cause the molecule to kink, so the fatty acids cannot pack together so closely.
  • Unsaturated fatty acids have a lower melting point.

Phospholipids

  • In a phospholipid, one fatty acid is replaced by a phosphate group.
  • The phosphate head is hydrophilic: it attracts water.
  • The hydrocarbon tails are hydrophobic: they repel 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
Phosphate groupNoneContains phosphate
Glycerol and fatty acidsBoth contain glycerol and fatty acids
Ester bondsBoth contain ester bonds
SaturationFatty acids may be saturated or unsaturated

Function of triglycerides

Compared with carbohydrate, a triglyceride has a high carbon and hydrogen content, so it stores more energy per gram and is less soluble.

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