1.5 Triglycerides & Ester Bonds

Lipids

• Lipids are macromolecules that contain carbon, hydrogen and oxygen atoms. Unlike carbohydrates, lipids contain a lower proportion of oxygen
• Lipids are non-polar and hydrophobic (insoluble in water)
• Triglycerides are a kind of lipid that forms the main component of fats and oils
• Lipids play an important role in energy yield, energy storage, insulation and hormonal communication

Triglycerides

• Are non-polar, hydrophobic molecules
• The monomers are glycerol and fatty acids
• Glycerol is an alcohol (an organic molecule that contains a hydroxyl group bonded to a carbon atom)
• Fatty acids contain a methyl group at one end of a hydrocarbon chain known as the R group (chains of hydrogens bonded to carbon atoms, typically 4 to 24 carbons long) and at the other is a carboxyl group
  • The shorthand chemical formula for a fatty acid is RCOOH

The basic structure of a triglyceride

Function of lipids

• Triglycerides are fats and oils
• Fatty acid and glycerol molecules are the components that make up triglycerides
• Fats and oils have a number of important functions in organisms: energy storage, insulation, buoyancy, and protection

Forming an ester bond: condensation reaction

• Triglycerides are formed by esterification
• An ester bond forms when the hydroxyl (-OH) group of the glycerol bonds with the carboxyl group (-COOH) of the fatty acid
  • The formation of an ester bond is a condensation reaction
  • For each ester bond formed a water molecule is released
  • Three fatty acids join to one glycerol molecule to form a triglyceride
  • Therefore for one triglyceride to form, three water molecules are released

Formation of a triglyceride from a glycerol molecule and three fatty acid molecules by the process of esterification

• Fatty acids contain a methyl group at one end of a hydrocarbon chain known as the R group (chains of hydrogens bonded to carbon atoms, typically 4 to 24 carbons long) and at the other is a carboxyl group
  • The shorthand chemical formula for a fatty acid is RCOOH
• Fatty acids can vary in two ways:
  • Length of the hydrocarbon chain (R group)
  • The fatty acid chain (R group) may be saturated (mainly in animal fat) or unsaturated (mainly vegetable oils, although there are exceptions e.g. coconut and palm oil)
• Saturated fatty acids contain no carbon-carbon double bonds
  • The hydrocarbon chain is saturated with hydrogen atoms due to the absence of double bonds
  • They form unbranched, linear chains 
• Unsaturated fatty acids can be mono or poly-unsaturated depending on the number of carbon-carbon double bonds present
  • If H atoms are on the same side of the double bond they are cis-fatty acids and are metabolised by enzymes
  • If H atoms are on opposite sides of the double bond they are trans-fatty acids and cannot form enzyme-substrate complexes, therefore, are not metabolised. They are linked with coronary heart disease

Examples of different types of fatty acids with the functional groups and presence of double bonds highlighted