Triglycerides & Ester Bonds (Edexcel International AS Biology)

Triglyceride Formation

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

Types of Fatty Acid in Lipids

• 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