Fatty Acids (HL IB Biology)

• Both triglycerides and phospholipids contain glycerol with molecules known as fatty acids attached
• These fatty acids have long hydrocarbon ‘tails’
  • Hydrocarbons are molecules that contain hydrogen and carbon
• Fatty acids occur in two forms:
  • Saturated fatty acids
  • Unsaturated fatty acids
    • Unsaturated fatty acids can be monounsaturated or polyunsaturated

Saturated fatty acids

• In saturated fatty acids the bonds between the carbon atoms in the hydrocarbon tail are all single bonds
• The fatty acid is said to be ‘saturated’ with hydrogen
  • This means that each carbon atom in the hydrocarbon tail (except for the final carbon atom) is bonded to two hydrogen atoms
• Saturated fatty acids are straight molecules, meaning that lipid molecules containing them are able to pack tightly together
  • This increases their melting point and causes them to be solid at room temperature
  • Saturated fatty acids are often used as storage molecules in animals for this reason, e.g. the fats in meat and butter

Saturated fatty acid diagram

Saturated fatty acids contain only single carbon-carbon bonds

Unsaturated fatty acids

• In unsaturated fatty acids the bonds between the carbon atoms in the hydrocarbon tail are not all single bonds
  • The fatty acid is said to be ‘unsaturated’ because the hydrocarbon tail does not contain the maximum number of hydrogen atoms possible; each carbon atom in a carbon-carbon double bond can only bond to one hydrogen atom instead of two
• These double bonds can cause the hydrocarbon tail of unsaturated fatty acids to kink, or bend, meaning they are not as straight as saturated fatty acids
  • Unsaturated fatty acids cannot pack as tightly together as saturated fatty acids, so fats containing unsaturated fatty acids are often liquids at room temperature
• Unsaturated fatty acids contain at least one carbon-carbon double bond
  • A fatty acid with one C=C double bond is known as monounsaturated fatty acid
    • Lipids that contain monounsaturated fatty acids have a lower melting point than saturated fatty acids, meaning that they form liquid oils; some animals and plants store energy in the form of oils
  • In some unsaturated fatty acids, there are many carbon-carbon double bonds; these are known as polyunsaturated fatty acids
    • Lipids containing polyunsaturated fats also have a low melting point, so form oils that are used for energy storage in plants

Mono- & polyunsaturated fatty acid diagrams

Monounsaturated fatty acids (top) contain only one carbon-carbon double bond, while polyunsaturated fatty acids (bottom) contain more than one