2 Lipids

• Lipids are a diverse group of organic molecules that differ widely in their size, structure and functions
• Classification of lipids

  • Simple lipids
  • Compound lipids
  • Steroids and sterols
• When a steroid nucleus has a hydroxyl group, it is called a sterol (e.g. cholesterol)
• Compound lipids

  • Consists of a lipid and a non-lipid component
  • Examples include phospholipids and glycolipids
• Simple lipids


  • Formed by joining fatty acids to an alcohol (e.g. glycerol) by ester linkages
  • Examples include triglycerides and waxes
• Lipids


  Similar to carbohydrates, they are made up of carbon, hydrogen and oxygen. They contain a higher proportion of hydrogen to oxygen
• Examples of steroids include sex hormones such as testosterone and oestrogen
• Steroids and sterols

  • Characterized by a carbon skeleton consisting of four fused rings
  • Different steroids contain different chemical groups attached to the carbon rings
  • When a steroid nucleus has a hydroxyl group, it is called a sterol (e.g. cholesterol)
• Simple and compound lipids are composed of glycerol and fatty acids
• Glycerol is a 3-carbon alcohol, each carbon bears a polar hydroxyl group, making it a polar molecule
• Saturated fatty acids do not possess any carbon-carbon double bond in the hydrocarbon tail and each carbon has the maximum number of hydrogen atoms attached
• Fatty acids are long-chained carboxylic acids containing an even number of carbon atoms (usually 14-22 carbon atoms in length)
• The hydrocarbon tails / chains of fatty acids are hydrophobic due to the non-polar C-H bonds, making lipids insoluble in water but soluble in organic solvents
• Unsaturated fatty acids possess one or more carbon-carbon double bonds in the hydrocarbon tail, creating a 'kink' in the tail where it occurs
• Different steroids contain different chemical groups attached to the carbon rings
• Glycolipids and cholesterol will be covered in greater detail during the topic of Transport across Membranes
• The polar hydroxyl groups allow glycerol to form hydrogen bonds with water molecules and glycerol is soluble in water. Glycerol is also viscous and has a higher density than water
• The structural formula of a fatty acid can be represented in several ways
• Fatty acids may be saturated or unsaturated
• A fatty acid consists of a long non-polar hydrocarbon chain and a functional group, the carboxyl (-COOH) group
• The greater the number of carbon atoms in a fatty acid, the higher its melting point due to increased hydrophobic interaction between hydrocarbon tails
• Functions of Triglycerides

  1. Storage energy molecule (major function)

  2. Source of metabolic water

  3. Good thermal insulator

  4. Protection against mechanical damage

  5. Provide buoyancy
• Comparison of several saturated and unsaturated fatty acids

  • Stearic acid: CH3(CH2)16COOH, 69.6°C
  • Oleic acid: CH3(CH2)7CH=CH(CH2)7COOH, 13.4°C
  • Linoleic acid: CH3(CH2)4CH=CHCH2CH=CH(CH2)7COOH, -5.0°C
  • Linolenic acid: CH3CH2CH=CHCH2CH=CHCH2CH=CH(CH2)7COOH, -11.0°C
• Unsaturated Fatty Acids

  • Possess one or more carbon-carbon double bonds in the hydrocarbon tail
  • The presence of a carbon-carbon double bond (C=C) creates a ‘kink’ (or rigid bend) in the hydrocarbon tail where it occurs
• Explanation for lower melting point in unsaturated fatty acids

  Greater number of kinks prevents close packing of fatty acid molecules, requiring less energy to weaken hydrophobic interactions
• Triglycerides act as a protective layer for delicate internal organs and provide buoyancy in aquatic mammals
• When molecules that do not mix with water interact together to exclude water
• Triglycerides are a compact energy storage due to a higher proportion of C-H bonds, releasing more ATP upon oxidation
• Observation from Table 2

  The greater the number of kinks or rigid bends in the molecule, the lower the melting point
• Unsaturated Fatty Acids:
• Kinks in the hydrocarbon tail prevent unsaturated fatty acid molecules from packing closely to one another, resulting in reduced hydrophobic interactions and a lower melting point for unsaturated fatty acids
• Triglycerides provide twice as much energy than starch or glycogen per unit mass
• Formation of Triglycerides:
• Triglycerides are a better source of metabolic water as they contain about twice the number of hydrogen atoms than carbohydrates
• Triglycerides:
• Classification of Triglycerides

  • Fats
  • Oils
• Condensation reaction in triglyceride formation

  Each hydroxyl group (-OH) in the glycerol reacts with the carboxyl group (-COOH) of a fatty acid. In each condensation reaction, one molecule of water is removed and an ester bond is formed between the glycerol and fatty acid. A total of 3 water molecules are lost during the formation of one triglyceride molecule
• Structure & Formation of Triglycerides

  Each triglyceride is formed from 3 fatty acids joined to 1 glycerol molecule via ester bonds
• Structure & Formation of Phospholipids

  Each phospholipid is formed from 1 phosphoric acid and 2 fatty acids combined with 1 glycerol. Each fatty acid forms an ester bond with glycerol, and phosphoric acid forms a phosphoester bond with glycerol
• Structural Adaptations of Triglycerides to Function

  Insoluble in water, 2. Compact energy storage molecule, 3. Easily hydrolysed