2.2.7 Triglycerides

Lipids

• Macromolecules which contain carbon, hydrogen and oxygen atoms. However, unlike carbohydrates lipids contain a lower proportion of oxygen
• Non-polar and hydrophobic (insoluble in water)
• Different types:
  • Fats and Oils (composed mainly of triglycerides)
  • Phospholipids
  • Steroids and waxes (considered lipids as they are hydrophobic thus insoluble in water)

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 (chains of hydrogens bonded to carbon atoms, typically 4 to 24 carbons long) and at the other is a carboxyl group
• Fatty acids can vary in two ways:
  • Length of the hydrocarbon chain
  • The fatty acid may be saturated (mainly in animal fat) or unsaturated (mainly vegetable oils, although there are exceptions e.g. coconut and palm oil)

• Unsaturated fatty acids can be mono or  poly-unsaturated
  • 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

• Triglycerides are formed by esterification
• An ester bond forms when the hydroxyl group of the glycerol bonds with the carboxyl group of the fatty acid
  • For each ester bond formed a water molecule is released
  • 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

Energy storage

• The long hydrocarbon chains contain many carbon-hydrogen bonds with little oxygen (triglycerides are highly reduced)
  • So when triglycerides are oxidised during cellular respiration this causes these bonds to break releasing energy used to produce ATP

• Triglycerides therefore store more energy per gram than carbohydrates and proteins (37kJ compared to 17kJ)
• As triglycerides are hydrophobic they do not cause osmotic water uptake in cells so more can be stored
  • Plants store triglycerides, in the form of oils, in their seeds and fruits. If extracted from seeds and fruits these are generally liquid at room temperature due to the presence of double bonds which add kinks to the fatty acid chains altering their properties
  • Mammals store triglycerides as oil droplets in adipose tissue to help them survive when food is scarce (e.g. hibernating bears)

• The oxidation of the carbon-hydrogen bonds releases large numbers of water molecules (metabolic water) during cellular respiration
  • Desert animals retain this water if there is no liquid water to drink
  • Bird and reptile embryos in their shells also use this water

Insulation

• Triglycerides are part of the composition of the myelin sheath that surrounds nerve fibres

  This provides insulation which increases the speed of transmission of nerve impulses

• Triglycerides compose part of the adipose tissue layer below the skin which acts as insulation against heat loss (eg. blubber of whales)

Buoyancy

• The low density of fat tissue increases the ability of animals to float more easily

Protection

• The adipose tissue in mammals contains stored triglycerides and this tissue helps protect organs from the risk of damage