Lipids & Ester Bonds (OCR A Level Biology)

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)
• There are two groups of lipid that you need to know:
  • Triglycerides (the main component of fats and oils)
  • Phospholipids

• Lipids play an important role in energy yield, energy storage, insulation and hormonal communication

Triglycerides

• Triglycerides are non-polar, hydrophobic molecules
• The monomers that make up triglycerides 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

• 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)

• 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, and therefore are not metabolised. 'Trans-fat' is linked with coronary heart disease

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

Phospholipids

• Phospholipids are a type of lipid, therefore they are formed from the monomers glycerol and fatty acids
• Unlike triglycerides, there are only two fatty acids bonded to a glycerol molecule in a phospholipid as one has been replaced by a phosphate ion (PO₄^3-)
• As the phosphate is polar it is soluble in water (hydrophilic)
• The fatty acid ‘tails’ are non-polar and therefore insoluble in water (hydrophobic)

Phospholipids are the major components of cell surface membranes. They have fatty acid tails that are hydrophobic and a phosphate head, that is hydrophilic, attached to a glycerol molecule.

• Phospholipids are amphipathic (they have both hydrophobic and hydrophilic parts)
• As a result of having hydrophobic and hydrophilic parts, phospholipid molecules form monolayers or bilayers in water

In the presence of water due to the hydrophobic and hydrophilic parts phospholipids will form monolayers or bilayers

Phospholipids v Triglycerides Table

• Triglycerides contain three ester bonds between the fatty acids and the glycerol molecule
• An ester bond forms when a hydroxyl (-OH) group from the glycerol bonds with the carboxyl (-COOH) group of the fatty acid
  • An H from glycerol combines with an OH from the fatty acid to make water
  • 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

• The ester bonds can break during breakdown of the triglyceride molecule (e.g. during digestion)
  • This process is a hydrolysis reaction and three water molecules are taken in the reverse reaction to the condensation

Formation of a triglyceride from a glycerol molecule and three fatty acid molecules in a condensation reaction