Lipid Diagrams & Properties (AQA A Level Biology)

• Triglycerides are a form of lipid
• They are made up of one molecule of glycerol with three fatty acids attached to it
• These fatty acids have long hydrocarbon ‘tails’
• Fatty acids occur in two forms:
  • saturated fatty acids
  • unsaturated fatty acids

• The difference between these fatty acid types is found in their hydrocarbon tails

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

An example of a saturated fatty acid

Unsaturated fatty acids

• In unsaturated fatty acids, the bonds between the carbon atoms in the hydrocarbon tail are not all single bonds
  • There is at least one carbon-carbon double bond (in some unsaturated fatty acids there are many carbon-carbon double bonds)
  • These double bonds cause the hydrocarbon tail of unsaturated fatty acids to kink (bend slightly), meaning they are not as straight as saturated fatty acids

• The fatty acid is said to be ‘unsaturated’ because the hydrocarbon tail does not contain the maximum number of hydrogen atoms possible
  • This is because each carbon atom in a carbon-carbon double bond can only bond to one hydrogen atom (instead of two)

An example of an unsaturated fatty acid

Properties of triglycerides

• Triglycerides are mainly used as energy storage molecules
• This is because the long hydrocarbon tails of the fatty acids in triglycerides contain large amounts of chemical energy, which can be released when the fatty acids are broken down
• Triglycerides are also suitable as energy storage molecules because they are insoluble, meaning that they don’t affect the water potential inside the cell
• Inside cells, triglycerides form insoluble droplets, with the hydrophobic (water-repelling) fatty acids on the inside and the glycerol molecules on the outside

Triglycerides are suitable as energy storage molecules as they form insoluble droplets inside cells

Properties of phospholipids

• Phospholipids are another kind of lipid
• Phospholipids are similar in structure to triglycerides
• In phospholipids, one of the three fatty acid molecules attached to glycerol is replaced by a phosphate group
• This phosphate group is hydrophilic (water-loving), whereas the two fatty acids are hydrophobic (like in triglycerides)
• This makes phospholipids suitable for making up the bilayer of cell membranes, with the fatty acids facing inwards and the phosphate groups facing outwards
• This is also useful as it means the centre of the phospholipid bilayer is hydrophobic, meaning water-soluble substances cannot easily pass through
• This allows the cell membrane to act as a barrier, controlling what substances enter and leave the cell

Phospholipids are suitable for making up cell membranes as they form a bilayer