IB Biology SL

Revision Notes

2.2.2 Fatty Acids

Lipid Diagrams & Properties

  • 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

[Saturated fatty acid example.png], downloadable AS & A Level Biology revision notes

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)

Unsaturated fatty acid example, downloadable AS & A Level Biology revision notes

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

Properties of triglycerides_2, downloadable AS & A Level Biology revision notes

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

Properties of phospholipids_2, downloadable AS & A Level Biology revision notes

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

Exam Tip

Make sure you learn the structures of triglycerides and phospholipids, as well as how to recognise from a diagram whether a fatty acid is saturated or unsaturated (check for any carbon-carbon double bonds)!

Make sure you also learn which parts of triglycerides and phospholipids are hydrophobic and which parts are hydrophilic, as this is what gives these molecules their useful properties in cells.


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