• 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