Lipids are a particular interest of mine as my PhD was based on fat metabolism. They have important functions in organisms.
Fatty acids
Fatty acids all have the same basic structure, but have a variable R group which is a long hydrocarbon chain. If there are no double covalent bonds between carbon molecules in the chain, they are saturated. If there is one or more double bonds in the carbon chain, they are unsaturated. A double bond causes a kink in the chain, as you can see in the below diagram.
Triglycerides
A triglyceride is a molecule of glycerol with three fatty acids attached to it. Each fatty acid joins to the glycerol molecule with a condensation reaction to form an ester bond. Each condensation reaction produces a water molecule. To break each ester bond and release the fatty acids, water is added in a hydrolysis reaction.
The properties of a triglyceride molecule are important for its function as an energy store:
- The fatty acid tails are hydrophobic (repel water). This means that they gather together to form droplets in cells; the glycerol end faces outwards and the fatty acid tails face inwards away from any water.
- The hydrocarbon tails release a lot of energy when they are broken down.
- They are insoluble (do not dissolve in water), which means they don’t affect the water potential of a cell and don’t draw in water by osmosis. This is good, otherwise cells storing lots of triglycerides would soon swell up.
Phospholipids
Another important molecule similar to a triglyceride is a phospholipid. This time, only two fatty acids are joined to a glycerol molecule, and there is a phosphate group attached to the glycerol molecule. Phospholipids make up cell membranes by forming a bilayer. As we know, fatty acids are hydrophobic so face inwards, whereas the phosphate group is hydrophilic (attracts water) and faces outwards. This forms a barrier to substances dissolved in water.
Saturated and unsaturated fatty acids can have very different effects on health, particularly the risk of cardiovascular disease.
Summary
- Fatty acids vary by having different length hydrocarbon tails which can be saturated (no C=C double bonds) or unsaturated (contain at least one C=C bond).
- Triglycerides are formed by joining three fatty acid molecules to one glycerol molecule in a condensation reaction (which produces three water molecules) to form ester bonds.
- The structure of triglycerides makes them a good energy store.
- Phospholipids also contain glycerol and fatty acids along with a phosphate group – they make up the phospholipid bilayer of the cell membrane.
