A level Biology B · Edexcel 9BI0 · Topic 1B
Lipids
Key points
- A triglyceride is one glycerol joined to three fatty acids by ester bonds, formed in condensation reactions.
- Saturated fatty acids have no C=C double bonds; unsaturated ones have one or more, which put kinks in the chain so the fat is more likely to be liquid (an oil) at room temperature.
- Triglycerides are good energy stores: they hold a lot of energy per gram, are insoluble and are compact.
- In a phospholipid, one fatty acid is replaced by a phosphate group. The phosphate head is hydrophilic and the fatty acid tails are hydrophobic, so phospholipids form a bilayer in water.
- Test for lipids: the emulsion test (shake with ethanol, then add water; a milky white emulsion is positive).
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Lipids are a varied group of molecules that are insoluble in water but dissolve in organic solvents such as ethanol. Triglycerides and phospholipids are the two you must know in detail, because their structures explain energy storage and…
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