Lipids: Fatty Acids, Diglycerides and Triglycerides

Revision notes

A varied molecular group

Lipids include fats, oils and waxes. They contain carbon, hydrogen and oxygen, though different lipid classes have different compositions and may contain other elements. They are generally poorly soluble in water and soluble in suitable non-polar solvents. Lipids are not all polymers made from one repeating monomer.

Fatty acids and saturation

A fatty acid has a carboxyl group and a hydrocarbon chain. A saturated fatty acid has no carbon–carbon double bonds in that chain. A monounsaturated fatty acid has one C=C bond; a polyunsaturated fatty acid has more than one. A cis double bond introduces a bend, reducing close packing and often lowering melting point relative to comparable saturated chains. Chain length and geometry also matter, so do not claim every unsaturated lipid has the same melting point.

Fats tend to be solid and oils liquid at room temperature, reflecting the mixture of fatty acids and molecular structures. Waxes typically contain long-chain fatty acids esterified to long-chain alcohols and serve protective waterproofing roles. A wax is not simply a triglyceride with a different name.

Esterification

Glycerol has three hydroxyl groups. Reaction of a fatty-acid carboxyl group with a glycerol hydroxyl forms an ester linkage and releases water in the simplified condensation model. Two fatty acids attached to glycerol form a diglyceride and release two water molecules overall. Three attached fatty acids form a triglyceride and release three waters. Hydrolysis uses water to break ester linkages.

The fatty acids in one triglyceride need not be identical. Biological esterification is enzyme controlled; the simplified glycerol-plus-fatty-acids equation communicates bond formation without describing every cellular intermediate. In diagrams, show the ester group as –C(=O)–O– rather than confusing it with a peptide or glycosidic bond.

Structure and properties

Long hydrocarbon regions make a triglyceride largely non-polar. Because it has no strongly polar head like a phospholipid, it does not form the same bilayer arrangement in water. A phospholipid instead typically contains two fatty-acid tails and a phosphate-containing polar head, linking lipid chemistry to membranes.

Worked application and data skills

One glycerol molecule plus three fatty acids forms one triglyceride and three water molecules in the simplified condensation model. A diglyceride has two ester linkages to fatty-acid residues and one unesterified glycerol hydroxyl. If five triglyceride molecules are formed this way, 15 ester bonds and 15 water molecules are produced.

Common misconceptions

Saturated refers to absence of C=C bonds in the fatty-acid chain, not absence of every double bond in the molecule. A triglyceride is not a phospholipid. Waxes have a different alcohol component.

Examination tips

Label glycerol, fatty-acid residues and ester links. Count one water per ester linkage formed in the simplified model.

Check your understanding

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Sources and scope

Scope: Unit 1 B5.1. Original BTECRevision notes, schematics and questions. Independent resource; not endorsed by Pearson.

Official Pearson specification. Scientific review references are listed in the accompanying QA report.