Lipids — Triglycerides and Ester Bonds

Lipids are a diverse group of organic molecules that share one key property: they are insoluble in water but soluble in organic solvents (e.g., ethanol, ether, chloroform). They include triglycerides (fats and oils), phospholipids, steroids such as cholesterol, and waxes. This lesson focuses on triglycerides — the main energy storage lipids in animals and plants — and covers OCR specification point 2.1.2 (c)(i)-(ii).

1. Overview of Lipids

Unlike carbohydrates and proteins, lipids are not polymers in the strict sense. They are built from smaller subunits but without the regular repeating structure characteristic of polymerisation. The defining feature is their hydrophobic character, due to a predominance of C–H and C–C bonds with few polar groups.

Lipid functions include:

Energy storage (triglycerides in adipose tissue, oil in seeds)
Thermal insulation (subcutaneous fat in mammals)
Physical protection (fat around kidneys and other organs)
Membrane structure (phospholipids and cholesterol)
Hormones (steroid hormones such as testosterone and oestrogen)
Waterproofing (cuticle waxes of plants and insects)
Metabolic water (lipid oxidation produces more water per gram than carbohydrate)

2. Structure of a Triglyceride

A triglyceride consists of one molecule of glycerol covalently bonded to three fatty acids by three ester bonds. It is formed by three condensation reactions, each releasing a water molecule.

Key Definition — Triglyceride: A lipid formed by the condensation of one glycerol and three fatty acids, linked by three ester bonds. The main energy storage molecule of many organisms.

2.1 Glycerol

Glycerol (propane-1,2,3-triol) is a three-carbon alcohol with three –OH groups, one on each carbon:

          H   H   H
          |   |   |
      H — C — C — C — H
          |   |   |
          OH  OH  OH

Glycerol is soluble in water because of its three hydroxyl groups, which form hydrogen bonds with water.

2.2 Fatty Acids

A fatty acid consists of a long hydrocarbon tail (typically 4–24 carbons) ending in a carboxyl group (–COOH):

    O
    ||
    C — (CH₂)ₙ — CH₃
   /
  OH

The hydrocarbon tail is non-polar and hydrophobic.
The carboxyl group is polar and hydrophilic, but in a triglyceride it forms the ester bond and is no longer exposed.
Fatty acids can be saturated (no C=C double bonds) or unsaturated (one or more C=C double bonds).

2.3 Formation of a Triglyceride

Each –OH of glycerol reacts with the –COOH of a fatty acid. Water is released (condensation) and an ester bond (–COO–) is formed. Three such reactions produce a triglyceride and three water molecules.

  Glycerol        Fatty acid 1
     OH   +   HOOC—(tail) →  O—CO—(tail) + H₂O
     OH   +   HOOC—(tail) →  O—CO—(tail) + H₂O
     OH   +   HOOC—(tail) →  O—CO—(tail) + H₂O

  Result: triglyceride with three ester bonds + 3 H₂O

The three fatty acid chains may be identical (simple triglyceride) or different (mixed triglyceride). Mixed triglycerides predominate in nature.

2.4 The Ester Bond

An ester bond is the covalent linkage formed between a hydroxyl group (–OH) and a carboxyl group (–COOH) with loss of water. Structurally, it is a –C(=O)–O– linkage.

Exam Tip: The carbonyl oxygen is part of the ester bond; students often forget to draw it.

3. Saturated vs Unsaturated Fatty Acids

Saturated Fatty Acids

All C–C bonds in the tail are single bonds.
The tail is straight and can pack tightly with neighbouring chains.
Higher melting point — solid at room temperature.
Common in animal fats (e.g., butter, lard, tallow).
Linked to increased blood cholesterol and cardiovascular disease when consumed in excess.

Lesson 6: Lipids — Triglycerides and Ester Bonds