state that triglycerides are non-polar hydrophobic molecules and describe the molecular structure of triglycerides with reference to fatty acids (saturated and unsaturated), glycerol and the formation of ester bonds

Published by Patrick Mutisya · 14 days ago

Cambridge A-Level Biology 9700 – Carbohydrates and Lipids: Triglycerides

Triglycerides – Structure and Properties

Key Objective

State that triglycerides are non‑polar, hydrophobic molecules and describe their molecular structure with reference to fatty acids (saturated and unsaturated), glycerol and the formation of ester bonds.

Why Triglycerides are Non‑Polar and Hydrophobic

  • Triglycerides consist mainly of long hydrocarbon chains (the fatty‑acid tails) which contain only C–H bonds.

  • C–H bonds are non‑polar; the electron density is evenly distributed, giving the molecule an overall non‑polar character.

  • Because water is polar, it cannot form favourable interactions with the non‑polar hydrocarbon region, making triglycerides insoluble (hydrophobic) in aqueous environments.

Molecular Components

The basic building blocks of a triglyceride are:

  1. Glycerol – a three‑carbon tri‑hydroxy alcohol: \$C3H8O_3\$.

  2. Fatty acids – long‑chain carboxylic acids. The general formula is \$CH3(CH2)_nCOOH\$, where \$n\$ varies (typically 14–22).

Fatty‑Acid Types

TypeStructureDegree of SaturationPhysical Property
Saturated fatty acid\$CH3(CH2)_{n}COOH\$ (no C=C double bonds)0 double bondsSolid at room temperature (e.g., stearic acid)
Unsaturated fatty acid\$CH3(CH2){m}CH=CH(CH2)_{p}COOH\$ (one or more C=C double bonds)1+ double bonds (mono‑ or poly‑unsaturated)Liquid at room temperature (e.g., oleic acid)

Formation of Ester Bonds

Triglycerides are formed by a condensation (esterification) reaction between the three hydroxyl groups of glycerol and the carboxyl groups of three fatty‑acid molecules.

Each ester bond is created as follows:

\$\text{R–COOH} + \text{HO–CH}2\text{–CH(OH)–CH}2\text{OH} \;\longrightarrow\; \text{R–COO–CH}2\text{–CH(OH)–CH}2\text{OH} + \text{H}_2\text{O}\$

Repeating this step three times yields a triglyceride (tri‑acylglycerol) with three ester linkages:

\$\text{Glycerol} + 3\,\text{Fatty acids} \;\xrightarrow{\text{esterification}}\; \text{Triglyceride} + 3\,\text{H}_2\text{O}\$

Overall Structure of a Triglyceride

A triglyceride can be represented schematically as:

\$\$\begin{aligned}

\text{Triglyceride} &= \text{CH}2\text{OCO–R}1\;-\;\text{CH}\text{OCO–R}2\;-\;\text{CH}2\text{OCO–R}_3 \\

\text{where } R_i &\text{ denotes the hydrocarbon tail of a fatty acid (saturated or unsaturated).}

\end{aligned}\$\$

The three \$R\$ groups may be identical or different, giving rise to a wide variety of triglycerides in nature.

Suggested diagram: Structural illustration of a triglyceride showing glycerol backbone, three ester linkages, and the three fatty‑acid tails (indicating saturated vs. unsaturated).

Summary Points

  • Triglycerides are non‑polar, hydrophobic molecules because they consist largely of long hydrocarbon chains.

  • They are composed of one glycerol molecule esterified to three fatty‑acid molecules.

  • Fatty acids can be saturated (no double bonds) or unsaturated (one or more double bonds), influencing physical state.

  • Each ester bond forms by a condensation reaction that releases a molecule of water.

  • The overall structure is a glycerol backbone with three ester‑linked fatty‑acid tails, giving the molecule its characteristic properties.