describe the molecular structure of phospholipids with reference to their hydrophilic (polar) phosphate heads and hydrophobic (non-polar) fatty acid tails

Published by Patrick Mutisya · 14 days ago

Cambridge A-Level Biology – Carbohydrates and Lipids: Phospholipid Structure

Carbohydrates and Lipids – Phospholipids

Learning Objective

Describe the molecular structure of phospholipids with reference to their hydrophilic (polar) phosphate heads and hydrophobic (non‑polar) fatty‑acid tails.

Key Features of a Phospholipid

  • Amphipathic molecule – contains both polar (water‑loving) and non‑polar (water‑repelling) regions.

  • Core structure: a glycerol backbone esterified to two fatty‑acid chains and one phosphate‑containing head group.

  • Forms the basic matrix of biological membranes, arranging into bilayers in aqueous environments.

Molecular Diagram (Suggested)

Suggested diagram: a phospholipid showing the glycerol backbone, two fatty‑acid tails, and a phosphate head with a choline (or other) group.

Detailed Structural Description

  1. Glycerol Backbone – a three‑carbon molecule (C₃H₈O₃). Each carbon bears a hydroxyl group (\$‑OH\$) that can form ester bonds.

  2. Fatty‑Acid Tails (Hydrophobic)

    • Two fatty acids are attached to carbon‑1 and carbon‑2 of glycerol via ester linkages.

    • Typical fatty‑acid chains are long hydrocarbon chains, e.g., \$C{16}H{32}O2\$ (palmitic acid) or \$C{18}H{34}O2\$ (stearic acid).

    • These tails are non‑polar, containing only C–C and C–H bonds, and therefore avoid water.

  3. Phosphate Head Group (Hydrophilic)

    • Attached to carbon‑3 of glycerol is a phosphate group (\$‑PO_4^{2-}\$) forming a phospho‑ester bond.

    • The phosphate can be further linked to a small polar “head” such as choline, ethanolamine, serine or inositol, giving the complete head group.

    • The head group contains charged or polar functional groups (e.g., \$‑NH_3^+\$, \$‑COO^-\$) that interact favorably with water.

Summary Table

ComponentStructurePolarityRole in Membrane
Glycerol Backbone\$C3H8O_3\$Amphipathic (central scaffold)Provides attachment points for tails and head
Fatty‑Acid Tail 1\$R1COO^-\$ (e.g., \$C{16}H_{31}COO^-\$)Non‑polar (hydrophobic)Forms interior of bilayer, barrier to polar molecules
Fatty‑Acid Tail 2\$R2COO^-\$ (e.g., \$C{18}H_{35}COO^-\$)Non‑polar (hydrophobic)Same as Tail 1
Phosphate Head\$‑PO4^{2-}\$ + \$R{head}\$ (e.g., choline)Polar/charged (hydrophilic)Interacts with aqueous environment, stabilises bilayer

Why Amphipathic Nature Is Crucial

The dual nature of phospholipids drives the spontaneous formation of bilayers in water. The hydrophobic tails aggregate away from water, while the hydrophilic heads remain in contact with the aqueous surroundings, creating a semi‑permeable barrier essential for cell function.

Key Points to Remember

  • Phospholipids are composed of a glycerol backbone, two fatty‑acid tails, and a phosphate‑containing head group.

  • The tails are long, non‑polar hydrocarbon chains – they are hydrophobic.

  • The phosphate head (often linked to choline, ethanolamine, etc.) is polar/charged – it is hydrophilic.

  • This amphipathic structure enables the formation of lipid bilayers, the fundamental architecture of cellular membranes.