A-Level Biology: Triglycerides and Phospholipids

Key Lipids in Biological Molecules

• Triglycerides

  • Consist of a glycerol molecule and three fatty acid chains.
  • Formed via three condensation reactions, producing three water molecules and three ester bonds.
  • Saturated vs Unsaturated Fatty Acids
    • Saturated: All carbon-carbon bonds in the hydrocarbon tail are single bonds.
    • Unsaturated: At least one carbon-carbon double bond, reducing the number of hydrogen atoms.

• Phospholipids

  • Composed of a glycerol molecule, two fatty acid chains, and one phosphate group.
  • Created through condensation reactions, forming two ester bonds.
  • Unique property: Contains a hydrophilic head (phosphate group) and hydrophobic tails (fatty acids).

Properties and Functions

Triglycerides

• Energy Storage
  • High energy due to numerous carbon-hydrogen bonds.
  • High hydrogen-to-oxygen ratio allows them to act as a metabolic water source.
  • Large, hydrophobic molecules are insoluble, preventing osmotic imbalance in cells.
  • Low mass relative to tissues like muscle, allowing for energy storage without adding bulk.

Phospholipids

• Membrane Structure
  • Form bilayers in aqueous environments, crucial for cell membranes.
  • Hydrophilic heads face outward toward water; hydrophobic tails face inward toward other lipids.
  • Polar nature due to charged phosphate group.

Testing for Lipids: The Emulsion Test

• Procedure
  • Dissolve the lipid sample in ethanol.
  • Add distilled water and shake.
  • Positive result: A white emulsion forms, indicating lipids present.
• Important Note
  • Use the term "emulsion" rather than "precipitate," as the result is not a solid.

Exam Tips

• Understand the structure and formation of triglycerides and phospholipids.
• Know the properties and functions of these lipids, including their role in energy storage and cell membrane structure.
• Be clear on how to conduct and interpret the emulsion test for lipids.