Explain how the structure of a triglyceride is related to its function as an energy storage molecule.

IB DP Biology Higher Level (2023 syllabus) — B1.1 Carbohydrates and lipids · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Triglycerides consist of a glycerol backbone esterified to three fatty‑acid chains, giving a large number of C–H bonds. The high proportion of C–H bonds means that, per gram, triglycerides release roughly twice the energy of carbohydrates when oxidised. Triglycerides are non‑polar and hydrophobic, so they do not associate with water and can be stored anhydrously in adipose tissue. The long, largely saturated hydrocarbon tails pack tightly together, allowing dense storage of energy in a small volume.

Examiner tips

Use the word ‘triglyceride’ and describe its glycerol backbone and three fatty acids; link C–H bonds to energy yield; mention non‑polarity and anhydrous storage; note tight packing in adipose tissue.
Show the chain of reasoning: structure → C–H bonds → energy yield; structure → hydrophobicity → storage; structure → packing → density.
Use UK spelling (e.g. ‘hydrocarbon’).

Common mistakes

Confusing triglycerides with phospholipids or cholesterol; forgetting to mention the glycerol backbone; not linking the high C–H bond content to energy yield; describing triglycerides as polar or aqueous.”

Mark scheme (4 marks)

Triglycerides contain three fatty acid chains (and a glycerol backbone), providing a large number of C–H bonds
The high proportion of C–H bonds means triglycerides have a higher energy yield per gram than carbohydrates (approximately twice)
Triglycerides are non-polar / hydrophobic, so they do not associate with water molecules and are stored anhydrously
The long, largely saturated hydrocarbon tails pack closely together in adipose tissue, allowing dense storage of energy in a small volume

Key terms in this question

triglyceride