Explain how the structure of a triglyceride makes it well suited for long-term energy storage in animals.

IB DP Biology Standard 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 are non‑polar, hydrophobic molecules, so they do not associate with water.
They consist of three fatty‑acid chains attached to a glycerol backbone, giving a high ratio of C–H bonds to oxygen.
The high proportion of C–H bonds means more energy is released per gram when oxidised than in carbohydrates.
Being stored as a compact, water‑free droplet allows a large amount of energy to be stored without a significant increase in body mass or volume.

Examiner tips

Use the exact terms ‘non‑polar’, ‘hydrophobic’, ‘C–H bonds’, and ‘compact, water‑free droplet’.
Show the logical link: structure → high C–H ratio → more energy per gram → efficient storage.
Keep the answer concise, 4 points, no extra words.

Common mistakes

Confusing triglycerides with phospholipids or saying they are polar.
Failing to mention the high C–H bond ratio or the energy per gram advantage.
Describing storage as ‘in fat cells’ without noting the compact, water‑free nature.

Mark scheme (4 marks)

Triglycerides are non-polar / hydrophobic molecules, so they do not associate with water molecules
Triglycerides are composed of three fatty acid chains bonded to glycerol, giving a high ratio of C–H bonds to oxygen
The high proportion of C–H bonds means more energy is released per gram upon oxidation compared with carbohydrates
Being stored as a compact, water-free droplet means a large amount of energy can be stored without significantly increasing body mass / volume

Key terms in this question

triglyceride