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Count the C-H bonds, find the water.

Six original Cambridge-style questions on the structure of triglycerides and phospholipids and on relating lipid structure to function.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Glycerol, three tails, three waters.

01
Recall
[3 marks]

Describe the molecular structure of a triglyceride.

  • One molecule of glycerol joined to three fatty acids. ✓
  • Each fatty acid is joined by an ester bond, formed by condensation between the COOH of the fatty acid and an OH of glycerol. ✓
  • The fatty acids have long non-polar hydrocarbon tails, which may be saturated or unsaturated. ✓
02
Explain
[2 marks]

Explain the difference between a saturated and an unsaturated fatty acid.

  • A saturated fatty acid has no C=C double bonds in its hydrocarbon chain (it holds the maximum number of hydrogen atoms). ✓
  • An unsaturated fatty acid has one or more C=C double bonds, which put kinks in the chain. ✓
03
Explain
[4 marks]

Relate the structure of triglycerides to their roles in living organisms.

  • Many C-H bonds, so a large amount of energy released per gram: an efficient energy store. ✓
  • Non-polar and insoluble, so they do not affect water potential and can be stored in large amounts. ✓
  • Produce metabolic water when oxidised. ✓
  • Low thermal conductivity and low density: insulation under the skin and buoyancy in aquatic animals. ✓
04
Explain
[3 marks]

Describe the structure of a phospholipid and explain why phospholipids form a bilayer in water.

  • Glycerol joined to two fatty acids and a phosphate group. ✓
  • The phosphate head is polar (hydrophilic); the fatty acid tails are non-polar (hydrophobic). ✓
  • In water the heads face the water on both sides and the tails point inwards away from water, forming a bilayer. ✓
05
Calculation
[2 marks]

Glycerol has Mr 92 and palmitic acid has Mr 256. Calculate the Mr of a triglyceride made from glycerol and three palmitic acid molecules. (Mr of water = 18)

  • Reactants: 92 + 3 × 256 = 860. ✓
  • Three water molecules are lost: 860 − 3 × 18 = 806. ✓
06
Data
[3 marks]

Triglyceride releases about 37 kJ g−1 and glycogen about 17 kJ g−1. A migrating bird stores 50 g of triglyceride. Calculate the mass of glycogen that would store the same energy, and suggest why birds store triglyceride.

  • Energy = 50 × 37 = 1850 kJ. ✓
  • Mass of glycogen = 1850 ÷ 17 = 109 g (about twice as heavy). ✓
  • Triglyceride stores the same energy in less than half the mass, which matters for flight. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.