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Learn9700Paper 2 · AS Level Structured QuestionsBiological moleculesCarbohydrates and lipids
Biological molecules · Subtopic 1 of 2

Carbohydrates and Lipids

Monosaccharides (glucose, fructose, galactose, ribose) join by condensation to form glycosidic bonds, releasing water; hydrolysis breaks them. Disaccharides: maltose (glucose + glucose), sucrose (glucose + fructose), lactose (glucose + galactose). α-glucose polymers store energy: amylose (unbranched, 1,4, helical, compact), amylopectin and glycogen (1,4 and 1,6 branches, many ends for rapid hydrolysis); insoluble, so no osmotic effect. β-glucose forms cellulose: straight chains, alternate glucose flipped, linked by hydrogen bonds into microfibrils: high tensile strength. Triglycerides = glycerol + 3 fatty acids (ester bonds): energy store, insulation. Phospholipids have a hydrophilic phosphate head and hydrophobic tails: membranes. Tests: Benedict's (reducing sugars), acid hydrolysis then Benedict's (non-reducing), iodine (starch), ethanol emulsion (lipids).

Cambridge 9700 Paper 1Cambridge 9700 Paper 2Edexcel YBI11 WBI11AQA 7402 Paper 1≈ 50 minRevision 1 · updated 2026-11-02
Section 1 of 14

Learning objectives

  • Describe glycosidic bond formation and hydrolysis.
  • Relate structures of starch, glycogen and cellulose to function.
  • Describe triglycerides and phospholipids.
  • Carry out tests for carbohydrates and lipids.
Pause and think

Starch and cellulose are both made of glucose. Why can we digest starch but not cellulose?

Section 2 of 14

Core idea

Start hereSame sugar, different jobs

Starch in potatoes, glycogen in your liver and cellulose in cotton are all made of glucose. The difference is how they're joined.

The big idea

α-glucose polymers are compact, branched energy stores; β-glucose forms straight, hydrogen-bonded fibres. Lipids store more energy per gram and build membranes.

Forming maltose
  1. C6H12O6+C6H12O6\mathrm{C_6H_{12}O_6+C_6H_{12}O_6}
    Two α-glucose.
  2. C1–OH+C4–OH→C1–O–C4\text{C1–OH}+\text{C4–OH}\rightarrow\text{C1–O–C4}
    Condensation forms a 1,4 glycosidic bond.
  3. C12H22O11+H2O\mathrm{C_{12}H_{22}O_{11}+H_2O}
    Water released; Mr=342M_r=342.
Polysaccharides compared

Structure suits function.

Worked exampleWater released

How many water molecules form when a triglyceride is made?

Goal
Number.
Strategy
Count ester bonds.
  1. 3 fatty acids→3 ester bonds3\ \text{fatty acids}\rightarrow3\ \text{ester bonds}
    One per fatty acid.
  2. 3 H2O3\ \mathrm{H_2O}
    One water per condensation.
Answer3.
✓ CheckGlycerol has three OH groups ✓.

Summary

  • Glycosidic bonds by condensation.
  • Starch, glycogen store; cellulose supports.
  • Triglycerides: 3 ester bonds.
  • Phospholipids form bilayers.
Key words
Condensation
Reaction joining molecules with the release of water.
Hydrolysis
Breaking a bond by adding water.
HistoryBenedict, 1909. Stanley Benedict devised his copper-based reagent to test urine for glucose, then a key sign of diabetes.
Fun factCotton is nearly pure cellulose. Cotton fibres are about 90% cellulose: the strength of your T-shirt comes from hydrogen-bonded β-glucose chains.
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