AS · Practice questions · Carbohydrates

Structure first, then function.

Six original Cambridge-style questions on monosaccharides, disaccharides and polysaccharides, from drawing glucose to explaining the plant cell wall.

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

Monomer, bond, shape, job.

α-glucose: C1 OH below the ring. β: above.
Condensation removes water; hydrolysis adds it.
Starch and glycogen: α-glucose, compact, insoluble stores.
Cellulose: β-glucose, straight chains, H-bonded microfibrils.

Recall

[3 marks]

Define the terms monomer, polymer and macromolecule.

Monomer: a small molecule that can be joined to many others of the same kind to make a polymer. ✓
Polymer: a large molecule made of many repeating monomer units joined by covalent bonds. ✓
Macromolecule: a very large molecule (for example a polysaccharide, protein or nucleic acid). ✓

Recall

[2 marks]

Describe the difference in structure between α-glucose and β-glucose.

Both are six-membered rings with the formula C₆H₁₂O₆. ✓
In α-glucose the OH group on carbon 1 is below the plane of the ring; in β-glucose it is above (with H and OH swapped). ✓

Explain

[3 marks]

Describe how a molecule of sucrose is formed.

An α-glucose and a fructose molecule join in a condensation reaction. ✓
A molecule of water is removed, from an OH on each monosaccharide. ✓
A (1,2) glycosidic bond forms, a covalent bond via an oxygen atom. ✓

Calculation

[2 marks]

Three glucose molecules (C₆H₁₂O₆) are joined in a chain by condensation. Give the molecular formula of the product.

Two glycosidic bonds form, so two H₂O are removed. ✓
3 × C₆H₁₂O₆ = C₁₈H₃₆O₁₈; minus 2 H₂O gives C₁₈H₃₂O₁₆. ✓

Explain

[4 marks]

Explain how the molecular structure of glycogen makes it suitable as an energy store in liver and muscle cells.

Made of α-glucose, which can be released and respired. ✓
Insoluble, so it does not affect the water potential of the cell or diffuse out. ✓
Highly branched (1,6 bonds) and coiled, so compact: a lot of glucose stored in a small space. ✓
Many chain ends, so enzymes can hydrolyse it to glucose rapidly. ✓

Explain

[4 marks]

Outline how the arrangement of cellulose molecules gives a plant cell wall its strength.

Cellulose is made of β-glucose with alternate units inverted, forming long straight unbranched chains. ✓
Many hydrogen bonds form between OH groups of neighbouring parallel chains. ✓
Chains group into microfibrils, which form fibres, laid down in layers at different angles. ✓
This gives high tensile strength, so the wall resists the pressure of a turgid cell without bursting. ✓

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.