Chemistry • Year 12 • Module 7 • Lesson 15b
Carbohydrate Biomolecules
Apply your understanding of glycosidic bond formation, disaccharide identification, condensation–hydrolysis equations, and the starch–cellulose distinction to real data, reasoning, and diagram analysis.
1. Identify disaccharides from hydrolysis data
A food scientist hydrolyses four unknown carbohydrate samples (A–D) using dilute hydrochloric acid and heat. She then identifies the monosaccharide products using chromatography. The table shows her results. 10 marks
| Sample | Monosaccharide products detected | Name of original carbohydrate | Class (mono-, di-, polysaccharide) |
|---|---|---|---|
| A | Glucose only | ||
| B | Glucose + fructose (equal amounts) | ||
| C | Glucose + galactose (equal amounts) | ||
| D | Glucose only (very large quantity) |
1.1 Complete the “Name” and “Class” columns. Note that sample A yields only glucose in small quantity (consistent with a disaccharide), while sample D yields a very large quantity (consistent with a polysaccharide). 8 marks (1 per cell)
1.2 Write the balanced hydrolysis equation for sample B using molecular formulas. State whether the reaction is acid-catalysed or enzyme-catalysed in the body. 2 marks
2. Interpret experimental data — enzyme specificity for starch vs cellulose
A biochemist tests two enzymes (amylase and cellulase) on two substrates (starch and cellulose) by measuring the rate of glucose production over time. The bar chart below summarises glucose yield (mg per g substrate) after 30 minutes. 7 marks
Figure 2.1. Glucose yield from starch and cellulose after 30 min with amylase or cellulase at pH 7, 37 °C. Illustrative data.
2.1 Describe the pattern shown in the graph regarding enzyme specificity. 2 marks
2.2 Explain, using knowledge of bond type, why amylase cannot effectively hydrolyse cellulose. 2 marks
2.3 A student concludes: “Humans cannot digest cellulose because it has a different monomer from starch.” Identify the error in this statement and provide the correct explanation. 3 marks
3. Compare starch and cellulose across five features
Complete the two-column table. For each feature, write a concise contrast between starch and cellulose. 10 marks (1 per cell)
| Feature | Starch | Cellulose |
|---|---|---|
| Bond type | ||
| Chain shape | ||
| Biological function | ||
| Digestibility by humans | ||
| Hydrolysis products (complete) |
4. Predict and justify — a lactose intolerance scenario
An adult with lactose intolerance lacks sufficient lactase enzyme. They drink a glass of milk containing 12 g of lactose. 7 marks
4.1 Write the balanced hydrolysis equation for lactose using molecular formulas. 2 marks
4.2 Predict what happens to the lactose in the digestive system of this person and explain why. Include reference to bond type and enzyme specificity. 3 marks
4.3 Lactose-free milk is produced by adding lactase to regular milk before packaging. Identify the chemical products present in lactose-free milk that are absent in regular milk, and state whether lactose-free milk is sweeter, less sweet, or the same sweetness as regular milk. Justify your answer. 2 marks
Q1.1 — Disaccharide identification table
A: Maltose / disaccharide. B: Sucrose / disaccharide. C: Lactose / disaccharide. D: Starch (or cellulose — accept either) / polysaccharide. Award: D is most likely starch given the context (complete hydrolysis to glucose). Accept cellulose if the student notes that cellulose would normally not be hydrolysed by dilute HCl under mild conditions without noting the extreme conditions used.
Q1.2 — Hydrolysis equation for sample B (sucrose)
C₁₂H₂₂O₁₁ + H₂O → C⁶H₁₂O₆ + C⁶H₁₂O₆ (1 mark for balanced equation with correct formulas). In the body this reaction is catalysed enzymatically by invertase (sucrase); the acid-catalysed version (dilute HCl + heat) is used in the laboratory (1 mark for stating enzyme-catalysed in the body).
Q2.1 — Enzyme specificity pattern
Amylase produces high glucose yield from starch (420 mg/g) but negligible yield from cellulose (8 mg/g). Conversely, cellulase produces high glucose yield from cellulose (415 mg/g) but negligible yield from starch (11 mg/g). Each enzyme is highly specific — it acts effectively only on its matching substrate. (1 mark for describing each enzyme&rsquos pattern; 2 marks total.)
Q2.2 — Why amylase cannot hydrolyse cellulose
Amylase is specific for α-1,4-glycosidic bonds (found in starch). Cellulose contains β-1,4-glycosidic bonds — a different bond geometry at C1. Amylase&rsquos active site is complementary in shape only to the α-configuration; it cannot bind and cleave the β-configuration bond in cellulose. (1 mark for identifying bond types differ; 1 mark for enzyme active site / specificity reasoning.)
Q2.3 — Error correction (3 marks)
Error: The statement is incorrect — both starch and cellulose have the same monomer, glucose (C⁶H₁₂O₆) [1]. Correct explanation: The difference is the orientation of the C1 –OH group forming the glycosidic bond: α-configuration in starch, β-configuration in cellulose [1]. Humans produce amylase, which cleaves α-1,4-glycosidic bonds (starch), but lack cellulase, the enzyme needed to cleave β-1,4-glycosidic bonds (cellulose) [1].
Q3 — Starch vs cellulose comparison table
Bond type: Starch: α-1,4-glycosidic bonds. Cellulose: β-1,4-glycosidic bonds.
Chain shape: Starch: coiled helix (coiled / compact). Cellulose: straight, rigid parallel chains.
Biological function: Starch: energy storage (in seeds, tubers, roots). Cellulose: structural support (plant cell walls).
Digestibility by humans: Starch: digestible; human amylase cleaves α-1,4-glycosidic bonds. Cellulose: indigestible; humans lack cellulase; passes through as dietary fibre.
Hydrolysis products (complete): Starch: glucose only (via maltose intermediate). Cellulose: glucose only (requires cellulase).
Q4.1 — Lactose hydrolysis equation
C₁₂H₂₂O₁₁ + H₂O → C⁶H₁₂O₆ + C⁶H₁₂O₆ (glucose + galactose). (1 mark for balanced equation; 1 mark for identifying glucose and galactose as the specific products.)
Q4.2 — Lactose in lactose-intolerant person
Without sufficient lactase, the glycosidic bond in lactose cannot be hydrolysed [1]. Lactose cannot be absorbed across the intestinal wall in its intact disaccharide form [1]. It passes into the large intestine where gut bacteria ferment it, producing gas and causing bloating and discomfort — the symptoms of lactose intolerance [1]. (Accept any 3 of the 3 points.)
Q4.3 — Lactose-free milk composition and sweetness
Lactose-free milk contains free glucose and galactose (the hydrolysis products of lactose) that are absent in regular milk [1]. Lactose-free milk is sweeter than regular milk because glucose and galactose are individually sweeter than the disaccharide lactose; monosaccharides generally bind taste receptors more strongly than the disaccharide from which they came [1].