HSC Exam Practice

Sample response. A condensation reaction joins two monosaccharides together by releasing one molecule of water (H₂O) as a by-product, forming a covalent glycosidic bond (C–O–C) between the two sugar units. Hydrolysis is the reverse: one molecule of water is added across the glycosidic bond as a reactant, breaking the bond and regenerating the original monosaccharide monomers.

Marking notes. 1 mark for condensation definition (water released, glycosidic bond formed); 1 mark for hydrolysis definition (water consumed as reactant, bond broken); 1 mark for explicitly contrasting the role of water in each reaction (released vs consumed).

Sample response.

(a) Maltose: C₁₂H₂₂O₁₁ + H₂O → 2 C⁶H₁₂O₆ (2 × glucose).

(b) Sucrose: C₁₂H₂₂O₁₁ + H₂O → C⁶H₁₂O₆ + C⁶H₁₂O₆ (glucose + fructose).

(c) Lactose: C₁₂H₂₂O₁₁ + H₂O → C⁶H₁₂O₆ + C⁶H₁₂O₆ (glucose + galactose).

Marking notes. 2 marks per disaccharide: 1 mark for correct balanced equation with molecular formulas; 1 mark for naming both hydrolysis products correctly. Total: 6 marks.

Sample response. When glucose (C⁶H₁₂O₆) and fructose (C⁶H₁₂O₆) react by condensation to form sucrose, one water molecule is released as a by-product. Therefore the formula of sucrose = 2 × C⁶H₁₂O₆ − H₂O = C₁₂H₂₄O₁₂ − H₂O = C₁₂H₂₂O₁₁. The loss of one H from one sugar and one OH from the other forms the glycosidic bond, accounting for the removal of H₂O. Balanced equation: C⁶H₁₂O₆ + C⁶H₁₂O₆ → C₁₂H₂₂O₁₁ + H₂O.

Marking notes. 1 mark for identifying that one H₂O is released per condensation; 1 mark for the correct calculation (2 × monomer formula − H₂O = disaccharide formula); 1 mark for writing the balanced condensation equation.

Sample response. An aldose has its carbonyl group (C=O) at the terminal carbon (C1), forming an aldehyde group. A ketose has its carbonyl group within the carbon chain (usually at C2), forming a ketone. Example aldose: glucose (carbonyl at C1). Example ketose: fructose (carbonyl at C2). Biological significance: glucose and fructose are both used as cellular fuels and as monomers in disaccharides (sucrose, lactose, maltose), but they are substrates for different enzymes because of their different structural configurations; enzymes are highly specific to the substrate they recognise.

Marking notes. 1 mark for correct definitions of aldose (terminal C=O) and ketose (internal C=O); 1 mark for naming a correct example of each; 1 mark for a valid biological significance related to enzyme specificity or different functional roles.

Sample response. The student is incorrect — both starch and cellulose are polymers of the same monomer: glucose (C⁶H₁₂O₆) [1]. What is actually different is the orientation (stereochemistry) of the C1 –OH group when forming the glycosidic bond: starch uses α-1,4-glycosidic bonds (C1 –OH in the axial position), while cellulose uses β-1,4-glycosidic bonds (C1 –OH in the equatorial position) [1]. This difference causes starch chains to coil into a helix (compact; digestible by human amylase) while cellulose chains are straight and rigid (form strong fibres in plant cell walls) [1]. The biological consequence for humans: amylase recognises and cleaves only α-1,4-glycosidic bonds, so humans can digest starch easily; humans lack cellulase, which cleaves β-1,4-glycosidic bonds, so cellulose cannot be digested and passes through as dietary fibre [1].

Marking notes. 1 = identifies the error (same monomer, glucose); 1 = correctly states what differs (α vs β glycosidic bond orientation); 1 = structural consequence (coiled vs straight); 1 = biological consequence for humans (amylase cleaves α; lacks cellulase; cellulose indigestible).

Sample response (a) — Trends and relationship (3 marks). The starch concentration decreases continuously from 1.0% at time zero to 0% at 50 min, with the rate of decrease fastest in the first 30 min and slowing as starch is consumed [1]. Simultaneously, glucose concentration increases from 0 mg/mL to approximately 9.0 mg/mL, with an inverse relationship to the starch decrease [1]. This inverse relationship reflects the conversion of starch to glucose via hydrolysis: as glycosidic bonds in starch are cleaved by acid and water, each starch polymer yields many glucose monomers. The total carbon in the system is conserved; starch is converted directly to glucose [1].

Sample response (b) — Iodine test pattern (2 marks). Iodine gives a blue-black colour by inserting into the helical structure of starch. Early in the experiment (0–20 min) sufficient intact starch remains with an intact helical chain to produce the colour change [1]. By 40–50 min, starch has been completely hydrolysed to glucose — there is no helical polymer left for iodine to insert into, so no colour change is observed (negative test) [1].

Sample response (c) — Cellulose prediction (3 marks). With cellulose under the same acid conditions: cellulose contains β-1,4-glycosidic bonds rather than α-1,4 bonds. Concentrated acid at high temperature can hydrolyse β-1,4 bonds, but the rate would be much slower than for starch because the straight, hydrogen-bonded crystalline structure of cellulose makes it more resistant to acid hydrolysis than the more accessible helical starch [1]. Therefore, the glucose line would rise much more slowly and may not reach the same final concentration in 50 min [1]. The cellulose does not give an iodine blue-black colour (iodine is specific to the starch helix), so an “iodine test” line would be absent/flat throughout regardless of cellulose concentration [1].

Marking notes. Part (a): 1 = trend of each line described with approximate values; 1 = inverse relationship identified; 1 = explanation (starch converted to glucose by hydrolysis, carbon conserved). Part (b): 1 = iodine positive because intact helical starch remains; 1 = iodine negative because starch fully hydrolysed. Part (c): 1 = cellulose has β-1,4-glycosidic bonds (slower acid hydrolysis than α-1,4); 1 = glucose line rises more slowly / lower plateau; 1 = no iodine colour change (cellulose lacks helical structure).

Sample response. The claim that condensation and hydrolysis are the unifying chemical principle of biomolecule assembly and breakdown is strongly supported for all three major biomolecule classes, though it requires careful qualification to fully capture biomolecular diversity.

Evidence for the claim — all three classes: Fats (triglycerides) are formed when glycerol and three fatty acids react by condensation, each forming an ester bond (C–O–C=O) and releasing one water molecule per bond; three water molecules are released in total. Hydrolysis regenerates glycerol and three free fatty acids. Proteins are formed by condensation between the amino group (–NH₂) of one amino acid and the carboxyl group (–COOH) of the next, releasing water and forming a peptide (amide) bond (–CO–NH–); hydrolysis yields the original amino acids. Carbohydrates are formed by condensation between –OH groups on adjacent monosaccharides, forming a glycosidic bond (C–O–C) and releasing water; hydrolysis regenerates monosaccharides. In all three cases, condensation releases H₂O (monomers → polymer) and hydrolysis consumes H₂O (polymer → monomers). This is a true unifying principle.

Starch vs cellulose — same monomer, different function: The most compelling evidence that bond geometry within the condensation framework matters is the starch–cellulose comparison. Both are glucose polymers assembled by condensation and broken by hydrolysis — the same underlying chemistry. Yet starch uses α-1,4-glycosidic bonds (C1 –OH axial), producing a coiled helix used for energy storage (compact, accessible to amylase), while cellulose uses β-1,4-glycosidic bonds (C1 –OH equatorial), producing straight parallel chains cross-linked by hydrogen bonds, serving as the structural component of plant cell walls. Humans can digest starch (amylase) but cannot digest cellulose (lack cellulase). The same condensation reaction, operating on the same monomer, produces biomolecules with radically different structures and functions solely because of the stereochemistry at C1.

Evaluation of the claim — validity and limitations: The claim is valid as a unifying principle: in every case, monomers are linked by condensation and released by hydrolysis, regardless of biomolecule class. However, condensation alone does not fully explain biomolecular diversity. Additional factors include: (1) the identity of the monomer — 20 different amino acids vs 3 common monosaccharides produce very different polymers; (2) the sequence (primary structure) in which monomers are arranged — protein function depends critically on the amino acid sequence, not just the bond type; (3) the three-dimensional folding of polymers (secondary and tertiary structure) driven by non-covalent interactions (hydrogen bonds, hydrophobic interactions) that are not described by condensation alone. Therefore, while condensation–hydrolysis is the correct and unifying chemistry of biomolecule assembly, structural and functional diversity requires understanding of monomer variety, sequence, and three-dimensional folding — aspects not captured by the claim alone.

Marking criteria (7 marks). 1 = correctly describes condensation and hydrolysis for all three biomolecule classes with monomer name, bond type, and hydrolysis products. 1 = explains starch vs cellulose with explicit reference to α vs β glycosidic bond orientation (NOT “different monomers”). 1 = explains the structural consequence (coiled helix vs straight chains) and functional consequence (digestible energy storage vs indigestible structural support). 1 = supports the claim with at least two specific pieces of evidence (e.g. all three classes use condensation; same monomer glucose can produce two very different polymers). 1 = correctly notes at least one limitation of the claim (monomer variety, sequence specificity, or 3D folding not captured by condensation alone). 1 = reaches an explicit evaluative judgement — the principle is valid as a chemical framework but does not fully account for biomolecular diversity. 1 = precise chemical terminology throughout (condensation, hydrolysis, glycosidic bond, ester bond, peptide/amide bond, α-1,4, β-1,4, stereochemistry, cellulase, amylase).