Biomolecules — Carbohydrates & Lipids

Question 1 (10 marks)

(a) (4 marks)

• A = starch (1); reason: α-glucose monomers / helical (coiled) chains that give blue-black iodine colour and are hydrolysed by amylase (1).
• B = cellulose (1); reason: β-glucose monomers forming straight, unbranched chains held by H-bonds into rigid microfibrils; insoluble/fibrous (1).

(b) (3 marks)

• Starch has α-1,4 glycosidic bonds (1); cellulose has β-1,4 glycosidic bonds where alternate glucose units are flipped 180° (1).
• Human amylase has an active site complementary only to the α bond, so it cannot bind/hydrolyse the β linkage of cellulose (1).

(c) (3 marks)

• The cow hosts symbiotic microorganisms (bacteria/protists) in its gut/rumen (1) that produce cellulase (1), which hydrolyses cellulose into glucose/short-chain fatty acids the cow can absorb and respire (1).

Question 2 (12 marks)

(a) (3 marks)

• Reaction: dehydration synthesis / condensation (1).
• Bond: ester bond (ester linkage) (1).
• 3 water molecules released per triglyceride (1).

(b) (4 marks)

• Hydrolysis reverses synthesis: each triglyceride requires 3 water molecules to break 3 ester bonds → yields 1 glycerol + 3 fatty acids (1 reasoning).
• Water consumed = 0.5 mol × 3 = 1.5 mol (1).
• Glycerol formed = 0.5 mol × 1 = 0.5 mol (1).
• Fatty acids formed = 0.5 mol × 3 = 1.5 mol (1).

(c) (5 marks)

• X (saturated) = solid; Y (unsaturated) = liquid (oil) (1 for correct assignment).
• Saturated chains have no C=C double bonds, so chains are straight and pack tightly together (1), maximising van der Waals contact → higher melting point → solid at room temp (1).
• Cis double bonds introduce kinks/bends in the chains (1), preventing tight packing, reducing intermolecular forces → lower melting point → liquid at room temp (1).

Question 3 (10 marks)

(a) (3 marks)

• Amphipathic = possessing both a hydrophilic (water-attracting) region and a hydrophobic (water-repelling) region (1).
• Phospholipid has a polar phosphate head (hydrophilic) (1) and two non-polar fatty acid tails (hydrophobic) (1).

(b) (4 marks)

• Two rows of phospholipids (bilayer) (1).
• Hydrophilic heads point outward — one layer facing external water, other facing internal water (1 for both surfaces).
• Hydrophobic tails point inward, facing each other, shielded from water (1).
• Correct labelling of heads/tails and inside/outside water (1).

(c) (3 marks)

• At normal body temperature cholesterol reduces fluidity by restraining phospholipid movement (1); (accept: at low temp it maintains/increases fluidity by preventing tight packing).
• Justification: cholesterol's rigid fused steroid ring structure inserts between phospholipid tails (1), restricting their movement / acting as a fluidity buffer (1).

Question 4 (10 marks)

(a) (3 marks)

• Polymer = chitin (1).
• Chitin is a polymer of a modified glucose (N-acetylglucosamine) — it contains a nitrogen-containing (amino/acetylamino) group, whereas cellulose is pure glucose with no nitrogen (2).

(b) (4 marks)

• Class = wax (1).
• Waxes are highly hydrophobic/non-polar long-chain lipids (esters of fatty acids + long alcohols) (1); they are water-insoluble and impermeable (1), forming a barrier that prevents evaporation of water from the leaf surface (1).

(c) (3 marks)

• Thickened chitin exoskeleton: provides a rigid impermeable barrier reducing water loss and physical protection (1).
• Heavy wax layer: waterproof coating minimising evaporative water loss through the cuticle (1).
• Together they reduce desiccation, aiding survival in dry/desert conditions (1).

Question 5 (8 marks)

(a) (3 marks)

• Fats/fatty acids are highly reduced — long hydrocarbon chains with many C–H bonds (1); carbohydrates are more oxidised (more C–O bonds/hydroxyl groups) (1).
• More C–H bonds means more energy released per gram upon oxidation → ~2× (≈38 vs 17 kJ g⁻¹) (1).

(b) (3 marks) — any two:

• Glycogen is more readily/rapidly hydrolysed to glucose (1).
• Glucose can enter respiration quickly, including anaerobically, for fast ATP (1).
• Glycogen's branched structure allows many glucose units to be released simultaneously (1). (max 3)

(c) (2 marks)

• Thermal insulation — subcutaneous adipose fat layer reduces heat loss (1); (accept blubber keeping body warm) (1).

[
  {
    "claim": "Hydrolysing 0.5 mol triglyceride consumes 1.5 mol water",
    "code": "trig = Rational(1,2); water = trig*3; result = (water == Rational(3,2))"
  },
  {
    "claim": "0.5 mol triglyceride yields 0.5 mol glycerol and 1.5 mol fatty acids",
    "code": "trig = Rational(1,2); glycerol = trig*1; fatty = trig*3; result = (glycerol == Rational(1,2) and fatty == Rational(3,2))"
  },
  {
    "claim": "One triglyceride synthesis releases 3 water molecules",
    "code": "bonds = 3; water = bonds; result = (water == 3)"
  },
  {
    "claim": "Fat energy density (~38 kJ/g) is roughly twice carbohydrate (~17 kJ/g)",
    "code": "fat = 38; carb = 17; result = (1.8 <= fat/carb <= 2.4)"
  }
]