Biomolecules — Carbohydrates & Lipids

Q1. (4 marks)

• (a) Monomer = a small, single molecular subunit that can bind to others to form a larger molecule. (1) Example: glucose / amino acid / nucleotide. (1)
• (b) Polymer = a large molecule made of many repeating monomer units joined together. (1) Example: starch / protein / DNA. (1) Why: Tests core definition; polymer = "many monomers," so example must be a chain molecule.

Q2. (4 marks)

• Dehydration synthesis (condensation): joins two monomers together, removing a water molecule. (2)
• Hydrolysis: breaks a bond between monomers by adding a water molecule. (2) Why: "Dehydration" = water lost; "hydro-lysis" = split by water. They are opposite reactions.

Q3. (3 marks)

• Elements: Carbon (C), Hydrogen (H), Oxygen (O) (2 — award 1 if only two correct)
• Ratio H:O = 2:1 (general formula $(CH_2O)_n$). (1) Why: Carbohydrate literally means "hydrated carbon," reflecting the 2:1 H:O ratio.

Q4. (6 marks) — 1 mark for definition + 1 for example each:

• Monosaccharide: single sugar unit (simplest carbohydrate). Example: glucose/fructose/galactose. (2)
• Disaccharide: two monosaccharides joined by a glycosidic bond. Example: sucrose/maltose/lactose. (2)
• Polysaccharide: many monosaccharides joined in a long chain. Example: starch/glycogen/cellulose. (2) Why: Classification is by number of sugar units (mono = 1, di = 2, poly = many).

Q5. (4 marks)

• Bond formed: glycosidic bond. (1)
• It forms by dehydration synthesis/condensation, in which an –OH from one sugar and an –H from another are removed. (2)
• By-product: water (H₂O). (1) Why: Glycosidic bond links C atoms of two sugars via an oxygen bridge, releasing water.

Q6. (6 marks) — ½ mark per correct cell (9 cells ≈ round to 6):

Why: Starch & glycogen use α-glucose (digestible storage); cellulose uses β-glucose giving straight, strong, indigestible fibres.

Q7. (4 marks)

• A triglyceride consists of 1 glycerol molecule (1) bonded to 3 fatty acid chains (1).
• The bonds are ester bonds, formed by dehydration synthesis (1), releasing 3 water molecules (1). Why: "Tri-" = three fatty acids; each ester bond forms with loss of one water (3 total).

Q8. (4 marks) — 2 marks per valid difference:

• Saturated: no C=C double bonds; unsaturated: one or more C=C double bonds. (2)
• Saturated: solid at room temperature (e.g. animal fats); unsaturated: liquid at room temperature (e.g. plant oils). (2)
• (Also accept: saturated chains are straight/pack tightly; unsaturated have kinks.) Why: Double bonds create kinks preventing tight packing, lowering melting point.

Q9. (3 marks)

• A phospholipid has a hydrophilic (polar) phosphate head (1) and two hydrophobic (non-polar) fatty acid tails (1).
• Because it has both a water-loving and a water-fearing region, it is described as amphipathic. (1) Why: This dual nature drives bilayer formation in membranes.

Q10. (2 marks)

• Any two functions: long-term energy storage / thermal insulation / cushioning organs / cell signalling (hormones) / component of membranes / waterproofing. (1 for two functions)
• Steroid in animal membranes: cholesterol. (1) Why: Cholesterol is the standard membrane steroid; lipids serve energy, insulation and signalling roles.

[
  {"claim":"Carbohydrate H:O ratio is 2:1","code":"H=2; O=1; result = (H/O == 2)"},
  {"claim":"A triglyceride forms with 3 fatty acids and releases 3 water molecules","code":"fatty_acids=3; ester_bonds=fatty_acids; water_released=ester_bonds; result = (water_released == 3)"},
  {"claim":"A disaccharide contains 2 monosaccharide units, one more than a monosaccharide","code":"mono=1; di=2; result = (di == mono + 1)"},
  {"claim":"A phospholipid has 2 hydrophobic tails and 1 hydrophilic head (3 key regions)","code":"tails=2; heads=1; result = (tails + heads == 3)"}
]