Level 3 — ProductionBiomolecules — Carbohydrates & Lipids

Biomolecules — Carbohydrates & Lipids

⏱ 45 minutes50 marksprintable — key stays hidden on paper

Level 3 — Production (from-scratch derivations, explain-out-loud reasoning) Time limit: 45 minutes Total marks: 50

Instructions: Answer all questions. Where a process is asked, reconstruct it fully from memory — do not merely name it. Diagrams may be drawn where helpful.

Question 1 — Dehydration synthesis & hydrolysis (8 marks)

(a) From scratch, explain the mechanism of dehydration synthesis (condensation) that joins two monomers into a dimer. State exactly what atoms/groups are removed and what molecule is released. (4)

(b) Two glucose molecules ( $C_6H_{12}O_6$ each) undergo dehydration synthesis to form maltose. Derive the molecular formula of the resulting disaccharide, showing your atom bookkeeping. (2)

(c) Explain why hydrolysis is described as the chemical reverse of dehydration synthesis, referencing the water molecule. (2)

Question 2 — Glycosidic bond & monosaccharides (9 marks)

(a) Name three common monosaccharides and state one biological source or role of each. (3)

(b) Describe, step by step, how a glycosidic bond forms between two monosaccharides. Identify the carbon atoms typically involved (e.g. in maltose vs sucrose) and the bond type. (4)

(c) Glucose and fructose share the same molecular formula $C_6H_{12}O_6$ yet differ in properties. Explain what this relationship is called and why it matters biologically. (2)

Question 3 — Polysaccharide comparison (10 marks)

Construct a comparison of starch, glycogen, and cellulose. For each, state: (a) the monomer and glycosidic linkage type ( $\alpha$ or $\beta$ , 1→4 / 1→6), (3) (b) branching pattern, (3) (c) biological function and organism(s) where found, (3) (d) Explain from structure why humans can digest starch but not cellulose. (1)

Question 4 — Triglycerides & fatty acids (9 marks)

(a) Draw or describe the full structure of a triglyceride, naming its two component parts and the bond that links them. State how many water molecules are released when one triglyceride forms. (4)

(b) Distinguish saturated from unsaturated fatty acids in terms of bonding, molecular shape (straight vs kinked), and physical state at room temperature. (3)

Question 5 — Phospholipids & membranes (8 marks)

(a) From memory, describe the structure of a phospholipid and explain precisely why it is amphipathic. (4)

(b) Predict and explain what arrangement phospholipids spontaneously adopt when placed in water, and link this to the structure of the cell membrane. (4)

Question 6 — Steroids, waxes & lipid functions (6 marks)

(a) Describe the general structural feature that defines a steroid, and give cholesterol as an example with one function. (3)

(b) Explain the structure of a wax and describe two biological roles, one in plants and one in animals. (3)

Answer keyMark scheme & solutions

Question 1 (8 marks)

(a) Dehydration synthesis: the –OH group is removed from one monomer and an –H is removed from the –OH of the second monomer (1). These combine to release one molecule of water, $H_2O$ (1). A covalent bond then forms directly between the two monomers at those positions (1). The process is endergonic/requires enzymes and energy input (1).

(b) Bookkeeping:

Total atoms of 2 glucose: $C_{12}H_{24}O_{12}$ (1)
Remove one $H_2O$ : $C_{12}H_{24}O_{12} - H_2O = C_{12}H_{22}O_{11}$ = maltose (1)

(c) Hydrolysis adds a water molecule across the bond: the –OH and –H of water are inserted back, splitting the dimer into two monomers (1). Since dehydration removes water to make the bond and hydrolysis adds water to break it, they are exact reverses (1).

Question 2 (9 marks)

(a) Any three, 1 mark each: Glucose — main respiratory fuel / blood sugar; Fructose — fruit sugar / found in honey; Galactose — component of lactose in milk. (3)

(b) Two monosaccharides align (1); an –OH on carbon of one sugar reacts with an –OH on a carbon of the other (1); a water molecule is removed (dehydration) (1); a glycosidic bond forms — e.g. an α-1,4 bond in maltose (C1–C4), or an α-1,β-2 bond in sucrose (1).

(c) They are isomers (structural isomers) — same molecular formula, different arrangement of atoms (1). This matters because different structures give different properties/functions (e.g. sweetness, metabolic handling) despite identical formula (1).

Question 3 (10 marks)

	Starch	Glycogen	Cellulose
Monomer/linkage (a)	α-glucose, α-1,4 (+α-1,6)	α-glucose, α-1,4 & α-1,6	β-glucose, β-1,4
Branching (b)	little (amylose none, amylopectin some)	highly branched	unbranched, straight
Function/organism (c)	energy storage in plants	energy storage in animals (liver/muscle)	structural in plant cell walls

(a) 1 mark each correct linkage. (3)
(b) 1 mark each correct branching. (3)
(c) 1 mark each correct function+organism. (3)

(d) Human enzymes (amylase) hydrolyse α-1,4 bonds in starch but cannot break the β-1,4 bonds of cellulose, so cellulose passes through as fibre. (1)

Question 4 (9 marks)

(a) A triglyceride = 1 glycerol (3-carbon alcohol with 3 –OH groups) (1) + 3 fatty acids (long hydrocarbon chains ending in –COOH) (1), joined by ester bonds (1). Forming the three ester bonds releases 3 water molecules (1).

(b) Saturated: only single C–C bonds, no double bonds, straight chains that pack tightly → solid at room temp (fats) (1.5). Unsaturated: one or more C=C double bonds causing kinks, cannot pack tightly → liquid at room temp (oils) (1.5). (3)

(c) Lipids have long hydrocarbon chains that are highly reduced (many C–H bonds, few oxygens) (1); oxidising these C–H bonds releases more energy per gram (~38 kJ/g vs ~17 kJ/g for carbs) than the more oxidised carbohydrates (1). (2)

Question 5 (8 marks)

(a) A phospholipid = glycerol + 2 fatty acid tails + a phosphate-containing head group (1). The phosphate head is polar/charged → hydrophilic (1); the fatty acid tails are non-polar → hydrophobic (1). Having both a water-loving and water-hating region in one molecule makes it amphipathic (1).

(b) In water, phospholipids self-assemble into a bilayer (or micelle), with hydrophilic heads facing the water on both sides and hydrophobic tails pointing inward away from water (2). This bilayer is the fundamental structure of the cell membrane, giving a stable, selectively permeable barrier (2). (4)

Question 6 (6 marks)

(a) Steroids have a characteristic four fused carbon ring structure (three 6-membered + one 5-membered) (1). Cholesterol is an example (1); it stabilises/fluidises the cell membrane (or is a precursor of steroid hormones) (1). (3)

(b) A wax = a long-chain fatty acid esterified to a long-chain alcohol, forming a highly hydrophobic, non-polar molecule (1). Plant role: waxy cuticle on leaves reduces water loss (1). Animal role: waterproofing feathers/fur or earwax protection (1). (3)

[
  {"claim":"Maltose formula from 2 glucose minus water is C12H22O11","code":"glc=(6,12,6)\nC=2*glc[0]\nH=2*glc[1]-2\nO=2*glc[2]-1\nresult=(C,H,O)==(12,22,11)"},
  {"claim":"Triglyceride formation releases 3 water molecules","code":"fatty_acids=3\nwater_released=fatty_acids\nresult=water_released==3"},
  {"claim":"Lipid energy density (~38 kJ/g) exceeds carbohydrate (~17 kJ/g)","code":"lipid=38\ncarb=17\nresult=lipid>carb"},
  {"claim":"Two glucose combined atoms before dehydration total C12H24O12","code":"result=(2*6,2*12,2*6)==(12,24,12)"}
]