SPEC 3.1.2 - BIOLOGICAL MOLECULES
Carbohydrates
Monosaccharides, disaccharides, polysaccharides, α and β glucose — and the biochemical tests that identify them
Monosaccharides — The Monomers
AQA SPEC (3.1.2)
Monosaccharides are the monomers from which larger carbohydrates are made. Glucose, galactose and fructose are common monosaccharides.
All three common monosaccharides share the same molecular formula: C₆H₁₂O₆ — they are structural isomers (same atoms, different arrangement).
MonosaccharideRoleWhere found
α-GlucosePrimary respiratory substrate; monomer of starch and glycogenBlood, plant sap, all living cells
β-GlucoseMonomer of celluloseProduced in plants; used to build cellulose in cell walls
GalactoseMonomer of lactose (with glucose)Milk, dairy products
FructoseMonomer of sucrose (with glucose)Fruit, honey
Synoptic Link — 1.01
Monosaccharides are the monomers of carbohydrates — recall from Lesson 1.01 that they are joined by condensation reactions forming glycosidic bonds, and separated by hydrolysis.
α-Glucose and β-Glucose
Glucose exists as two structural isomers that differ only in the position of the –OH group on carbon-1:
α-GLUCOSE
The –OH group on carbon-1 points downward (below the ring plane).
→ Chains coil into a helix → starch and glycogen
β-GLUCOSE
The –OH group on carbon-1 points upward (above the ring plane).
→ Chains remain straight → cellulose
Examiner Alert
The difference between α and β glucose is only the orientation of the –OH on carbon-1. Both have the formula C₆H₁₂O₆. This tiny difference determines whether the polymer is helical (starch) or straight (cellulose) — a massive functional consequence.
Disaccharides
AQA SPEC (3.1.2)
Disaccharides are formed by the condensation of two monosaccharides, forming a glycosidic bond and eliminating one water molecule.
DisaccharideMonomersBondReducing sugar?
MaltoseGlucose + GlucoseGlycosidicYes ✓
SucroseGlucose + FructoseGlycosidicNo ✗
LactoseGlucose + GalactoseGlycosidicYes ✓
Examiner Alert — Sucrose is NOT a reducing sugar
Sucrose is the only common disaccharide that is a non-reducing sugar. This is because the glycosidic bond between glucose and fructose involves both anomeric carbon-1 positions — there is no free reducing group. This is why sucrose requires a two-stage test (hydrolysis first).
Memory Aid
Maltose = Malt (barley brewing) — two glucose. Sucrose = Sugar (table sugar) — glucose + fructose. Lactose = Lac (Latin for milk) — glucose + galactose.
Polysaccharides — Starch
Starch is the energy storage molecule in plants. It is formed from α-glucose by condensation reactions.
Amylose
Unbranched chain of α-glucose
Joined by 1,4-glycosidic bonds
Coils into a helix due to α-glucose geometry
~20–30% of starch
Amylopectin
Branched chain of α-glucose
1,4-glycosidic bonds along chain; 1,6-glycosidic bonds at branch points
~70–80% of starch
Why is starch suited to energy storage in plants?
Insoluble — no osmotic effect on cells (does not lower water potential)
Compact — helix coiling and branching allows dense packing
Easily hydrolysed — many free terminal ends (especially amylopectin) for rapid enzyme access
Does not diffuse out of cells
Polysaccharides — Glycogen
Glycogen is the energy storage molecule in animals and fungi. Like starch, it is formed from α-glucose, but it is more highly branched.
Structure
α-glucose monomers joined by 1,4-glycosidic bonds
1,6-glycosidic bonds at frequent branch points — more branched than amylopectin
Very compact structure; stored as granules in liver and muscle cells
Why is glycogen more branched than starch?
More branch points = more free terminal ends = more sites for simultaneous enzymatic hydrolysis. This allows rapid glucose release when muscles are working or blood glucose falls — a critical adaptation for animal metabolism.
Starch vs Glycogen — Key Difference
Both are α-glucose polymers. Glycogen is more highly branched than amylopectin. This makes glycogen better suited to animals, which require faster glucose mobilisation than plants.
Polysaccharides — Cellulose
Cellulose is the main structural component of plant cell walls. It is formed from β-glucose.
1
β-glucose monomers are joined by 1,4-glycosidic bonds via condensation reactions
2
Because the –OH on C1 of β-glucose points upward, each alternate monomer is rotated 180° — this produces straight, unbranched chains
3
Straight chains run parallel and form hydrogen bonds between adjacent chains
4
Many chains together form microfibrils — bundles with very high tensile strength, ideal for structural support
Why can't humans digest cellulose?
Humans lack cellulase — the enzyme with an active site complementary to β-1,4-glycosidic bonds. Our amylase only hydrolyses α-1,4-glycosidic bonds. The β-glucose geometry produces a bond of different shape that amylase cannot catalyse the hydrolysis of.
Polysaccharide Comparison
FeatureStarchGlycogenCellulose
Monomerα-glucoseα-glucoseβ-glucose
Bond typeα-1,4 (amylose); α-1,4 and α-1,6 (amylopectin)α-1,4 and α-1,6β-1,4-glycosidic
BranchingUnbranched (amylose); branched (amylopectin)Highly branchedUnbranched
Chain shapeHelix (amylose); branched (amylopectin)Highly branched, compactStraight, parallel, unbranched
H-bonds between chains?NoNoYes → microfibrils
FunctionEnergy storage (plants)Energy storage (animals/fungi)Structural (plant cell walls)
SolubilityInsolubleInsolubleInsoluble
Common Error
Students often say starch/glycogen are "insoluble because they are large." The correct explanation is: insolubility is a property of the polymer structure (glycosidic bonds, compact form) — and insolubility means they exert no osmotic effect on cells. Always link structure to function.
Benedict's Test — Reducing Sugars
REDUCING SUGAR
A reducing sugar is one that can donate electrons to (reduce) another chemical. All monosaccharides and some disaccharides (maltose, lactose) are reducing sugars. Sucrose is not a reducing sugar.
Benedict's Test — Reducing Sugars (Required Practical)
1
Add equal volumes of sample solution and Benedict's reagent (blue, alkaline copper(II) sulfate) to a test tube
2
Heat in a water bath at 80°C for 5 minutes (or boil)
3
Observe colour change: blue → green → yellow → orange → brick-red precipitate
Result Interpretation
Brick-red precipitate = positive (reducing sugar present). The Cu²⁺ ions in Benedict's are reduced to Cu⁺ (copper(I) oxide — the red precipitate) by the reducing sugar. No colour change (remains blue) = negative.
Testing for Non-Reducing Sugars (Sucrose)
Sucrose gives a negative Benedict's test (remains blue). To test for a non-reducing sugar, you must first hydrolyse it:
Non-Reducing Sugar Test — Two Stages
1
Stage 1: Test the original sample with Benedict's — it should remain blue (confirms no reducing sugar present)
2
Add a small volume of dilute hydrochloric acid (HCl) to the original sample and heat in a boiling water bath for 5 minutes — this hydrolyses the glycosidic bond, releasing the monosaccharides
3
Neutralise the acid by adding sodium hydrogencarbonate solution (NaHCO₃) — this is essential; acid destroys Benedict's reagent
4
Stage 2: Add Benedict's reagent and heat — if brick-red precipitate now forms, a non-reducing sugar was present
Examiner Alert — Neutralisation Step
You must state both the hydrolysis AND the neutralisation steps for full marks. Many students omit neutralisation. Also: you must state both results — negative then positive — to demonstrate you understand the full procedure.
Iodine Test for Starch
Iodine / Potassium Iodide Test
1
Add a few drops of iodine solution (iodine dissolved in potassium iodide solution) to the sample
2
Observe colour: yellow-brown → blue-black = positive (starch present)
3
Remains yellow-brown = negative (no starch)
Why does starch turn blue-black?
Iodine (I₃⁻ ions) becomes trapped inside the helical coils of amylose, forming a charge-transfer complex that absorbs light and appears blue-black. This only works with the helical structure — β-glucose polymers such as cellulose do not give a blue-black result because their straight chains form no helix, so I₃⁻ cannot be trapped.
Synoptic Link — α vs β glucose
The iodine test gives blue-black with starch (amylose) only. Glycogen gives a reddish-brown colour (not blue-black) because its highly branched structure means chain segments are too short to form a stable helix for trapping I₃⁻. Cellulose gives no colour change (remains yellow-brown) — straight chains form no helix at all. This is a direct structural–functional link.
Biochemical Tests — Summary Table
Substance tested forReagentConditionsPositive resultNegative result
Reducing sugar (glucose, maltose, lactose)Benedict's reagentHeat in water bath ~80°CBrick-red precipitateRemains blue
Non-reducing sugar (sucrose)1. Dilute HCl + heat; 2. NaHCO₃ to neutralise; 3. Benedict's + heatBoiling water bath (both stages)Stage 1: blue (negative); Stage 2: brick-red precipitateRemains blue after Stage 2
StarchIodine/potassium iodide solutionRoom temperature (no heating needed)Blue-black colourRemains yellow-brown
Which sugars are reducing sugars?
All monosaccharides (glucose, galactose, fructose) are reducing sugars. Of the common disaccharides: maltose and lactose are reducing sugars; sucrose is NOT. All polysaccharides (starch, glycogen, cellulose) are non-reducing.
DEEP Framework: Extended Answer
Question: "Explain why starch is well suited to its role as an energy storage molecule in plants." [4 marks]
D
Define: Starch is a polysaccharide made from α-glucose monomers joined by glycosidic bonds via condensation reactions. It exists as amylose (unbranched helix) and amylopectin (branched).
E
Explain mechanism: Starch is insoluble — it does not dissolve in cell sap and therefore exerts no osmotic effect on the cell (does not lower water potential). Its compact helical/branched structure allows large amounts of glucose to be stored in a small space. It does not diffuse out of cells.
E
Evidence/Example: Amylopectin's many free terminal ends provide multiple sites for simultaneous enzymatic hydrolysis by amylase — allowing rapid glucose release when respiration demands increase. Stored in starch grains in chloroplasts and amyloplasts.
P
Practical/Quantitative: Presence of starch is confirmed by a positive iodine test (yellow-brown → blue-black), due to iodine ions entering the helical coil of amylose. The disappearance of blue-black colour (iodine clock experiment) demonstrates starch being hydrolysed by amylase over time.
Practice Question
(a) A student tests an unknown solution with Benedict's reagent and observes no colour change. The student then adds dilute hydrochloric acid and heats, neutralises the solution, then tests with Benedict's again. This time a brick-red precipitate forms.
Identify the type of sugar present and explain each stage of the procedure. [4 marks]
(b) Explain why the iodine test gives a blue-black colour with starch but not with cellulose, even though both are glucose polymers. [3 marks]
D:
(a) The sugar present is a non-reducing sugar, most likely sucrose (1).
E:
Stage 1 Benedict's negative confirms no reducing sugar (1). HCl + heat hydrolyses the glycosidic bond, releasing monosaccharides (glucose and fructose) (1). Neutralisation with NaHCO₃ prevents acid destroying Benedict's reagent (1). Stage 2 brick-red precipitate confirms reducing sugars (monosaccharides) are now present (1). [max 4]
E:
(b) Starch (amylose) is made from α-glucose, which forms a helical coil (1). Iodine molecules (I₃⁻) become trapped inside the helix, forming a charge-transfer complex that appears blue-black (1). Cellulose is made from β-glucose and forms straight unbranched chains — no helix is formed, so iodine cannot be trapped and no blue-black colour develops (1).
P:
Quantitative note: Benedict's test is semi-quantitative — greater concentration of reducing sugar gives a more intense brick-red precipitate (assessed using a colorimeter with a calibration curve).
Common Exam Errors
Alert 1 — Non-reducing sugar test
Describing the non-reducing sugar test without both stages loses marks. You must state: Stage 1 negative, then hydrolysis with HCl, then neutralisation with NaHCO₃, then Stage 2 positive. Omitting neutralisation is the most common error.
Alert 2 — Starch insolubility explanation
Never write "starch is a good storage molecule because it is large." Write: "starch is insoluble so it has no osmotic effect on the cell." Large molecules that are soluble (like glucose) do lower water potential — it is insolubility specifically that matters.
Alert 3 — Glycogen vs starch branching
Glycogen is more highly branched than starch (amylopectin). Do not say glycogen is "the same as starch." The higher branching of glycogen allows faster glucose release — this functional link must be stated in exam answers.
Alert 4 — "Sucrose is a polysaccharide"
Sucrose is a disaccharide, not a polysaccharide. It is made of only two monosaccharides. Students frequently confuse this in extended answers about food and digestion.
Summary
Key Points
TopicKey Fact
MonosaccharidesGlucose, galactose, fructose — all C₆H₁₂O₆; structural isomers
α vs β glucoseα: –OH on C1 points down → helix (starch/glycogen); β: –OH on C1 points up → straight chains (cellulose)
DisaccharidesMaltose (glu+glu), Sucrose (glu+fru) — non-reducing, Lactose (glu+gal)
Starchα-glucose; amylose (unbranched helix) + amylopectin (branched); insoluble; energy storage in plants
Glycogenα-glucose; highly branched; energy storage in animals/fungi; rapid hydrolysis
Celluloseβ-glucose; straight chains; H-bonds → microfibrils; structural in plant cell walls
Benedict's (reducing)Brick-red precipitate with glucose, maltose, lactose
Non-reducing sugar testHCl hydrolysis + NaHCO₃ neutralisation + Benedict's → brick-red
Iodine testBlue-black with starch (amylose helix traps I₃⁻); glycogen → reddish-brown; cellulose → yellow-brown (no colour change)
Exam Tip
For any question linking structure to function: (1) name the monomer and bond type, (2) describe the resulting shape (helix/straight/branched), (3) link that shape to a specific functional property (insoluble/compact/tensile strength/rapid hydrolysis). Every mark requires a structural–functional link.
Next: Monosaccharides — The Monomers