Carbohydrates are the sugars and the sugar polymers. Three monosaccharide types, three named disaccharides, three named polysaccharides. One isomer of glucose builds compact stores; the other builds rigid structure. Two biochemical tests identify them.
Monosaccharides are small sugar units, and one glucose isomer switches entire polymer function.
Monosaccharides are the small sugar units from which every carbohydrate is built. They contain three elements only — carbon, hydrogen, oxygen — with the general formula (CH₂O)ₙ. Three monosaccharides are named at AQA 3.1.2: glucose, fructose, and galactose. All three are hexoses; all three share the molecular formula C₆H₁₂O₆ and differ only in the arrangement of their atoms.
The same molecular formula, different positions of the –OH group on carbon 1. In α-glucose the –OH on C1 sits on the opposite side of the ring from the CH₂OH on C5. In β-glucose it sits on the same side. This single positional difference is the engineering decision behind the entire storage-versus-structure split: α-glucose builds compact stores, β-glucose builds rigid structure.
Small, soluble in water, sweet-tasting, transportable across cell membranes. Glucose is the most biologically important monosaccharide because it is the primary respiratory substrate. Three named at AQA: glucose, fructose, galactose. All hexoses with formula C₆H₁₂O₆.
Write
α-glucoseoralpha-glucose.a-glucosewith a lowercase Roman letter is rejected. The Greek symbol α or the spelled-out word are both acceptable; the Roman letter is not. The same rule applies to β-glucose.
Two monosaccharides join by condensation through a glycosidic bond.
Two monosaccharides join by a condensation reaction to form a disaccharide. As in all condensation reactions, a covalent bond forms between the two monomers and one water molecule is released. The bond formed is a glycosidic bond — the only bond name AQA credits for any carbohydrate. Hydrolysis reverses the process: a water molecule is added across the glycosidic bond, the bond breaks, and the two monosaccharides separate.
Write
glycosidic bondfor any carbohydrate bond.Peptide bond,phosphodiester bond, andester bondare all rejected here. Glycosidic is the only credited bond name in a carbohydrate context.
The three named disaccharides at AQA 3.1.2.
| Disaccharide | Monosaccharide pair | Reducing sugar? |
|---|---|---|
| Maltose | Two α-glucose | Yes |
| Sucrose | Glucose + fructose | No (non-reducing) |
| Lactose | Glucose + galactose | Yes |
Lactose is glucose + galactose, not glucose + fructose. The
glucose + fructoseanswer for lactose is an explicit reject on the 2018 mark scheme. Lactose pairs glucose with galactose; sucrose pairs glucose with fructose.
Maltose is the disaccharide; maltase is the enzyme. Writing
maltaseas a disaccharide name is an explicit reject. Maltase hydrolyses maltose into two glucose molecules; the spelling matters for the mark.
Starch and glycogen store α-glucose in compact, mobilisable forms.
Starch (in plants) and glycogen (in animals) are both polymers of α-glucose joined by glycosidic bonds. Both serve as energy stores. The storage properties — insoluble, compact, non-diffusible — emerge from the α-glucose chemistry and the coiled or branched architecture each polymer takes.
Starch, glycogen, and cellulose compared.
| Polysaccharide | Components | Branching |
|---|---|---|
| Starch | Amylose (unbranched, 1,4-bonds, coiled) + amylopectin (1,4- and 1,6-bonds) | Branched in amylopectin only |
| Glycogen | Single branched polymer of α-glucose (no subcomponents) | More highly branched than amylopectin |
| Cellulose | Single straight polymer of β-glucose | Unbranched |
Glycogen has no amylose or amylopectin components. Both names are starch-only. Glycogen is described as a
highly branched α-glucose polymer— it has no subcomponent names of its own.
Three credited reasons. Insoluble, so storage does not affect cell water potential. Compact, so many glucose units pack into a small volume (coiled in amylose, branched in amylopectin and glycogen). Non-diffusible, so the polymer cannot leak through the cell membrane. Branching adds a fourth functional advantage: many free chain ends allow many hydrolytic enzymes to act at once, releasing glucose rapidly when demand spikes.
Energy release ends with respiration, not hydrolysis. Hydrolysis releases glucose; glucose is then used in respiration.
Hydrolysis releases energyis a zero-mark drop. The two-step structure is required: hydrolysis releases glucose, respiration releases energy from glucose.
Cellulose's strength comes from β-glucose, parallel chains, and hydrogen bonds.
Cellulose is the structural polysaccharide of the plant cell wall, built from β-glucose. The β-glucose chemistry and the parallel-chain architecture together produce a fibre with high tensile strength, strong enough to resist the osmotic pressure of water entering a plant cell.
- Alternate β-glucose monomers are rotated 180° relative to each other so successive units can form a bond.
- The monomers are joined by 1,4-glycosidic bonds only; no 1,6-bonds and no branching occur.
- The result is a straight, linear, unbranched chain — not a coil and not a tree.
1,6-glycosidic bonds belong to amylopectin and glycogen only. Applying
1,6 bondsorbranchedto cellulose, or to any novel β-glucose polymer in a question stem, is an explicit reject. β-glucose polymers run straight.
Many parallel cellulose chains run alongside one another and form hydrogen bonds between hydroxyl groups on adjacent chains. Each hydrogen bond is individually weak, but the very large number of them along the chains gives the assembled microfibrils very high tensile strength — the property that lets plant cell walls resist the osmotic pressure of water entering the cell.
Pitfall — Only credit what the figure shows
Only credit what the figure shows.
When AQA introduces a novel β-glucose polymer with a figure, write the three locked mark points from the chain above: alternate monomers rotated 180°, 1,4-glycosidic bonds, straight unbranched chains. Do not add
hydrogen bonds between chainsas a fourth point unless the figure depicts inter-chain bonds. AQA credits only what the figure shows; volunteered structure-function content does not score.
Two biochemical tests identify reducing sugars, non-reducing sugars, and starch.
AQA examines three test procedures at 3.1.2. Benedict's reagent detects reducing sugars; an extended Benedict's sequence detects non-reducing sugars; iodine in potassium iodide solution detects starch. Mark schemes credit method, result, and reason as three independent points for every test.
- Heat the sample with Benedict's reagent in a water bath. (Method — heating is required, not optional.)
- A brick-red, red, or orange precipitate forms; the solution changes from blue to brick-red. (Result.)
- The named sugar is a reducing sugar — it reduces Cu²⁺ to Cu⁺, which precipitates as copper(I) oxide. (Reason.)
Heat is part of the method.
Water bathalone, without explicit heating, does not score. The heating step is the mark point; the water bath is the apparatus.
- Perform a direct Benedict's test on the sample. A negative (blue) result confirms no reducing sugar is present.
- Take a fresh aliquot. Add dilute hydrochloric acid and heat to acid-hydrolyse the glycosidic bonds in the disaccharide.
- Neutralise the hydrolysed sample with sodium hydrogencarbonate, because Benedict's reagent requires alkaline conditions.
- Add Benedict's reagent to the neutralised sample and heat in a water bath.
- A brick-red precipitate at this step confirms a non-reducing sugar was present in the original sample.
The two routinely dropped steps are the initial negative Benedict's and the acid hydrolysis. On 2019 P1 Q10.2 the examiner reported
hardly anycandidates included the acid-hydrolysis step. Both steps are required for the full sequence mark.
Add iodine in potassium iodide solution (orange-brown reagent) to the sample. If starch is present, the iodine molecules lodge in the helical coils of amylose and produce a blue-black colour. The test is specific to starch — monosaccharides, disaccharides, glycogen, and cellulose do not produce the blue-black change at A-level.
Key terms
- glucose
- α-glucose
- β-glucose
- monosaccharide
- polysaccharide
- polymer
- glycosidic bond
- condensation reaction
- hydrolysis
- starch
- branched
- reducing sugar