Disaccharides — Maltose, Sucrose, Lactose and Glycosidic Bonds

Spec mapping — OCR H420 Module 2.1.2 — Biological molecules. This lesson covers the synthesis and breakdown of disaccharides via condensation and hydrolysis of glycosidic bonds, with the three biologically important disaccharides — maltose, sucrose, lactose — as named exemplars. The mechanism connects directly to Lesson 2 (monosaccharide stereochemistry) and forward to Lessons 4–5 (polysaccharide architecture) (refer to the official OCR H420 specification document for exact wording).

When two monosaccharides join together, they form a disaccharide. The covalent bond between them is called a glycosidic bond, and its formation releases a molecule of water. This lesson develops the condensation/hydrolysis paradigm that underpins every later polymerisation step in this course — peptide bonds, ester bonds, phosphodiester bonds all follow the same condensation logic.

1. The Glycosidic Bond

A glycosidic bond is the covalent bond formed between two monosaccharide units in a condensation reaction. When the bond forms between C1 of one monosaccharide and C4 of the next, it is called a 1,4-glycosidic bond. Bonds can also form between other carbons (e.g., 1,6 in branching points of amylopectin and glycogen, 1,2 in sucrose).

Key Definition — Condensation reaction: A reaction in which two molecules are joined by the formation of a new covalent bond, with the elimination of a water molecule.

Key Definition — Hydrolysis reaction: A reaction in which a covalent bond is broken by the addition of a water molecule, splitting one molecule into two.

1.1 Condensation: Forming a Glycosidic Bond

Condensation equation: $\text{glucose} + \text{glucose} \rightarrow \text{maltose} + H_2O$

The hydroxyl on C1 of one glucose reacts with the hydroxyl on C4 of another. The two oxygens contribute one shared O (which becomes the glycosidic bridge C1–O–C4); the other –OH plus a hydrogen leave as a water molecule.

The hydroxyl group (–OH) on C1 of one glucose reacts with the hydroxyl group on C4 of another glucose. One –OH provides the oxygen that bridges the two rings; the other –OH (and an H from the first) leave as water.

1.2 Hydrolysis: Breaking a Glycosidic Bond

Hydrolysis equation: $\text{maltose} + H_2O \rightarrow \text{glucose} + \text{glucose}$

Hydrolysis is the reverse of condensation. A water molecule is added across the glycosidic bond: the –OH is added to one monosaccharide and the –H to the other. Hydrolysis reactions are catalysed by specific enzymes (e.g., maltase, sucrase, lactase).

2. The Three Most Important Disaccharides

2.1 Maltose — α-glucose + α-glucose

flowchart LR
  A["α-glucose"] -- "1,4-glycosidic bond" --> B["α-glucose"]

Formed by a 1,4-glycosidic bond between two α-glucose molecules.
Found in germinating seeds (e.g., barley), produced by the hydrolysis of starch by the enzyme amylase.
A reducing sugar — the C1 of the second glucose remains free and can open to reveal an aldehyde group.
Hydrolysed by the enzyme maltase to yield two α-glucose units.

2.2 Sucrose — α-glucose + fructose

flowchart LR
  A["α-glucose"] -- "1,2-glycosidic bond" --> B["fructose"]

Formed by a 1,2-glycosidic bond between α-glucose and fructose.
The main transport sugar in the phloem of plants — it is soluble, chemically less reactive than glucose, and osmotically less active than an equivalent mass of separate hexoses.
Extracted commercially from sugar cane and sugar beet; the familiar "table sugar".
A non-reducing sugar — both anomeric carbons (C1 of glucose and C2 of fructose) are locked up in the glycosidic bond, so neither monosaccharide can open to reveal a reducing group.
Hydrolysed by the enzyme sucrase (also called invertase) to yield glucose and fructose.

2.3 Lactose — β-galactose + α-glucose

flowchart LR
  A["β-galactose"] -- "1,4-glycosidic bond" --> B["α-glucose"]

Formed by a 1,4-glycosidic bond between β-galactose and α-glucose.
The principal sugar in mammalian milk; provides energy for young mammals.
A reducing sugar — the anomeric carbon of the glucose remains free.
Hydrolysed by the enzyme lactase in the small intestine. Adults with low lactase activity suffer from lactose intolerance, as undigested lactose is fermented by gut bacteria producing gas and osmotic discomfort.

3. Summary Table

Disaccharide	Monomers	Bond	Reducing?	Enzyme	Biological Role
Maltose	α-glucose + α-glucose	1,4-glycosidic	Yes	Maltase	Product of starch digestion; germinating seeds
Sucrose	α-glucose + fructose	1,2-glycosidic	No	Sucrase	Main transport sugar in phloem
Lactose	β-galactose + α-glucose	1,4-glycosidic	Yes	Lactase	Energy source for young mammals

4. Reducing Sugars — A Closer Look

A reducing sugar is one that can donate electrons to (reduce) another chemical. In the Benedict's test, reducing sugars reduce blue Cu²⁺ ions (copper(II) sulfate) to brick-red Cu⁺ ions (copper(I) oxide precipitate).

All monosaccharides are reducing sugars.
Many disaccharides are reducing (maltose, lactose, cellobiose) — they have at least one free anomeric carbon.
Sucrose is non-reducing because both of its anomeric carbons are involved in the glycosidic bond.

To test for a non-reducing sugar such as sucrose, the sample must first be hydrolysed by boiling with dilute hydrochloric acid, then neutralised with sodium hydrogencarbonate, before repeating the Benedict's test. If the sample contained a non-reducing sugar, it will now give a positive Benedict's result.

5. Why Cells Make Disaccharides

Disaccharides serve three main biological purposes:

Transport — sucrose is translocated in the phloem because it is soluble but chemically inert.
Storage precursor — disaccharides can be rapidly polymerised into storage polysaccharides or broken down for respiration.
Nutrition — lactose provides energy for suckling young, maltose for germinating seeds.

6. Worked Example: Identifying a Disaccharide

A student was given an unknown sugar. Benedict's test on the unknown gave a brick-red precipitate. The student then boiled the sample with an enzyme that hydrolyses 1,4-glycosidic bonds between α-glucose and α-glucose. TLC (thin-layer chromatography) showed only glucose in the hydrolysed sample. Name the disaccharide.

Answer: Maltose. Reasoning:

Positive Benedict's → reducing sugar → not sucrose.
Hydrolysed by an enzyme specific to α1,4 glucose-glucose bonds → not lactose (contains galactose).
Product is glucose only → maltose (α-glucose + α-glucose).

7. Digestion of Dietary Disaccharides

In humans and most mammals, disaccharides from the diet are hydrolysed in the small intestine by membrane-bound disaccharidase enzymes located on the brush border (microvilli) of the ileum epithelial cells. The resulting monosaccharides are then absorbed across the epithelium.

7.1 Sequence of Carbohydrate Digestion

graph TD
  A[Dietary starch] --> B["Salivary amylase<br/>mouth"]
  B --> C[Maltose and dextrins]
  C --> D["Pancreatic amylase<br/>small intestine"]
  D --> E[Maltose]
  E --> F["Maltase<br/>brush border"]
  F --> G[α-glucose]
  H[Dietary sucrose] --> I["Sucrase<br/>brush border"]
  I --> J[Glucose + fructose]
  K[Dietary lactose] --> L["Lactase<br/>brush border"]
  L --> M[Galactose + glucose]
  G --> N["Absorbed by SGLT1<br/>Na+ co-transport"]
  J --> N
  M --> N

7.2 Lactose Intolerance

Most mammals lose lactase activity after weaning. In humans, lactase persistence — continued production of lactase into adulthood — is a relatively recent adaptation that arose independently in several populations with dairy farming traditions (notably northern Europeans and some African pastoralists).

Individuals without lactase persistence cannot digest lactose. Undigested lactose passes into the large intestine where:

It draws water in by osmosis, causing diarrhoea.
Gut bacteria ferment it, producing gases (CO₂, H₂, CH₄) that cause bloating and flatulence.

Treatment includes avoiding dairy products, consuming lactase-treated milk, or taking lactase enzyme tablets.

8. Why the Body Uses Different Disaccharides

Each disaccharide has evolved for a specific biological context:

Maltose is a transient intermediate in starch digestion — it is rapidly hydrolysed to glucose and rarely accumulates.
Sucrose is ideal for long-distance transport in plant phloem because it is soluble, chemically unreactive (non-reducing), and osmotically less active per unit mass than free glucose or fructose.
Lactose provides energy for young mammals. Its relative sweetness is lower than sucrose, avoiding excessive stimulation of sweet taste receptors in infants.

Exam Tips

Always state which carbons form the glycosidic bond (1,4 or 1,2).
Use the word condensation (not "dehydration" alone) and always mention that a water molecule is released.
For hydrolysis, mention the enzyme where possible (maltase, sucrase, lactase).
When explaining reducing vs non-reducing sugars, refer to the anomeric carbon.
For sucrose, remember to state it is non-reducing AND why it is used for phloem transport.

Common Exam Mistakes

Writing "glycosidic bond" without specifying the carbons (1,4 or 1,2).
Confusing condensation (water released) with hydrolysis (water added).
Saying sucrose is reducing — it is non-reducing.
Writing "lactose is made of two galactoses" — lactose is galactose + glucose.
Forgetting to neutralise with NaHCO₃ after acid hydrolysis before repeating Benedict's test.
Saying humans "don't produce enzymes for disaccharides" — we produce maltase, sucrase and lactase (though lactase may decline after weaning).

9. The Condensation Mechanism in Detail

A glycosidic bond is formed by a nucleophilic substitution at the anomeric carbon. The C1 hydroxyl of one sugar (typically activated by protonation to form a good leaving group) is attacked by the C4 hydroxyl oxygen of the second sugar. The transition state requires a precise geometric approach — which is why enzymes (glycosyltransferases) almost always catalyse glycosidic-bond formation in vivo, rather than the bond forming spontaneously by mass action. The reverse reaction (hydrolysis) requires a water molecule to attack the bridging oxygen of the glycosidic bond, regenerating the two free –OH groups. Hydrolase enzymes (maltase, sucrase, lactase, amylase, cellulase) accelerate this step at physiological temperature without the boiling acid that non-enzymatic hydrolysis would require.

The free energy change for glycosidic bond formation is modestly endergonic (ΔG ≈ +16 kJ mol⁻¹) — which is why biological synthesis uses activated sugars (UDP-glucose, ADP-glucose) rather than free monosaccharides. UDP-glucose in particular is the universal activated donor for glucose transfer in glycogen synthesis and many glycoprotein/glycolipid biosyntheses; ADP-glucose is the equivalent in plant starch synthesis. The hydrolysis of the UDP/ADP–glucose bond is exergonic enough to drive glycosidic bond formation forward.

10. Synoptic Links

This lesson connects across the OCR H420 specification:

ocr-alevel-biology-biological-molecules Lessons 4–5 — polysaccharides. Maltose is the disaccharide repeating unit of amylose; cellobiose (β-1,4 glucose-glucose) is the equivalent repeating unit of cellulose.
ocr-alevel-biology-exchange-transport — phloem transport. Sucrose is the exemplar non-reducing transport disaccharide; companion-cell loading of sucrose drives the mass-flow hypothesis of phloem transport.
ocr-alevel-biology-nucleic-acids-enzymes — enzyme specificity. Maltase, sucrase and lactase are tightly substrate-specific; lactase deficiency (lactose intolerance) demonstrates the consequences of enzyme absence.
ocr-alevel-biology-diseases-immunity — gut microbiome. Undigested lactose fermented in the colon produces SCFAs (short-chain fatty acids) and gases; gut microbes complete the digestion that mammalian enzymes cannot.

11. Specimen Question (Modelled on the OCR H420 Paper Format)

Q (6 marks): Sucrose is the principal sugar translocated in the phloem of flowering plants. Using your knowledge of disaccharide structure, explain why sucrose is better suited to this role than glucose or fructose.

AO breakdown

AO	Marks	Earned by
AO1	2	Identifying sucrose as glucose + fructose joined by 1,2-glycosidic bond
AO2	3	Linking to non-reducing nature, solubility, osmotic activity
AO3	1	Synthesis comparison vs free monosaccharides

Mid-band response

Sucrose is a disaccharide made from glucose and fructose joined by a 1,2-glycosidic bond. It is soluble in water so it dissolves in the phloem sap. It is also non-reducing because the anomeric carbons are both used in the bond, so it doesn't react with other things in the plant. This means the sugar is transported safely. If glucose were used, it might react with other molecules. Also, sucrose has less osmotic effect per gram than the same mass of glucose, so plants can pack more sugar without too much water following by osmosis.

Examiner commentary: M1 for sucrose composition, M1 for 1,2 bond, M1 for non-reducing, M1 for osmotic argument. Around 4/6. The candidate omits the AO2 detail that both anomeric carbons being locked is what makes sucrose non-reducing, and never makes the explicit comparison with glucose's reactivity at C1.

Lesson 3: Disaccharides — Maltose, Sucrose, Lactose and Glycosidic Bonds