Carbohydrates: Monosaccharides and Disaccharides

This lesson covers the structure, properties and biological importance of monosaccharides and disaccharides as required by the Edexcel A-Level Biology B specification (9BI0), Topic 1: Biological Molecules. You need to understand the general formula of carbohydrates, the structures of key sugars, condensation and hydrolysis reactions, and the tests used to identify reducing and non-reducing sugars.

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What Are Carbohydrates?

Carbohydrates are organic molecules containing the elements carbon (C), hydrogen (H) and oxygen (O). They have the general formula:

(CH₂O)ₙ

where n is the number of carbon atoms. For example, glucose has the formula C₆H₁₂O₆ (n = 6).

Carbohydrates are classified by size:

Category	Number of Sugar Units	Examples
Monosaccharide	1	Glucose, fructose, galactose, ribose, deoxyribose
Disaccharide	2	Maltose, sucrose, lactose
Polysaccharide	Many (hundreds to thousands)	Starch, glycogen, cellulose

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Monosaccharides

Monosaccharides are the simplest carbohydrates — they are single sugar units that cannot be hydrolysed into smaller carbohydrates. They are the monomers from which larger carbohydrates are built.

Classification by Number of Carbon Atoms

Number of Carbons	Name	Examples
3	Triose	Glyceraldehyde (G3P / GALP — an intermediate in respiration and photosynthesis)
5	Pentose	Ribose (in RNA and ATP), deoxyribose (in DNA)
6	Hexose	Glucose, fructose, galactose

Properties of Monosaccharides

All monosaccharides are:

• Soluble in water — because they have many polar –OH groups that form hydrogen bonds with water.
• Sweet-tasting — they stimulate taste receptors on the tongue.
• Reducing sugars — they can donate electrons to (reduce) another molecule, such as Benedict's reagent.

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Glucose — The Most Important Hexose

Glucose (C₆H₁₂O₆) is the most biologically important monosaccharide. It is the primary respiratory substrate — the molecule that is broken down during cellular respiration to release energy for ATP synthesis.

Alpha and Beta Glucose

Glucose exists in a ring form in aqueous solution. There are two structural isomers depending on the position of the –OH group on carbon 1:

• α-glucose (alpha-glucose): The –OH group on carbon 1 is below the plane of the ring (same side as the –OH on carbon 4 in a Haworth projection).
• β-glucose (beta-glucose): The –OH group on carbon 1 is above the plane of the ring (opposite side to the –OH on carbon 4).

Feature	α-glucose	β-glucose
–OH on C1	Below the ring	Above the ring
Polymerises to form	Starch (amylose, amylopectin) and glycogen	Cellulose
Biological role of polymer	Energy storage	Structural support

Exam Tip: You must be able to draw the ring structures of α-glucose and β-glucose and clearly show the difference at carbon 1. Label the carbon atoms and the position of the –OH group. This is a frequently examined skill in the 9BI0 specification.

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Other Important Monosaccharides

Fructose

• Molecular formula: C₆H₁₂O₆ (same as glucose — they are structural isomers).
• Found in fruit and honey.
• Forms a five-membered ring (furanose ring) rather than the six-membered ring of glucose.
• Combined with glucose to form sucrose (table sugar).

Galactose

• Molecular formula: C₆H₁₂O₆.
• An epimer of glucose — differs only in the arrangement of the –OH group at carbon 4.
• Combined with glucose to form lactose (milk sugar).

Ribose and Deoxyribose (Pentoses)

• Ribose (C₅H₁₀O₅): a component of RNA, ATP, NAD and FAD.
• Deoxyribose (C₅H₁₀O₄): a component of DNA. It has one fewer oxygen atom than ribose (at carbon 2, the –OH is replaced by –H).

Key Definition: Structural isomers are molecules with the same molecular formula but different structural arrangements of atoms. Glucose, fructose and galactose are all C₆H₁₂O₆ but have different structures and properties.

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Condensation Reactions and Glycosidic Bonds

Two monosaccharides can join together in a condensation reaction to form a disaccharide. During this reaction:

1. An –OH group from one monosaccharide reacts with an –OH group from another.
2. A molecule of water (H₂O) is released.
3. A glycosidic bond is formed between the two sugar residues.

The type of glycosidic bond is named according to the carbon atoms involved:

• 1,4-glycosidic bond: Links carbon 1 of one sugar to carbon 4 of another (e.g. in maltose).
• 1,2-glycosidic bond: Links carbon 1 of one sugar to carbon 2 of another (e.g. in sucrose).
• 1,6-glycosidic bond: Creates branch points in amylopectin and glycogen.

The Condensation Reaction (General)

Monosaccharide A–OH + HO–Monosaccharide B → Monosaccharide A–O–Monosaccharide B + H₂O

Exam Tip: Always use the term glycosidic bond (not just "bond") when describing the link between sugar molecules. Specify the type (e.g. α-1,4 or β-1,4) whenever possible — this demonstrates precise knowledge that gains marks.

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Hydrolysis of Disaccharides

The reverse of condensation is hydrolysis. A water molecule is added across the glycosidic bond, breaking it and releasing the two monosaccharide units.

Disaccharide + H₂O → Monosaccharide A + Monosaccharide B

Hydrolysis occurs:

• During digestion — enzymes in the gut hydrolyse disaccharides into monosaccharides for absorption.
• In the laboratory — by boiling with dilute acid (e.g. dilute HCl).

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The Three Key Disaccharides

Disaccharide	Monosaccharide Components	Glycosidic Bond	Where Found
Maltose	Glucose + Glucose	α-1,4	Produced during starch digestion (by amylase); germinating seeds
Sucrose	Glucose + Fructose	α-1,2	Table sugar; transported in phloem sap in plants
Lactose	Glucose + Galactose	β-1,4	Milk — the main sugar in mammalian milk

Maltose

• Formed from two α-glucose molecules joined by an α-1,4-glycosidic bond.
• It is a reducing sugar (free anomeric carbon on the second glucose can open and reduce Benedict's reagent).
• Hydrolysed by the enzyme maltase in the small intestine.

Sucrose

• Formed from α-glucose and fructose joined by an α-1,2-glycosidic bond.
• It is a non-reducing sugar — both anomeric carbons are involved in the bond, so neither can open to act as a reducing agent.
• Hydrolysed by the enzyme sucrase (also called invertase).
• The main transport sugar in plants (transported in phloem).

Lactose

• Formed from β-galactose and glucose joined by a β-1,4-glycosidic bond.
• It is a reducing sugar.
• Hydrolysed by the enzyme lactase in the small intestine.
• People with lactose intolerance produce insufficient lactase, so undigested lactose passes to the large intestine where bacteria ferment it, producing gas and causing symptoms.

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Testing for Sugars — Required Practical Knowledge

Benedict's Test for Reducing Sugars

All monosaccharides and some disaccharides (maltose, lactose) are reducing sugars. They can be detected using Benedict's reagent (an alkaline solution of copper(II) sulfate).

Method:

1. Add the food sample to a test tube with an equal volume of Benedict's reagent.
2. Heat in a water bath at approximately 80 °C for 5 minutes.
3. Observe the colour change.

Results:

Colour	Sugar Concentration
Blue (no change)	No reducing sugar present
Green	Low concentration
Yellow	Moderate concentration
Orange	High concentration
Brick-red (precipitate)	Very high concentration

The colour change occurs because the reducing sugar reduces the Cu²⁺ ions (blue) to Cu⁺ ions, which form copper(I) oxide (Cu₂O), an insoluble brick-red precipitate.

Exam Tip: Benedict's test is semi-quantitative — the colour gives a rough indication of concentration. For a truly quantitative result, you can filter and weigh the precipitate, or use a colorimeter on the remaining solution. The more precipitate formed, the more reducing sugar was present.

Testing for Non-Reducing Sugars

Sucrose does not give a positive Benedict's test directly. To test for non-reducing sugars:

1. First, perform a Benedict's test to confirm the result is negative (stays blue).
2. Add dilute hydrochloric acid to a fresh sample and boil to hydrolyse the glycosidic bond.
3. Neutralise the acid by adding sodium hydrogencarbonate (NaHCO₃) until it stops fizzing.
4. Perform the Benedict's test again.
5. A positive result (colour change from blue to green/yellow/orange/red) indicates a non-reducing sugar was present — it has been hydrolysed into reducing sugars.

Exam Tip: You must neutralise the acid before adding Benedict's reagent because Benedict's reagent is alkaline and the acid would neutralise it, preventing the reaction. Always mention the neutralisation step in your method — this is a common mark that students miss.

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Biological Importance of Monosaccharides and Disaccharides

Sugar	Biological Role
Glucose	Primary respiratory substrate; transported in blood (blood glucose ≈ 4–8 mmol dm⁻³); monomer for starch, glycogen and cellulose
Fructose	Sweetener in fruits to attract animals for seed dispersal; component of sucrose
Galactose	Component of lactose (energy source for infant mammals)
Ribose	Component of RNA, ATP, NAD⁺ and FAD
Deoxyribose	Component of DNA
Sucrose	Transport sugar in plant phloem (highly soluble, chemically unreactive)
Lactose	Energy source in mammalian milk for newborns
Maltose	Intermediate product of starch digestion

Exam Tip: Sucrose is the transport sugar in plants (not glucose) because it is a non-reducing sugar — it is less chemically reactive, so less likely to interfere with other metabolic reactions during transport. This is a commonly tested explanation.

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Summary

• Carbohydrates contain C, H and O with the general formula (CH₂O)ₙ.
• Monosaccharides are single sugar units classified by the number of carbon atoms (trioses, pentoses, hexoses).
• α-glucose and β-glucose differ in the position of the –OH group on carbon 1; this small difference determines whether the polysaccharide formed is used for storage or structure.
• Two monosaccharides join by a condensation reaction to form a disaccharide linked by a glycosidic bond, with the release of water.
• Hydrolysis is the reverse — water breaks the glycosidic bond.
• Reducing sugars are detected by Benedict's test (blue to brick-red precipitate).
• Non-reducing sugars must first be hydrolysed with dilute acid before testing.

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A-Level Deep Dive: Monosaccharides and Disaccharides

Spec mapping

This lesson sits in Edexcel 9BI0 Topic 1 — Biological Molecules, specifically the sub-strand on carbohydrate structure and properties. The relevant content statements paraphrase to: describe the general formula of carbohydrates and classify them as mono-, di- and polysaccharides; describe the ring structures of α-glucose and β-glucose, and recognise fructose, galactose, ribose and deoxyribose; explain condensation of monosaccharides into disaccharides with release of water and formation of a glycosidic bond; explain hydrolysis as the reverse; and describe the Benedict's test for reducing and non-reducing sugars (refer to the official Pearson Edexcel 9BI0 specification for exact wording). The material is examined directly on Paper 1 and reactivated synoptically on Paper 2 (sucrose transport in phloem, Topic 7) and Paper 3 (required practical method evaluation, including semi-quantitative Benedict's and colorimetric calibration).

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Worked example with full mark scheme

Question (8 marks): Lactose is a disaccharide found in mammalian milk.

(a) Draw an annotated diagram or write a labelled equation to show the formation of lactose from its monosaccharide components, including the type of glycosidic bond formed. (3)

(b) Explain why a sample of pure sucrose gives a negative result with Benedict's reagent, but a positive result if the sample is first boiled with dilute hydrochloric acid and then neutralised. (5)

Solution with mark scheme:

(a) Step 1 — name the monomers correctly. Lactose is formed from one molecule of β-galactose and one molecule of glucose.

M1 (AO1) — naming both monomers. Many candidates lose marks here by writing "two glucose molecules" (that is maltose) or by omitting the β designation on galactose.

Step 2 — identify the bond and the carbons involved. A β-1,4-glycosidic bond forms between carbon 1 of galactose and carbon 4 of glucose, with the release of one molecule of water.

A1 (AO2) — explicit β-1,4 designation plus naming carbons 1 and 4. Writing "glycosidic bond" without specifying β-1,4 is only a partial mark; "1,4 link" without the β prefix loses the discriminator.

Step 3 — show the condensation product. The –OH on C1 of galactose and the –OH on C4 of glucose react; one H₂O is expelled and the remaining oxygen bridges the two rings.

A1 (AO2) — water clearly shown as a product, with the bridging oxygen retained in the bond. A common pitfall is to draw water leaving from C1 of both sugars, which is geometrically impossible.

(b) Step 1 — locate the reducing group. A reducing sugar carries a free anomeric carbon (C1 in glucose, C2 in fructose) that can open from the ring form to expose a free aldehyde or ketone group, which then donates electrons to Cu²⁺.

M1 (AO1) — naming the anomeric carbon / open-chain form / aldehyde or ketone group.

Step 2 — explain why sucrose is non-reducing. In sucrose the α-1,2-glycosidic bond links the anomeric C1 of glucose to the anomeric C2 of fructose. Both anomeric carbons are committed to the bond, so neither sugar can open into its reducing aldehyde/ketone form.

A1 (AO2) — explicit "both anomeric carbons are involved in the bond." Many candidates lose marks here by writing "sucrose has no –OH groups" (it has plenty) instead of identifying the locked anomeric carbons.

Step 3 — describe acid hydrolysis. Boiling with dilute HCl provides H⁺ which protonates the bridging oxygen of the glycosidic bond and supplies water to break it, releasing free glucose and free fructose.

M1 (AO1) — naming hydrolysis / release of free monosaccharides.

Step 4 — explain the neutralisation step. Benedict's reagent is alkaline (contains NaOH/Na₂CO₃ alongside CuSO₄). Without neutralisation by NaHCO₃, residual HCl titrates the alkali and Cu²⁺ precipitates as Cu(OH)₂ before any redox can occur, blocking the test.

A1 (AO3) — connecting the chemistry of Benedict's alkalinity to the necessity of neutralisation. This is the AO3 evaluation mark and the discriminator for top band.

Total: 8 marks (a: M1 A1 A1; b: M1 A1 M1 A1). A clean A* response names anomeric carbons explicitly and finishes with the alkaline-reagent justification for neutralisation.

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Specimen question modelled on the Edexcel 9BI0 paper format

Question (6 marks): Glucose, fructose and ribose are all monosaccharides. Compare and contrast their structures and biological roles.

Mark scheme decomposition by AO:

Mark	AO	Awarded for
1	AO1	Naming the carbon counts: glucose and fructose are hexoses (6C), ribose is a pentose (5C).
2	AO1	Identifying ring geometry: glucose forms a six-membered (pyranose) ring; fructose forms a five-membered (furanose) ring; ribose forms a five-membered (furanose) ring.
3	AO2	Glucose is the universal respiratory substrate for glycolysis (Topic 5).
4	AO2	Fructose is found in fruit and combines with α-glucose to form sucrose, the plant transport sugar (Topic 7 phloem).
5	AO2	Ribose is the pentose component of RNA, ATP, NAD⁺ and FAD — its 2′-OH distinguishes it from deoxyribose in DNA.
6	AO3	Synthesis: explicit comparison statement such as "the same C₆H₁₂O₆ formula yields functionally distinct molecules because biology exploits subtle structural differences (ring size, hydroxyl orientation) rather than gross composition."

Total: 6 marks split AO1 = 2, AO2 = 3, AO3 = 1. The AO3 mark rewards the candidate who articulates why structural isomerism matters biologically rather than listing the three sugars in parallel.

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Synoptic links

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