# Carbohydrates

> CIE A-Level Biology · Unit 2: Biological Molecules
> Source: https://www.owlsprep.com/study/cie-9700-u2-carbohydrates/

This module covers classification, structure and function of carbohydrates, core biological macronutrients. You will learn bond formation, key polysaccharide adaptations and biochemical tests for sugars, all frequently assessed in CIE exams.

**Prerequisites:** [Introduction to biological molecules](https://www.owlsprep.com/study/cie-9700-u2-biological-molecules-intro/)

## Learning objectives

- Classify carbohydrates into monosaccharides, disaccharides and polysaccharides
- Explain glycosidic bond formation via condensation reactions
- Compare the structure and function of starch, glycogen and cellulose
- Interpret results of biochemical tests for sugars and starch

## Classification and Structure of Monosaccharides

**Monosaccharide** — The simplest carbohydrate monomer, soluble in water and sweet-tasting. Monosaccharides are the building blocks for larger carbohydrates.

*Notation:* General formula: $(CH_2O)_n$

*Example:* Glucose, a hexose (6-carbon) monosaccharide, has molecular formula $C_6H_{12}O_6$

Glucose has two structural isomers: α-glucose and β-glucose. They differ only in the position of the hydroxyl (-OH) group on carbon 1 of the ring structure. In α-glucose, the -OH group is below the plane of the ring; in β-glucose, it sits above the plane. This small difference leads to massive differences in the structure and function of polymers formed from each isomer.

**Worked example:** State the key difference between α-glucose and β-glucose, and explain why this difference matters.

1. 1. The only difference is the position of the hydroxyl (-OH) group on carbon 1 of the glucose ring.
2. 2. For α-glucose: C1 -OH is below the ring; for β-glucose: C1 -OH is above the ring.
3. 3. This difference changes how glucose molecules bond together, leading to very different polymer structures: α-glucose forms storage polysaccharides (starch/glycogen), while β-glucose forms structural cellulose.

## Disaccharides and Glycosidic Bond Formation

Two monosaccharides are joined together by a condensation reaction, which forms a covalent glycosidic bond and releases one molecule of water. To break a glycosidic bond, water is added in a hydrolysis reaction. Common disaccharides you need to recall are: maltose (2 × α-glucose), sucrose (α-glucose + fructose) and lactose (α-glucose + galactose).

**Glycosidic Bond** — A covalent bond that links two sugar molecules, formed by a condensation reaction between two hydroxyl groups on adjacent sugars.

**Worked example:** Explain how an α 1-4 glycosidic bond forms between two α-glucose molecules.

1. 1. The hydroxyl group on carbon 1 of the first α-glucose aligns with the hydroxyl group on carbon 4 of the second α-glucose.
2. 2. A condensation reaction occurs: one oxygen atom remains between the two carbons, and a water molecule ($H_2O$) is released (one molecule loses -OH, the other loses -H).
3. 3. The covalent bond linking C1 of the first glucose and C4 of the second is an α 1-4 glycosidic bond. The product of this reaction is the disaccharide maltose.

## Polysaccharides: Structure and Function

Polysaccharides are long chains of monosaccharide monomers linked by glycosidic bonds. Their structure is uniquely adapted to their biological function, as shown in the table below.

| Polysaccharide | Monomer | Glycosidic Bonds | Key Function |
| --- | --- | --- | --- |
| Amylose (starch) | α-glucose | α 1-4 only | Plant energy storage |
| Amylopectin (starch) | α-glucose | α 1-4 + α 1-6 | Plant energy storage |
| Glycogen | α-glucose | α 1-4 + α 1-6 | Animal energy storage |
| Cellulose | β-glucose | β 1-4 only | Plant cell wall structure |

**Worked example:** Explain how cellulose structure is adapted to its function as a structural component of plant cell walls.

1. 1. Cellulose is made of β-glucose monomers linked by β 1-4 glycosidic bonds. Each consecutive β-glucose must flip 180° to allow the bond to form.
2. 2. Flipping produces straight, unbranched parallel chains of cellulose.
3. 3. Many hydrogen bonds form between hydroxyl groups on adjacent chains, cross-linking them into strong microfibrils.
4. 4. This high tensile strength allows cell walls to resist bursting when cells take in water by osmosis, and provides structural support for the plant.

## Biochemical Tests for Carbohydrates

CIE regularly assesses the method and interpretation of results for three core tests for carbohydrates, outlined below.

- **Benedict's Test for reducing sugars**: Add Benedict's reagent, heat to ~95°C. Brick-red precipitate = positive result (reducing sugar present).
- **Test for non-reducing sugars**: If initial Benedict's is negative, boil sample with dilute HCl to hydrolyse bonds, neutralise with sodium hydrogencarbonate, repeat Benedict's. Positive result = non-reducing sugar present.
- **Iodine Test for starch**: Add iodine in potassium iodide solution. Blue-black colour change = positive result (starch present).

**Worked example:** An unknown solution gives a negative Benedict's test, then a positive Benedict's test after boiling with HCl and neutralisation. What does this result mean?

1. 1. A negative initial Benedict's test confirms no reducing sugars are present in the original solution.
2. 2. Boiling with HCl hydrolyses any glycosidic bonds, breaking disaccharides/polysaccharides into monosaccharides.
3. 3. A positive Benedict's after hydrolysis means the original solution contained a non-reducing sugar. This is most commonly sucrose in CIE exam questions.

## Common pitfalls

- **Wrong:** Claiming starch is made of β-glucose monomers
  - Why it fails: Only cellulose uses β-glucose; all storage polysaccharides (starch, glycogen) use α-glucose
  - Correct: Starch and glycogen are polymers of α-glucose; cellulose is a polymer of β-glucose
- **Wrong:** Mixing up condensation and hydrolysis for bond formation
  - Why it fails: Many candidates incorrectly state hydrolysis forms glycosidic bonds, when it actually breaks them
  - Correct: Condensation forms glycosidic bonds and releases water; hydrolysis breaks glycosidic bonds using water
- **Wrong:** Stating glycogen is unbranched
  - Why it fails: Glycogen is more branched than amylopectin, which allows faster release of glucose for energy
  - Correct: Glycogen is highly branched, with α 1-6 glycosidic bonds at all branch points
- **Wrong:** Claiming a positive Benedict's test confirms sucrose is present originally
  - Why it fails: Sucrose is a non-reducing sugar, so it only gives a positive result after hydrolysis
  - Correct: Negative initial Benedict's + positive after hydrolysis confirms non-reducing sugar (e.g. sucrose) is present
- **Wrong:** Drawing α-glucose with C1 -OH above the ring
  - Why it fails: CIE expects you to correctly identify the difference between α and β glucose in diagrams
  - Correct: α-glucose has C1 -OH *below* the ring; β-glucose has C1 -OH *above* the ring

## Cheatsheet

| Carbohydrate Type | Key Features | Core Function |
| --- | --- | --- |
| Monosaccharides | Soluble, reducing | Respiratory substrate |
| Maltose | 2 α-glucose, α 1-4 | Digestion product of starch |
| Sucrose | Glucose + fructose, α 1-2, non-reducing | Plant transport sugar |
| Amylose | α-glucose, α 1-4, unbranched | Plant energy storage |
| Amylopectin | α-glucose, α 1-4/1-6, branched | Plant energy storage |
| Glycogen | α-glucose, highly branched | Animal energy storage |
| Cellulose | β-glucose, β 1-4, cross-linked | Plant cell wall structure |

## What's next

Carbohydrates are the first of four core biological macromolecules you study for CIE A-Level Unit 2. The principles of condensation reactions and covalent bond formation you learned here apply to all other biological polymers, helping you understand how structure links to function across the entire unit. Next, you will build on this foundation to study lipids, then proteins and nucleic acids, before learning more about how macromolecules interact in biological systems.

- [Lipids](https://www.owlsprep.com/study/cie-9700-u2-lipids/)
- [Amino acids and proteins](https://www.owlsprep.com/study/cie-9700-u2-amino-acids-and-proteins/)
- [Protein structure](https://www.owlsprep.com/study/cie-9700-u2-protein-structure/)

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