1.4Chemistry of Life

Carbohydrates — AP BIOLOGY Revision Notes & Topical Questions

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1.4 Carbohydrates — Notes — AP Biology notes: Describe the structure and function of carbohydrates.Carbohydrates — they contain C, H and O, usually in the ratio 1:2:1, with the formula (CH₂O)ₙMonosaccharides = single sugars such as glucose (C₆H₁₂O₆), fructose and galactose, and r…

Glucose forms
Disaccharides
Polysaccharides
Dehydration synthesis
Distribution
Recognition

Section 1 of 11Monosaccharides and disaccharides

Carbohydrates

Carbohydrates they contain C, H and O, usually in the ratio 1:2:1, with the formula (CH₂O)ₙ

Monosaccharides

Monosaccharides single sugars such as glucose (C₆H₁₂O₆), fructose and galactose, and ribose has five carbons

Glucose forms

α-glucose and β-glucose differ in the position of the –OH on carbon 1

Disaccharides

Disaccharides two monosaccharides joined by a glycosidic bond through dehydration synthesis
Maltose is glucose + glucose.
Sucrose is glucose + fructose.
Lactose is glucose + galactose.

Section 2 of 11Forming a polysaccharide

Forming a polysaccharide

Polysaccharides

Polysaccharides complex carbohydrates formed when monosaccharides join by covalent bonds, and they may be linear or branched

Dehydration synthesis

Dehydration synthesis a new covalent bond forms between two monomers and a molecule of water is produced
The process repeats many times to build a polysaccharide.

Structure and function

Structure and function the type of monomer and the nature of the covalent bonds determine the properties and function of a carbohydrate

Section 3 of 11Cellulose

Cellulose

Role

Role a structural polysaccharide of glucose found in plant cell walls

Structure

Structure long chains of β-glucose joined by β-1,4 glycosidic bonds

Rotation

Rotation alternate β-glucose molecules must be rotated through 180° to form the bonds
The chains are straight and unbranched.
Many hydrogen bonds form between neighbouring chains and give microfibrils their structural strength.

Digestion

Digestion most animals cannot hydrolyse β-1,4 bonds, so herbivores rely on gut microbes with cellulase and humans get cellulose only as fibre

Section 4 of 11Starch and glycogen

Starch and glycogen

Role

Role starch and glycogen are storage polysaccharides

Distribution

Distribution glycogen is the storage polysaccharide of animals and fungi and starch is the storage polysaccharide of plants
- (liver and muscle)
Amylose is unbranched with α-1,4 bonds and coils into a helix.
Amylopectin is a branched form of starch.

Structure

Structure both are polymers of α-glucose joined by 1,4 glycosidic bonds, with 1,6 glycosidic bonds at branches

Shared properties

They are compact, so large quantities are stored in a small space.
They are insoluble, so they do not affect the water potential of cells.
They can be branched, with glycogen always branched and starch either branched or unbranched.

Branching

Branching it gives many free ends where glucose can be added or removed, so storage and release suit the energy demands of the cell

Section 5 of 11Comparing polysaccharides

Polysaccharide	Monomer	Linkage	Shape	Function	Organism
Starch (amylose)	α-glucose	α-1,4	Helix	Energy storage	Plants
Amylopectin	α-glucose	α-1,4 and α-1,6	Branched	Energy storage	Plants
Glycogen	α-glucose	α-1,4 and α-1,6	Highly branched	Energy storage	Animals and fungi
Cellulose	β-glucose	β-1,4	Straight fibres	Cell wall structure	Plants
Chitin	Modified glucose with nitrogen	β-1,4	Straight, strong fibres	Exoskeleton and cell wall	Arthropods and fungi

Section 6 of 11Other roles of carbohydrates

Recognition

Recognition carbohydrates on proteins (glycoproteins) and lipids (glycolipids) on the cell surface act as recognition and signalling molecules

Nucleic acids

Nucleic acids ribose and deoxyribose are the sugars in RNA and DNA

Transport

Transport plants transport sugar in the phloem as sucrose

Energy

Energy glucose provides quick energy

Section 7 of 11Digestion and use of carbohydrates

Starch digestion

Starch digestion amylase hydrolyses α-1,4 bonds in starch to maltose, and maltase converts maltose to glucose in the small intestine

Lactose

Lactose lactase hydrolyses lactose, and people who make too little are lactose intolerant

Fibre

Fibre humans lack cellulase, so cellulose passes through as dietary fibre

Glycogen control

Glycogen control glycogen is built when blood glucose is high and hydrolysed when glucose is needed (stimulated by glucagon)
- (stimulated by insulin)

Plants

Plants sugars made in photosynthesis are converted to starch in chloroplasts and in storage organs such as potato tubers

Section 8 of 11Tests for carbohydrates

Starch

Starch iodine in potassium iodide turns from orange to blue-black
Iodine is dissolved in potassium iodide because iodine alone is insoluble.
Fading of the blue-black colour shows that starch has been digested.

Reducing sugars

Reducing sugars heated Benedict's reagent changes from blue through green, yellow and orange to brick red, depending on concentration
The sugar reduces Cu²⁺ to copper(I) oxide, so the test is semi-quantitative.
Glucose, galactose, fructose and maltose are reducing sugars.

Sucrose

Sucrose it is a non-reducing sugar, so it only gives a positive result after boiling with dilute HCl and neutralising with NaHCO₃

Estimating concentration

Make a serial dilution of glucose standards, for example 0–10 mmol dm⁻³.
Treat every tube and the unknown identically, with the same volume, excess Benedict's, set temperature and about 5 minutes of heating.
Match the colour of the unknown with the closest standard, or use a colorimeter with a blue filter and a blank at zero.
Plot absorbance of the standards as a calibration curve and read the unknown from it.

Section 9 of 11More from the specification

Section 10 of 11Key terms

Term	Meaning
Monosaccharide	A single sugar unit such as glucose, fructose or galactose.
Disaccharide	Two sugars joined, such as sucrose, lactose or maltose.
Polysaccharide	Many sugars joined, such as starch, glycogen, cellulose or chitin.
Glycosidic bond	Covalent bond between sugars.
α and β glucose	Isomers that differ in the position of the –OH on carbon 1.
Chitin	Polysaccharide of fungal cell walls and arthropod exoskeletons.
Microfibril	Bundle of cellulose chains held by hydrogen bonds.

Section 11 of 11Exam technique

Confusing α and β glucose — cellulose has β-1,4 bonds.

Saying cellulose is a storage molecule — it is structural.

Writing that animals store starch — they store glycogen.

Forgetting that sucrose is a non-reducing sugar.

Last updated: 14 October 2026