What Are the Three Types of Carbohydrates?
Learn the three types of carbohydrates for AQA A Level Biology, with examples of each and a clear explanation of glycosidic bonds, condensation and hydrolysis.
Carbohydrates are grouped into monosaccharides, disaccharides and polysaccharides according to how many sugar units they contain. Monosaccharides are single sugar units; disaccharides contain two joined units; polysaccharides contain many joined units.
Monosaccharides
Monosaccharides include glucose, fructose and galactose. They are the building blocks for larger carbohydrates. A monosaccharide is a monomer: a small molecule that can join others to build a larger molecule. Glucose is also a respiratory substrate.
Glucose occurs in two forms, alpha-glucose and beta-glucose, which differ in the arrangement of atoms around one carbon atom. That small structural difference matters because it affects which polysaccharides can be made.
Disaccharides
A disaccharide forms when two monosaccharides join by a condensation reaction. This forms a glycosidic bond and releases water. The combinations to know are:
- Maltose: glucose and glucose
- Sucrose: glucose and fructose
- Lactose: glucose and galactose
For example, joining two glucose molecules produces maltose. The reverse process, hydrolysis, uses water to break the glycosidic bond and separate them again.
Polysaccharides
Polysaccharides are long chains of monosaccharides joined by glycosidic bonds. Starch and glycogen are made from alpha-glucose. Starch stores glucose in plants, while glycogen stores it in animals. Cellulose is made from beta-glucose and provides strength to plant cell walls. Its chains are arranged differently from those in starch or glycogen, giving it a different function.
Starch contains coiled amylose and branched amylopectin. Glycogen is also branched, providing many ends where enzymes can act to release glucose. Cellulose has straight chains held together by hydrogen bonds, forming strong fibres. This is why the glucose form matters: alpha-glucose makes storage polysaccharides, whereas beta-glucose makes structural cellulose.
Exam mistake to avoid: do not call a disaccharide a single sugar unit, or say that condensation uses water to break a bond. Condensation joins sugar units and releases water; hydrolysis breaks their bond using water.
