A Level Biology Lesson 3:
Monosaccharides and Disaccharides
Task 1. Check Where this Lesson fits into your Exam Specification!
Task 2. Watch the Revision Notes Lesson in Full.
00:00 Learning Outcomes
00:43 Recap of condensation reaction to make a disaccharide.
02:11 Difference between alpha and beta glucose
02:55 Disaccharides you Must Know!
Download, Print out & complete the accompanying Interactive Worksheet.
A-Level Biology "Monosaccharides and Disaccharides" Lesson 3. Summary.
Monosaccharides join together to form disaccharides and in turn polysaccharides via condensation reactions.
The resulting bond linking the monosaccharides is known as a glycosidic bond.
Isomers have the same molecular formula, but different molecular arrangements.
α-glucose and β-glucose both have the molecular formula C6H12O6– However, the location of a Hydrogen at Carbon 1 differs (they are isomers!). Pay attention to the ‘ring diagram’ of a glucose molecule: -
Notice that α-glucose has an Hydrogen located at the “top” of carbon number 1.
Whereas β-glucose has a Hydroxyl group [OH] at the top of Carbon number 1.
When only 2 monosaccharides join via a condensation reaction one molecule of water (H2O) is removed and a glycosidic bond between the monosaccharides is formed.
The Disaccharides you have to know, and the monosaccharides that make them up are:
Maltose. Which is composed of 2 α-glucoses molecules.
Lactose. Which is composed of Glucose and Galactose.
Sucrose. Which is composed of Glucose and Fructose
Check Your Spec!
Specification Reference: - 3.1.2 Carbohydrates: Monosaccharides are the monomers from which larger carbohydrates are made. Glucose, galactose and fructose are common monosaccharides. A condensation reaction between two monosaccharides forms a glycosidic bond. Disaccharides are formed by the condensation of two monosaccharides: maltose is a disaccharide formed by condensation of two glucose molecules sucrose is a disaccharide formed by condensation of a glucose molecule and a fructose molecule lactose is a disaccharide formed by condensation of a glucose molecule and a galactose molecule. Glucose has two isomers, α-glucose and β-glucose.
Specification Reference: - 2 Biological molecules: 2.2 Carbohydrates. a) describe the ring forms of α-glucose and β–glucose. c) describe the formation of a glycosidic bond by condensation, with reference both to polysaccharides and to disaccharides, including sucrose.
★ Edexcel (Biology A – Salters-Nuffield)
Specification Reference: - Topic 1: 1.12 i) Know the difference between monosaccharides, 1.13 Know how monosaccharides join to form disaccharides (sucrose, lactose and maltose) through condensation reactions forming glycosidic bonds, and how these can be split through hydrolysis reactions.
★ Edexcel (Biology B)
Specification Reference: - Topic 1: Biological Molecules. 1.1 Carbohydrates i) Know the difference between monosaccharides, disaccharides and polysaccharides. ii) Know the structure of the hexose glucose (alpha and beta). iii) Understand how monosaccharides (glucose, fructose, galactose) join to form disaccharides (sucrose, lactose and maltose) through condensation reactions forming glycosidic bonds, and how these can be split through hydrolysis reactions.
★ OCR (Biology A)
Specification Reference: - 2.1.2 Biological molecules. the ring structure and properties of glucose as an example of a hexose monosaccharide. To include the structural difference between an α- and a β-glucose molecule. To include the disaccharides sucrose, lactose and maltose. The synthesis and breakdown of a disaccharide and polysaccharide by the formation andbreakage of glycosidic bonds.
★ OCR (Biology B)
Specification Reference: - 2.1.2 Water and its importance in plants and animals. (e) the structure of the ring form of α-glucose as an example of a simple monosaccharide, and lactose as a disaccharide.
Specification Reference: - Core Concepts 1. Chemical elements are joined together to form biological compounds.(c) the structure, properties and functions of carbohydrates: monosaccharides. disaccharides (sucrose, lactose, maltose); (d) alpha and beta structural isomerism in glucose.