Carbohydrates contain the elements Carbon, Hydrogen and Oxygen. The Carbohydrate 'group' includes:  Monosaccharides (monomers), Disaccharides (Dimers ) and Polysaccharides (polymers). The diagram below summarises the carbohydrates with examples from key examples each subcategory.

Download & Complete your own Carbohydrate Chart (PDF)

Showing common examples of Monosaccharides 

Disaccharides &

Polysaccharides

Polysaccharides are long chains of many monosaccharides joined together by glycosidic bonds.

 

Three important polysaccharides are: -

  1. Starch

  2. Glycogen

  3. Cellulose

 

A Level Biology: Monosaccharides and Disaccharides

Watch the monosaccharides and disaccharides interactive lesson (walkthrough), then download the interactive lesson folder (.zip) to open up and work through the lesson on your own. 

 

  • Don't delete any assets from the folder :)

 

Check Where this Lesson fits into your Exam Specification!

In This A-Level Biology Lesson “Monosaccharides and Disaccharides” you will learn that monosaccharides join together to make disaccharides (which in turn make polysaccharides) which they do by means of condensation reactions. Also, as illustrated with very simple diagrams you'll be shown how to recognise the difference between alpha and beta glucose (a very common exam question!). Finally we'll list the important disaccharides you have to know about. Once you've finished watching the lesson, download the accompanying revision notes available as an A3 Poster (PDF). The notes are also available in the A4 PDF, so you can easily follow along with the video.

When you’re confident you know this topic, complete the accompanying Monosaccharides and Disaccharides knowledge check questions and check your answers with my answers in the back of the work book (no vague mark schemes - You’ll see exactly how you should be writing your answers so you gain maximum marks in the exams!)

A Level Biology Monosaccharides and disaccharides Revision Notes Poster A3 PDF
A Level Biology Monosaccharides and disaccharides revision notes with knowledge check Q and A PDF

A-Level Biology "Monosaccharides and Disaccharides"

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.

Remember!

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:

  1. Maltose. Which is composed of 2 α-glucoses molecules.

  2. Lactose. Which is composed of Glucose and Galactose.

  3. Sucrose. Which is composed of Glucose and Fructose

A Level Biology: Polysaccharides - The Structure and Function of Starch, Glycogen and Cellulose.

 

Check Where this Lesson fits into your Exam Specification!

In This A-Level Biology Lesson “polysaccharides: the structure of and function of starch, glycogen and cellulose you will learn about Amylose and the alpha 1,4 glycosidic bonds of Amylose You'll find out why Amylose is an Ideal Storage Molecule. We'll go on to discuss Amylopectin and its alpha 1,6 Glycosidic bonds, and understand what makes Amylopectin  a great Energy Storage Molecule. Next we'll see that Starch is Made up of both Amylose and Amylopectin  and identify the Key Features of Starch

 

You'll learn that Animal Cells Store Excess Glucose in the form of Glycogen.

 

The Structure and Function of Cellulosemicrofibrils and beta 1,4 glycosidic bonds.

 

Finally you'll see that weak hydrogen bonds link the long unbranched chains of Cellulose together as we finish the lesson with some of the Key features of Cellulose.

When you’re confident you know all of this topic complete the accompanying

polysaccharides: the structure of and function of starch, glycogen and cellulose knowledge check questions and check your answers with my answers in the back of the work book (no vague mark schemes - You’ll see exactly how you should be writing your answers so you gain maximum marks in the exams!)

A-Level Biology Polysaccharides: The Structure and Function of Starch, Glycogen and Cellulose Revision notes with knowledge check questions and answers pdf
A-Level Biology: Polysaccharides: The Structure and Function of Starch, Glycogen and Cellulose Revision Notes Poster A3 PDF

A-Level Biology: Polysaccharides: The Structure and Function of Starch, Glycogen and Cellulose.

Polysaccharides are formed via condensation reactions.

 

Amylose is a compact cylindrical polysaccharide composed of many a-glucose molecules. The Glycosidic bond linking the α-glucose molecules is an α-1,4 glycosidic bond.

 

Amylose is a compact, energy storage molecule.

 

Amylopectin is a compact, branched polysaccharide composed of many a-glucose molecules. The Glycosidic bonds linking the α-glucose molecules are α-1,4 glycosidic bonds and α-1,6  glycosidic bonds.

It is the α-1,6 glycosidic bonds which give rise the branched structure of Amylopectin.

 

Starch is a compact “spiral” molecule composed of Amylose and Amylopectin.

Starch is an ideal energy storage molecule, found in plant cells.

Starch is insoluble in water and does not affect water potential of cells.

 

Animal cells store excess glucose in the form of Glycogena highly branched polysaccharide.

Glycogen is an excellent energy reserve molecule – due to the highly branched structure.

The α-1,6 glycosidic bonds are readily hydrolysed releasing the α-glucose molecules.

 

Cellulose is a long unbranched polysaccharide composed of β-glucose monosaccharides, which form plant cell walls. Cellulose chains are linked together via weak hydrogen bonds, which ‘hold’ the cellulose molecules together in structures known as microfibrils. These strong microfibrils form very strong microfibers. Cellulose is strong and provides structural support for plant cells – helping prevent cell lysis (bursting) when they fill with water via osmosis.

A Level Biology - Carbohydrates: Starch

 

What is Starch?

Starch is a polysaccharide found in plants (i.e starch (carbohydrate) rich foods such as rice). It is insoluble and forms starch granules inside many plant cells. Due to the insolubility of starch it does not change the water potential of cells - which means it does not cause the cells to take up water by osmosis. 

 

Describe the structure and function of starch.

Starch is a polysaccharide composed of a mixture of Amylose and Amylopectin.

 

Amylose is simply a straight chain polysaccharide composed of a-glucose monomers. you may see it referred to as 'poly-1-4-' (remember this is because of those α-1,4 glycosidic bonds which make up Amylose.  Amylose is a straight chain but is quite ‘loose’  so it tends to coil up into a helix (which make it great for storage!)

Amylopectin is also “poly-1-4” composed of a-1,4 glycosidic bonds forming long straight chains. However unlike Amylose, Amylopectin has approximately 4% a-1,6 glycosidic bonds which result in this polysaccharide having a more ‘branched’ and ‘open’ molecular structure. Now, because Amylopectin is more open and branched it has more ‘ends’ than Amylose - which means Amylopectin can be broken down (hydrolysed) more quickly than Amylose by amylase enzymes. You should know that both Amylose and Amylopectin are hydrolysed by the enzyme Amylase into the disaccharide Maltose - though they are broken down at different rates (and you should now be able to explain why).

A Level Biology - Carbohydrates: Glycogen
 

Glycogen is very similar in structure to amylopectin (so take care if asked to identify this polysaccharide). Glycogen is a polysaccharide composed of glucose monomers with a-1,4 glycosidic bonds and approximately 9% a-1,6 glycosidic bond (so is more 'highly branched' than the similar looking Amylopectin). Glycogen is made by animals as their storage polysaccharide and is found mainly in muscle and liver. Because Glycogen is so 'highly branched' and 'open' it can be hydrolysed very quickly - i.e. Glycogen can broken down (hydrolysed) into glucose monomers and used as a source of energy very quickly. Glycogen is broken down into a-glucose monomers by the enzyme glycogen phosphorylase.

A Level Biology - Carbohydrates: Cellulose
 

Cellulose is only found in plants and is the main component of cell walls. It is a polysaccharide made up from a different isomer of glucose (β-glucose). Thus, the poly-1-4 glycosidic bonds are β-1,4 glycosidic bonds! You already know the difference between α-glucose and  β-glucose  -  and this small difference makes a massive difference to the molecular structure and properties of cellulose. The difference in the way β-glucose monomers form a polysaccharide chain, means that alternate glucose molecules are inverted (which results in the  β-1,4-glycosidic bonds. Remember the α-1,4 glucose polymer in starch coils up (forming storage granules), well… the β-1,4 glucose polymer in cellulose forms long straight chains.

Hundreds of these chains are then linked together by hydrogen bonds to form cellulose microfibrils. Microfibrils are very strong and rigid, giving strength and structural integrity to plant cells. β-1,4 glycosidic bonds cannot be broken by amylase, rather the hydrolysis of cellulose (and those β-1,4 glycosidic bonds) requires a specific enzyme called cellulase. We humans cannot digest cellulose (we refer to as cellulose as fibre). The only organisms that do possess cellulase enzymes are bacteria (prokaryotes), so herbivorous animals like cows and termites whose diet is primarily cellulose rely upon their symbiotic relationship with the mutualistic bacteria in their guts - allowing them to digest cellulose.

Check Your Exam Specification

 

AQA A Level Biology 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.

Polysaccharides are formed by the condensation of many glucose units. Glycogen and starch are formed by the condensation of α-glucose. Cellulose is formed by the condensation of β-glucose. The basic structure and functions of glycogen, starch and cellulose. The relationship of structure to function of these substances in animal cells and plant cells.

 

CIE A Level Biology 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. Define the terms monomer, polymer, macromolecule, monosaccharide, disaccharide and polysaccharide. e) describe the molecular structure of polysaccharides including starch (amylose and amylopectin), glycogen and cellulose and relate these structures to their functions in living organisms.

 

Edexcel A Level Biology  (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. Know the difference between monosaccharides, disaccharides and polysaccharides, including glycogen and starch (amylose and amylopectin). Be able to relate the structures of monosaccharides, disaccharides and polysaccharides to their roles in providing and storing energy. Topic 4: Biodiversity and Natural Resources: 4.9 Understand the structure and function of the polysaccharides starch and cellulose, including the role of hydrogen bonds between β -glucose molecules in the formation of cellulose microfibrils.

 

Edexcel A Level Biology (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. Understand how monosaccharides join to form polysaccharides (starch formed from amylose and amylopectin; glycogen) through condensation reactions forming glycosidic bonds, and how these can be split through hydrolysis reactions. Understand how the structure of glucose, starch, glycogen and cellulose relates to their function.

 

OCR A Level Biology (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 and breakage of glycosidic bonds. 

The structure of starch (amylose and amylopectin), glycogen and cellulose molecules - how the structures and properties of glucose, starch, glycogen and cellulose molecules relate to their functions in living organisms.

 

OCR A Level Biology  (Biology B) Specification Reference: - The structure of the ring form of α-glucose as an example of a simple monosaccharide and lactose as a disaccharide.

the importance of hydrolysis and condensation of biological molecules in cell metabolism - To include the concept of monomers and polymers in a range of biological molecules. The formation of polysaccharides by condensation - To include glycogen and starch (amylose and amylopectin) AND the formation of 1,4- and 1,6-glycosidic bonds and reference to the significance of branching on solubility.

 

WJEC  A Level Biology 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. The structure, properties and functions of carbohydrates: polysaccharides (starch, glycogen, cellulose) alpha and beta structural isomerism in glucose and its polymerisation into storage and structural carbohydrates, illustrated by starch and cellulose. The chemical and physical properties which enable the use of starch and glycogen for storage and cellulose.

★ BTEC Level 3 Nationals in Applied Science. Unit 10: Biological Molecules and Metabolic Pathways: A: Understand the structure and function of biological molecules and their importance in maintaining biochemical processes.  A2 Carbohydrates: Structure and features: contain carbon (C), hydrogen and oxygen atoms. Monosaccharides, e.g. α and β glucose, galactose, fructose, ribose and deoxyribose. Disaccharides, e.g. lactose, maltose and sucrose. Polysaccharides, e.g. amylose, amylopectin, cellulose. Use of iodine and Benedicts’ solution as tests for presence of carbohydrates.

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