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. 

What are Polysaccharides?

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

Three important polysaccharides are: -

​1. Starch

2. Glycogen

3. Cellulose

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

The Structure and Function of Cellulose, microfibrils 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.

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How are Polysaccharides like starch, glycogen and cellulose formed?

Polysaccharides like starch, glycogen and cellulose are formed via condensation reactions.

Can you describe the structure of 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).

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.

Starch is the energy storage molecule found in plant cells - remember starch is made up from many glucose monomers which can be used to make ATP (Adenosine triphosphate the energy currency molecule).

Can you describe the structure of Amylose?

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.

Can you describe the structure of Amylopectin?

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.

How do animals store excess glucose?

Animal cells store excess glucose in the form of glycogen – a 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.

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.

Can you Describe the structure of cellulose?

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.

​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.