polysacchrides

What are polysaccharides?

Carbohydrates made from many monosaccharides joined together by glycosidic bonds.

How are polysaccharides formed?

By condensation reactions between monosaccharides, forming glycosidic bonds and releasing water.

What are the three main polysaccharides in A-level Biology?

Starch, glycogen and cellulose.

What are the general structural forms of polysaccharides?

They may be branched, coiled or straight.

What is starch?

A storage polysaccharide found in plants, made from alpha-glucose.

What are the two polysaccharides that make up starch?

Amylose and amylopectin.

What is amylose?

An unbranched polymer of alpha-glucose joined by 1,4-glycosidic bonds.

What shape does amylose form?

A coiled helix.

Why is amylose compact?

Its coiled structure allows many glucose molecules to be stored in a small space.

Why is starch suitable for storage?

It is compact, insoluble in water and therefore does not affect water potential significantly.

What is amylopectin?

A branched polymer of alpha-glucose containing 1,4- and 1,6-glycosidic bonds.

What type of glycosidic bond creates branches in amylopectin?

1,6-glycosidic bonds.

Why is amylopectin useful for energy storage?

Its branches provide many ends where enzymes can hydrolyse glucose from the molecule.

How does branching affect the rate of glucose release?

More branches provide more sites for enzymes to act, allowing glucose to be released more rapidly.

Where is starch stored?

In plants, often in storage organs such as seeds and tubers.

Why is starch insoluble?

Its insolubility prevents it from drawing water into cells by osmosis.

Why is starch a suitable storage molecule?

It is compact, insoluble and can be hydrolysed to release glucose for respiration.

What is glycogen?

A storage polysaccharide found mainly in animals, made from alpha-glucose.

Where is glycogen stored in animals?

Mainly in liver and muscle cells.

What glycosidic bonds are found in glycogen?

1,4-glycosidic bonds and 1,6-glycosidic bonds.

How is glycogen structured?

It is highly branched and compact.

Why is glycogen more highly branched than amylopectin?

It needs to allow rapid release of glucose when energy is required.

Why is glycogen suitable for storage?

It is compact and insoluble, so large quantities can be stored without significantly affecting water potential.

What is the difference between glycogen and starch?

Glycogen is generally more highly branched than starch, allowing glucose to be released more rapidly.

What is cellulose?

A structural polysaccharide found in plant cell walls, made from beta-glucose.

What type of glycosidic bond joins beta-glucose molecules in cellulose?

1,4-glycosidic bonds.

Why does cellulose form a straight chain?

Beta-glucose molecules alternate orientation when joined by 1,4-glycosidic bonds.

What happens to alternate beta-glucose molecules in cellulose?

They are inverted relative to one another.

What bonds form between cellulose chains?

Hydrogen bonds form between hydroxyl groups on adjacent chains.

What are cellulose microfibrils?

Bundles of cellulose chains held together by hydrogen bonds.

Why is cellulose strong?

Hydrogen bonds between many cellulose chains give the microfibrils high tensile strength.

What is the function of cellulose in plants?

It provides strength and support to plant cell walls.

Why are cellulose fibres suitable for plant cell walls?

They have high tensile strength and resist being pulled apart.

What is the difference between alpha-glucose and beta-glucose?

They differ in the orientation of the hydroxyl group attached to carbon 1.

Which form of glucose makes starch and glycogen?

Alpha-glucose.

Which form of glucose makes cellulose?

Beta-glucose.

What is the main difference between starch and cellulose?

Starch is made from alpha-glucose and cellulose is made from beta-glucose.

Why do starch and cellulose have different structures?

They are made from different isomers of glucose, causing different glycosidic bond arrangements and structures.

Why can humans digest starch but not cellulose?

Humans produce enzymes that hydrolyse the glycosidic bonds in starch but do not produce cellulase to hydrolyse the bonds in cellulose.

What is the role of polysaccharides in storage?

They allow large amounts of glucose to be stored compactly and insolubly.

Why must storage polysaccharides be insoluble?

They do not affect the water potential of cells and therefore do not cause excessive water uptake.

Why are branched polysaccharides useful?

They have many ends where enzymes can act, allowing rapid hydrolysis and glucose release.