Polysaccharides

Chemistry for Biologists

What is a polysaccharide?

A complex carbohydrate made from many monosaccharide units joined together by glycosidic bonds.

How are polysaccharides formed?

Many monosaccharides join together by condensation reactions, which remove water and form glycosidic bonds.

What is an oligosaccharide?

A carbohydrate molecule containing between 3 and 10 monosaccharide units.

How many monosaccharides are found in a true polysaccharide?

11 or more monosaccharide units.

How are glycosidic bonds formed?

By condensation reactions in which a molecule of water is removed.

How are glycosidic bonds broken?

By hydrolysis reactions in which a molecule of water is added.

What is hydrolysis?

A reaction in which a bond is broken by the addition of a molecule of water.

What is the relationship between condensation and hydrolysis?

They are opposite reactions: condensation removes water to form a bond, while hydrolysis adds water to break a bond.

What happens when starch and glycogen undergo hydrolysis?

They are gradually broken into shorter carbohydrate chains and eventually into individual glucose molecules.

Where does hydrolysis of carbohydrates occur during digestion?

In the gut.

Why are stored carbohydrates hydrolysed in muscles and the liver?

To release glucose that can be used in cellular respiration.

What is ATP?

Adenosine triphosphate, a molecule that acts as a universal energy supply in cells.

How is ATP produced from glucose?

Glucose is broken down during cellular respiration and chemical energy is transferred from glucose to ATP.

Why should you not say that respiration creates energy?

Energy cannot be created; chemical energy is transferred from glucose to ATP.

What are the main end products of aerobic respiration?

Carbon dioxide and water.

Why are monosaccharides such as glucose useful for immediate energy?

They can be absorbed and used directly by cells in cellular respiration.

What happens to other absorbed monosaccharides and disaccharides?

They are rapidly converted to glucose.

Why are foods containing monosaccharides and disaccharides good sources of relatively instant energy?

Their sugars can be rapidly absorbed or converted to glucose and used in cellular respiration.

Why are monosaccharides and disaccharides unsuitable for long-term energy storage?

They are chemically active and very soluble in water, so they affect the water balance of cells.

Why are polysaccharides suitable as energy-storage molecules?

They are compact, physically and chemically inactive, and not very soluble in water.

Why is being compact useful for an energy-storage molecule?

It allows a large amount to be stored in a small space and prevents it interfering with cell structures.

Why is being chemically inactive useful for a storage polysaccharide?

It prevents the molecule interfering with other chemical reactions in the cell.

Why is being insoluble useful for storage polysaccharides?

They have little effect on the water potential of cells and therefore do not cause excessive osmotic movement of water.

What is starch?

The main carbohydrate energy-storage molecule in plants, made from α-glucose.

Where is starch produced in plants?

Starch is produced from sugars made during photosynthesis.

Why is starch suitable for energy storage in plants?

It is insoluble and compact but can be broken down to release glucose when needed.

What are the two components of starch?

Amylose and amylopectin.

What is amylose?

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

Approximately how many glucose units can an amylose molecule contain?

Between about 200 and 5000 glucose units.

Why does amylose form a compact molecule?

Its long unbranched chain coils into a spiral.

What type of glycosidic bonds are present in amylose?

Only 1,4-glycosidic bonds.

Why is amylose unbranched?

It contains only 1,4-glycosidic bonds.

What is amylopectin?

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

What type of glycosidic bonds form the main chains of amylopectin?

1,4-glycosidic bonds.

What type of glycosidic bonds form the branch points in amylopectin?

1,6-glycosidic bonds.

Why is amylopectin branched?

Some glucose molecules are joined by 1,6-glycosidic bonds, producing branch points.

Why can amylopectin release glucose rapidly?

Its branches provide many terminal glucose units where hydrolysis can occur simultaneously.

How do amylose and amylopectin differ?

Amylose is an unbranched, coiled chain containing only 1,4-glycosidic bonds, whereas amylopectin is branched and contains both 1,4- and 1,6-glycosidic bonds.

Why does starch contain both amylose and amylopectin?

Amylopectin can release glucose rapidly when needed, while amylose releases glucose more slowly over time.

How do you test for starch?

Add a few drops of iodine solution to the sample.

What colour is iodine solution before testing for starch?

Reddish-brown.

What is a positive result for the iodine test for starch?

The iodine solution turns blue-black.

What does it mean if iodine solution remains reddish-brown?

Starch is not present.

What is glycogen?

A highly branched storage polysaccharide made from α-glucose and used as an energy store in animals.

Why is glycogen sometimes called animal starch?

It is the main carbohydrate energy store in animals and has a structure similar to amylopectin in starch.

Where is glycogen stored in animals?

Particularly in cells of the liver and muscles.

What type of glucose forms glycogen?

α-glucose.

What types of glycosidic bonds are found in glycogen?

1,4-glycosidic bonds in the chains and many 1,6-glycosidic bonds at branch points.

How is glycogen structurally similar to amylopectin?

Both are branched polymers of α-glucose containing 1,4- and 1,6-glycosidic bonds.

How does glycogen differ from amylopectin?

Glycogen has more 1,6-glycosidic bonds and therefore has many more branches.

Why is glycogen highly branched?

It contains many 1,6-glycosidic bonds that create branch points.

Why can glycogen be broken down very rapidly?

Its many branches provide many terminal glucose units where hydrolysis can occur simultaneously.

Why is the rapid breakdown of glycogen useful to animals?

It allows glucose to be released quickly for respiration when there is a high energy demand.

Why is glycogen a suitable energy-storage molecule?

It is compact, insoluble and highly branched, so it does not affect water potential and can release glucose rapidly.

Compare amylose, amylopectin and glycogen.

Amylose is unbranched and contains only 1,4-glycosidic bonds; amylopectin is branched with 1,4- and some 1,6-glycosidic bonds; glycogen has 1,4- and many 1,6-glycosidic bonds and is therefore more highly branched.

Which polysaccharide is the most highly branched: amylose, amylopectin or glycogen?

Glycogen.

Which of amylose, amylopectin and glycogen is unbranched?

Amylose.

Which bonds cause branching in amylopectin and glycogen?

1,6-glycosidic bonds.

Which bonds form the main chains of amylose, amylopectin and glycogen?

1,4-glycosidic bonds.

Why does increased branching increase the rate at which glucose can be released?

More branches create more terminal ends where enzymes can hydrolyse glycosidic bonds simultaneously.

Define an oligosaccharide.

A molecule containing between 3 and 10 monosaccharide units.

Define hydrolysis.

A reaction in which bonds are broken by the addition of a molecule of water.

Define ATP.

Adenosine triphosphate, the molecule that acts as a universal energy supply in all cells.

Define end products.

The final products of a chemical reaction.

Define amylose.

A complex carbohydrate containing only α-glucose monomers joined by 1,4-glycosidic bonds, forming long unbranched chains.

Define amylopectin.

A complex carbohydrate made of α-glucose monomers joined mainly by 1,4-glycosidic bonds with some 1,6-glycosidic bonds, causing repeated branching.

Define glycogen.

A complex carbohydrate containing many α-glucose units joined by 1,4-glycosidic bonds with many 1,6-glycosidic bonds, giving it many side branches.