Polysaccarides

Starch and Glycogen

Learning Outcomes

Describe the formation and breakage of glycosidic bonds in the synthesis and hydrolysis of a polysaccharide.

Describe the structure of starch

Describe the structure of glycogen.

Explain how the structures relate to the functions

of starch and glycogen

Starch

• The most abundant storage chemical in plants

• The single largest provider of energy for the world’s population

Properties

• Compact

• Insoluble

• Readily accessible when needed(quickly hydrolysed)

Starch

• A mixture of two compounds

- AMYLOSE

- AMYLOPECTIN

How is Amylose made?

2 α-glucose molecules bond together Via a

Condensation reactionto form a

1,4-Glycosidic bondto make a

Disaccharidecalled

Maltose

Condensation reaction

O

C C

C C

C

CO

C C

C C

C

C

OHH OHH

Condensation reaction

O

C C

C C

C

CO

C C

C C

C

C

OH OH

Condensation reaction

O

C C

C C

C

CO

C C

C C

C

C

O

H2O

Condensation reaction

O

C C

C C

C

CO

C C

C C

C

C

O

disaccharide1,4 glycosidic bond

41

How is amylose made …

• The same condensation reaction is repeated over and over again to join thousands of glucose molecules together

to make

AMYLOSE

STRUCTURE OF AMYLOSE

COMPACT COIL

WHY?• BONDS ARE BROUGHT

TOGETHER AT A SLIGHT ANGLE when repeated many times, a spiral molecule is made

• Inward pointing H bonds stabilise the structure

AMYLOPECTIN

• Branched molecule – 1,6 glycosic bonds every so often cause side chains

• Molecule is a tightly packed and brush-like

• Can be broken down more quickly than amylose

Starch Insoluble store of glucose in plantsformed from two glucose polymers:

• Amylose

-glucose

1,4 glycosidic bonds

Spiral structure

• Amylopectin

-glucose

1,4 and some 1,6 glycosidic bonds

Branched structure

GLYCOGEN

• Major storage carbohydrate in animals

• Large molecule made of α-glucose units

• Can be broken down to release the glucose

• Like amylopectin in structure has many more branches though

• More compact than starch

• Stored in liver and muscle cells

Structure of Glycogen

Structure linked to functionBoth glycogen and starch….

• Are insoluble – will not bring about large movement of water by osmosis

• Compact – lots stored in a small space

• Easily broken down/hydrolysed at the ‘ends’ of the chains – to release glucose for respiration

Questions

1. Glycogen is more branched than amylopectin. Explain how this difference is important to animals

2. Why are glycogen and starch storage molecules whereas glucose is an energy source?

Answers

1. Animals have a higher /rapid demand for energy to support movement of muscles. Increased branching in glycogen means more ‘ends’ to release more glucose for respiration

2. Glycogen and starch must be broken down first before the glucose can be respired. Glucose can be respired immediately.