Biology- Chapter 1.4(Starch, Glycogen, Cellulose)

What is Starch?

It is a polysaccharide found in many parts of a plant in the form of small grains

Monomer of Starch

Alpha glucose

Bond/s in starch

Glycosidic bonds

Structure of Starch

Large polymer

The chains may be branched or unbranched;

The unbranched chain is wound into a tight coil, making the molecule very compact

Where is starch found?

• Plastids of flowering plants

• Storage structures

• Green chloroplasts

What are the 2 types of starch

• Amylose

• Amylopectin

Structure of Amylose

1,4 glycosidic bond 

What is amylose?

A relatively long, linear polysaccharide composed of glucose units linked by α-1,4 glycosidic bonds.

It is coiled like a spring and is less soluble in water than amylopectin. 

Amylopectin

A highly branched polysaccharide where glucose units are linked by α-1,4 bonds in the main chains

With additional α-1,6 glycosidic bonds creating branch points. 

Function(s) of Starch

• Energy Storage 

• A source of organic carbon

5 reasons starch is suitable for energy storage

• It is insoluble and therefore doesn’t affect water potential and does not diffuse out of cells

• It is compact, therefore it can be stored in a small space

• It forms alpha glucose when hydrolysed, which is both easily transported and used in respiration 

• The branched form (amylopectin) has many ends which can each be acted on by enzymes simultaneously which allows glucose monomers to be released rapidly

Monomer of cellulose

Beta glucose

Bond/s in Cellulose

Beta 1-4 glycosidic bonds

Hydrogen bonds between the cellulose chains

Structure of Cellulose

• Has straight unbranched chains which run parallel to each other

• Hydrogen bonds form cross linkages between adjacent chains 

• The large number of these hydrogen bonds are what strengthen cellulose

• The cellulose chain has adjacent glucose molecules rotated by 180 degrees.

• This allows the hydrogen bonds to be formed between the hydroxyl groups on adjacent parallel chains that help to give cellulose its structural stability

Functions of cellulose

• A major component of plant cell walls providing rigidity to the cell

• Prevents the cell from bursting as water enters the cell via osmosis by exerting inward pressure preventing any further influx of water

• As a result, non woody parts of the plant are semi rigid. 

• Important in maintaining stems and leaves in the turgid state to maximise surface area for photosynthesis

Where is cellulose found?

The cell wall of plant cells

3 Reasons cellulose is suitable for its function of providing support and rigidity

• Cellulose molecules are made up of beta glucose, forming long, straight unbranched chains

• These cellulose molecular chains run parallel to each other and are cross linked by hydrogen bonds which add collective strength

• These molecules are then grouped to form microfibrils which are then grouped to form fibres with more strength

Monomer of glycogen

Alpha glucose

Bond/s in glycogen

1-4 glycosidic bonds with extra 1-6 glycosidic bonds between the branches

Structure of glycogen

• Similar to starch (amylopectin) however, it has shorter chains and is much more branched (has more (has more frequent branching)

• Less dense and more soluble than starch. 

Function of glycogen

• A storage polysaccharide found in animals which can be rapidly broken down to produce glucose monomers

Where is glycogen found?

• In animals and bacteria

• In animals; it is found in small granules in liver and muscle cells

Glycogen is suitable for its function because…

• It is insoluble and therefore does not tend to draw water in or out of the cell due to osmosis and due to it being insoluble it does not diffuse out of cells

• It is compact, therefore it can be stored in a small space

• It is more highly branched than starch therefore it has more ends which can be simultaneously acted on by enzymes

• It is therefore more rapidly broken down into glucose monomers. This is important to animals which have a higher metabolic rate than plants.