Carbohydrates - Polysaccharides

What are Polysaccharides?

Polysaccharides are complex carbohydrates made up of many monosaccharides joined via glycosidic bonds

What are some examples of polysaccharides?

• Starch

• Glycogen

• Cellulose

What is starch?

Starch is an example of a polysaccharide used by plants to store excess glucose.

This means that starch can be hydrolysed back into glucose when plants require energy.

How/ what is starch made up of?

Its made up of many alpha-glucose monomers joined via glycosidic bonds to form chains.

What are the 2 forms ( and their percentages) that Starch chains can come in

1.) Amylose - 20%

2.) Amylopectin - 80%

What is Amylose and its structure

It’s a long, unbranched chain of alpha- glucose joined by 1-4 glycosidic binds.

The angles of these bonds cause the chain to coil into a helix to make a compact structure

What’s the estimate of the number of alpha glucose units in 1 amylose molecule ?

There’s 200 - 500 alpha glucose units

What causes the shape of amylose?

the bonds lie below the rings, so the molecules naturally coils into a spiral, held together by hydrogen bonds

What is Amylopectin and its structure?

It’s a long, branched chain of alpha- glucose joined by both 1-4 and 1-6 glycosidic bonds.

Its side branches allows enzymes to hydrolyse alpha-glucose monomers easily

What’s the estimated number of alpha glucose units in 1 molecule of amylopectin?

one amylopectin molecule contains 5000 - 100,000 alpha glucose units

Features of starch that helps it to function as a energy STORE: Solubility & Reactions

• Insoluble - It does not affect the water potential, so water does not enter cells by osmosis.

• Hydrolysis releases alpha-glucose monomers - These are readily used in respiration.

Features of starch that helps it to function as a energy STORE: Size

Large - It cannot diffuse out of cells.

Features of starch that helps it to function as a energy STORE: Structure

Many side branches - These allow enzymes to hydrolyse the glycosidic bonds easily to rapidly release glucose.

Features of starch that helps it to function as a energy STORE: Shape

Coiled - This makes it compact so that a lot of glucose can be stored in a small space

What is Glycogen?

Glycogen is an example of a polysaccharide used by animals to store excess glucose.

This means that glycogen can be hydrolysed back into glucose when animals require energy.

Glycogen is very similar to starch, but it is used by animals rather than plants.

How is Glycogen formed/ what’s its structure?

Glycogen is made up of many alpha-glucose monomers joined via 1-4 and 1-6 glycosidic bonds to form highly branched chains. (Mainly amylopectins)

Features that allow glycogen to function as a energy STORE: Solubility & Reactions

• Insoluble - It does not affect the water potential of cells, and so water does not enter cells by osmosis.

• Hydrolysis releases alpha-glucose monomers - These are readily used in respiration.

Features that allow glycogen to function as a energy STORE: Size

• Large - It cannot diffuse out of cells.

• Compact - A lot of glucose can be stored in a small space

Features that allow glycogen to function as a energy STORE: Structure

• More highly branched than starch - Enzymes can easily hydrolyse the glycosidic bonds to rapidly release glucose.

What is cellulose?

Cellulose is a polysaccharide formed from beta-glucose.

Primary use of cellulose

To provide structural support for plant cell walls.

How is cellulose formed?

Cellulose is made up of many beta-glucose monomers joined together via glycosidic bonds.

What is the problem with bonding two beta-glucose molecules together?

• If two beta-glucose monomers line up next to each other, the hydroxyl groups (OH) on carbon 1 and carbon 4 are too far from each other to react.

What’s the solution to opposite hyrdoxyl groups on the beta glucose?

Every other beta-glucose molecule is inverted by 180° (flipped upside down). This brings the hydroxyl groups (OH) close enough together to react.

What happens when many beta-glucose molecules are bonded together?

They form long, straight, unbranched chains.

What is the general order of a straight, ubranched chain of a beta glucose molecule?

• Every other glucose unit has to be turned upside down (first is right side up, the other is upside down).

• The glycosidic bonds alternate above and below the ring therefore the molecule doesn’t coil like starch but forms a straight chain.

What provides the great strength to cellulose as a whole?

The hydrogen bonds - Although each hydrogen bond itself is relatively weak, the huge number of these bonds provides great strength to cellulose as a whole.

Where exactly do these hydrogen bonds form (i.e on which carbon specifically)?

Forms between the partially negative Oxygen on carbon 3 on one chain and the partially positive hydrogen on carbon 6 of another. This provides strength

What is the straight chain supported by?

It’s supported by hydrogen bonds between the oxygen atom in the ring and the O-H on carbon 3 of another glucose unit. As a result, it allows many chains to lie parallel.

What are Microfibrils?

They are bundles of (60 -70) cellulose molecules. So, multiple cellulose chains become tightly cross linked via hydrogen bonds to form bundles called Microfibrils.

When Microfibrils join together, what do they make?

Macrofibrils aka cellulose fibers. (400 microfibrils form macrofibrils)

What are cell walls made of and what holds them together?

They are made of several layers of fibres running in all directions

Pectin holds he fibres together

Adaptations of cellulose for its role - Numerous hydrogen bonds

These allow for high tensile strength to be achieved as plant cell walls need to be able to withstand high pressure without breaking or stretching.

• these bonds form between parallel molecules due to the fact that they are straight.

Adaptations of cellulose for its role - Long, straight and unbranched chains

These provide rigidity to the cell wall.

Adaptations of cellulose for its role - Microfibrils

Provide additional strength

Adaptations of cellulose for its role - Digestion difficulty

The beta-glucose molecules are difficult to digest because of the different orientation of the bonds.

Adaptations of cellulose for its role - Gaps between Macrofibrils

This allows for the cell wall to be fully permeable.