2.2.5 Polysaccharides

Question 1

Polysaccharide Structure

Answer 1

Starch, glycogen and cellulose are polysaccharides
Polysaccharides are macromolecules (polymers) that are formed by many monosaccharides joined by the glycosidic bonds in a condensation reaction to form chains
These chains can be:
- branched or unbranched
- folded (making the molecule compact which is ideal for storage e.g. glycogen)
- straight (making the molecules suitable to construct cellular structures e.g. cellulose) or coiled

Question 2

Starch Structure

Answer 2

Starch is constructed from two different polysaccharides:
Amylose (10%-30% of starch)
- unbranched helix-shaped chain with 1,4 glycosidic bonds between alpha glucose molecules
- the helix shape enables it to be more compact and thus it is more resistant to digestion
Amylopectin (70%-90% of starch)
- 1,4 glycosidic bonds between alpha glucose molecules but also 1,6 glycosidic bonds form between glucose molecules, creating a branched molecule

Question 3

Glycogen Structure

Answer 3

Glycogen is a polysaccharide found in animals
It is made up of alpha glucose molecules
- there are 1,4 glycosidic bonds between alpha glucose molecules and also 1,6 glycosidic bonds between glucose molecules, creating a branched molecule
Glycogen had a similar structure to amylopectin but it has more branches

Question 4

Summary of Storage Polysaccharides

Answer 4

Starch
Amylose:
Monomer = alpha glucose
Branched = no
Helix (coiled) shape = yes
Glycosidic Bonds Present = 1,4
Source = plant
Amylopectin:
Monomer = alpha glucose
Branched = yes (every 20 monomers)
Helix (coiled) shape = no
Glycosidic Bonds Present = 1,4 and 1,6
Source = plant
Glycogen
Monomer = alpha glucose
Branched = yes (every 10 monomers)
Helix (coiled) shape = no
Glycosidic Bonds Present = 1,4 and 1,6
Source = animal

Question 5

Cellulose Structure

Answer 5

Cellulose is a polysaccharide found in plants
It consists of long chains beta glucose joined together by 1,4 glycosidic bonds
Beta glucose is an isomer of alpha glucose, so in order to form the 1,4 glycosidic bonds consecutive beta glucose molecules must be rotated 180 degrees to each other
Due to the inversion of the beta glucose molecules, many hydrogen bonds form between the long chains, giving cellulose its strength

Question 6

Polysaccharide Function

Answer 6

Starch and glycogen are storage polysaccharides because they are:
- compact - so large quantities can be stored
- insoluble - so they will be no osmotic effect, unlike glucose which would lower the the water potential of a cell causing more water to move into cells

Question 7

Starch Function

Answer 7

Starch is the storage polysaccharide of plants
It is stored as granules in plastids such as amyloplasts and chloroplasts
- plastids are membrane-bound organelles that can be found in plant cells
- they have a specialised function e.g. amyloplasts store starch granules
Due to the many monomers in a starch molecules, it takes longer to digest that glucose
The amylopectin in starch has branches that result in many terminal glucose molecules that can be easily hydrolysed for use during cellular respiration or added form storage

Question 8

Glycogen Function

Answer 8

Glycogen is the storage of polysaccharides in animals and fungi
It is highly branched and not coiled
Liver and muscle cells have a high concentration of glucose, present as disable granules, as the cellular respiration rate is high in these cells (due to animals being mobile)
- glycogen is more branched than amylopectin, making it more compact which helps animals store more
- the branching enables more free ends where glucose can either be added or removed, allowing for condensation and hydrolysis reactions to occur more rapidly, thus the storage or release of glucose can suit the demands of the cell

Question 9

Cellulose Function

Answer 9

Cellulose is the main structural component of cell walls due to its strength which is a result of the many hydrogen bonds found between the parallel chains of microfibrils
The high tensile strength of cellulose allows it to be stretched without breaking which makes it possible for cell walls to withstand turgor pressure
The cellulose fibres and other molecules (e.g. lignin) found in the cell wall forms a matrix which increases the strength of the cell walls
The strengthened cell walls provide support to the plant
Cellulose fibres are freely permeable which allows water and solutes to leave or reach the cell surface membrane
As few organisms have the enzyme (cellulase) to hydrolyse cellulose, it is a source of fibre