1.4 - Starch, glycogen and cellulose

Question 1

What is starch?

Answer 1

Starch is a polysaccharide that is found in many parts of a plant in the form of small grains.

Question 2

Where is starch the most abundant?

Answer 2

Large amounts occur in seeds and storage organs, such as potato tubers.

Question 3

What is starch used for?

Answer 3

It forms an important component of food and is the major energy source in most diets.

Question 4

What is starch made up from?

Answer 4

Starch is made up of chains of a-glucose monosaccharides linked by glycosidic bonds that are formed by condensation reactions.

Question 5

Can starch chains be branched?

Answer 5

The chains may be branched or unbranched. The unbranched chain is wound into a tight coil that makes the molecule very compact.

Question 6

How is starch suited for energy storage?

Answer 6

1. insoluble - doesn’t affect water potential - water not drawn into cell by osmosis
2. being large and insoluble, it does not diffuse out of cells
3. it is compact, so a lot of it can be stored in a small space
4. When hydrolysed it forms a-glucose, which is both easily transported and readily used in respiration
5. The branched form has many ends, each of which can be acted on by enzymes simultaneously meaning that glucose monomers are released very rapidly

Question 7

Is starch found in animal cells?

Answer 7

Starch is never found in animal cells. Instead a similar polysaccharide, called glycogen, serves the same role.

Question 8

What is Glycogen?

Answer 8

“Human Starch”

Question 9

Where is Glycogen found?

Answer 9

Glycogen is found in animals and bacteria but never in plant cells.

Question 10

How does the structure of glycogen differ to starch?

Answer 10

Glycogen is very similar in structure to starch but has shorter chains and is more highly branched.

Question 11

How is glycogen stored?

Answer 11

In animals it is stored as small granules mainly in the muscles and
the Liver.

Question 12

Why is the mass of glycogen stored relatively low?

Answer 12

The mass of carbohydrate that is stored is relatively small because fat is the main storage molecule in animals.

Question 13

How does the structure of Glycogen suit its role?

Answer 13

1. insoluble - does not tend to draw water into cells by osmosis
2. being insoluble, it does not diffuse out of cells
3. it is compact, so a lot of it can be stored in a small space
4. It is more highly branched than starch and so has more ends that can be acted on simultaneously by enzymes. It is therefore more rapidly broken down to form glucose monomers, which are used in respiration.

Question 14

Suggest why glycogen is hydrolysed faster than starch.

Answer 14

animals have a higher metabolic rate and therefore respiratory rate than plants because they are more active.

Question 15

How does cellulose differ to starch and glycogen?

Answer 15

Cellulose differs from starch and glycogen in one major respect: it is made of monomers of β-glucose rather than ⍺-glucose.

Question 16

How does β-glucose affect the structure of cellulose?

Answer 16

β-glucose forms long straight, unbranched chains

Question 17

Structure of cellulose.

Answer 17

β-glucose chains run parallel to one another, allowing hydrogen bonds to form cross-linkages
between adjacent chains. While each individual hydrogen bond
adds very little to the strength of the molecule, the sheer overall number of them makes a considerable contribution to strengthening cellulose, making it the valuable structural material that it is. The cellulose molecules are grouped together to form microfibrils which, in turn, are arranged in parallel groups called fibres.

Question 18

What is Cellulose?

Answer 18

Cellulose is a major component of plant cell walls and provides
rigidity to the plant cell.

Question 19

How does cellulose prevent the cell from bursting as water enters it?

Answer 19

The cellulose cell wall also prevents the cell from bursting as water enters it by osmosis. It does this by exerting
an inward pressure that stops any further influx of water. As a result, living plant cells are turgid and push against one another, making non-woody parts of the plant semi-rigid.

Question 20

Why is maintaining a turgid state important?

Answer 20

This is especially important in maintaining stems and leaves in a turgid state so that they can provide the maximum surface area for photosynthesis.

Question 21

Summarise the structure of cellulose

Answer 21

• cellulose molecules arc made up of β-glucose and so form long straight, unbranched chains
• these cellulose molecular chains run parallel to each other and are crossed linked by hydrogen bonds which add collective strength
• these molecules are grouped to form microfibrils which in turn are grouped to form fibres all of which provides yet more strength.