Starch, Glycogen and Cellulose - 1.4

Question 1

What is starch?

Answer 1

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

Question 2

Where are especially large quantities of starch found?

Answer 2

In seeds and storage organs, such as potato tubers.

Question 3

Why is starch important?

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 of?

Answer 4

Chains of alpha-glucose monosaccharides linked by glycosidic bonds that are formed by condensation reactions.

Question 5

What is the main role of starch?

Answer 5

Energy storage.

Question 6

How is the structure of starch suited for energy storage?

Answer 6

• It is insoluble and therefore doesn’t affect water potential, so water is not drawn into the cells by osmosis.
• Being large and insoluble, it does not diffuse out of cells.
• It is compact, so a lot of it can be stored in a small space.
• When hydrolysed it forms alpha-glucose, which is both easily transported and readily used in respiration.
• 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

Where is starch never found?

Answer 7

Animal cells.

Question 8

What molecule serves the same role as starch but in animal cells?

Answer 8

Glycogen.

Question 9

Where is glycogen found?

Answer 9

In animals and bacteria.

Question 10

How does the structure of glycogen differ from that of starch?

Answer 10

Glycogen has shorted chains but is more highly branched.

Question 11

Why is glycogen referred to as ‘animal starch’?

Answer 11

It is the major carbohydrate storage product of animals.

Question 12

How is glycogen stored?

Answer 12

As small granules, mainly in the muscles and the liver.

Question 13

Why is the mass of carbohydrate stored in glycogen relatively small?

Answer 13

Fat is the main storage molecule in animals.

Question 14

How is the structure of glycogen suited for its storage?

Answer 14

• It is insoluble and therefore doesn’t tend to draw water into cells by osmosis.
• Being insoluble, it does not diffuse out of cells.
• It is compact, so a lot of it can be stored in a small space.
• It is more highly branched than starch and so has more ends that can be acted in simultaneously by enzymes. It is therefore more rapidly broken down to form glucose monomers, which are used in respiration. This is important to animals which have a higher metabolic rate and therefore respiratory rate than plants because they are more active.

Question 15

What is the major difference between starch/glycogen and cellulose?

Answer 15

Cellulose is made of monomers of beta-glucose rather than alpha-glucose.

Question 16

What is the structure of cellulose?

Answer 16

Straight, unbranched chains that run parallel to one another, allowing hydrogen bonds to form cross-linkages between adjacent chains.

Question 17

How does cellulose gain strength?

Answer 17

While each individual hydrogen bond (formed between parallel chains) 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.

Question 18

What are cellulose molecules grouped together to form?

Answer 18

Microfibrils which, in turn, are arranged in parallel groups called fibres.

Question 19

What is the role of cellulose?

Answer 19

It is a major component of plant cell walls and provides rigidity to the plant cell. It also prevents the cell from bursting as water enters it by osmosis.

Question 20

How does cellulose prevent cells from bursting as water enters by osmosis and what is the result of this?

Answer 20

It exerts 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 21

Why is maintaining stems and leaves in a turgid state important?

Answer 21

So they can provide the maximum surface area for photosynthesis.

Question 22

How is the structure of cellulose suited to its function?

Answer 22

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