2.2. 3-5 Carbohydates

Question 1

What is a Carbohydrate?

Answer 1

a group of molecules containing C, H, and O

Question 2

What is a glycosidic bond

Answer 2

a bond formed between two monosaccharides by a condensation reaction

Question 3

What are the functions of a Carbohydrate?

Answer 3

1. Source of energy (e.g glucose)
2. Store of energy (e.g starch & glycogen)
3. Structural units (e.g cellulose in plants and chitin in insects)
4. Part of other molecules such as Nucleic Acids

Question 4

What are Monosaccharides?

Answer 4

1. Simplest carbohydrates
2. Important for the source of energy
3. Soluble in water
4. Exist in straight chains or in rings or in cyclic forms
5. Glucose can exist as a number of different isomers.
   - H and -OH can be revered in straight chains.
6. Two isomers of glucose αlpha and βeta

Question 5

What are Disaccharides

Answer 5

1. Like monosaccharides they are soluble
2. Most common types are:
   - Maltose, Lactose (Reducing Sugars)
   - Sucrose (non- reducing sugar)
3. Two hydroxyl groups line up next to each other, from which water is removed.

Question 6

[Disaccharides]

α-glucose + α-glucose —>

Answer 6

maltose

Question 7

[Disaccharides]

α-glucose + fructose —>

Answer 7

sucrose

Question 8

[Disaccharides]

β-galactose + α-glucose —>

Answer 8

lactose

Question 9

[Disaccharides]

β-glucose + β-glucose

Answer 9

Cellobiose

Question 10

[α-glucose] What is the molecular formula, the role and the type of sugar

Answer 10

Molecular Formula: C6H12O6
Role in the body: Energy source, Component of starch and glycogen which act as energy stores
Type of sugar: Hexose

Question 11

[β-glucose] What is the molecular formula, the role and the type of sugar

Answer 11

Molecular Formula: C6H12O6
Role in the body: Energy source, Component of cellulose, which provides structural support in plant cell walls.
Type of sugar: Hexose

Question 12

[Ribose] What is the molecular formula, the role and the type of sugar

Answer 12

Molecular Formula: C5H10O5
Role in the body: Component of RNA, ATP, NAD
Type of sugar: Pentose

Question 13

[deoxyRibose] What is the molecular formula, the role and the type of sugar

Answer 13

Molecular Formula: C5H10O4
Role in the body: Component of DNA
Type of sugar: Pentose

Question 14

What are homopolysacchardies?

Answer 14

Polysaccharides made solely of one kind of monosaccharide e.g starch

Question 15

What happens when you join lots of glucose molecules together into polysaccharides

Answer 15

1. You create a store of energy.
2. Plants store energy as starch in chloroplasts and in membrane bound starch grains
3. Humans store energy as glycogen in cells of muscles and livers

Question 16

Why are polysacchardies good energy stores?

Answer 16

1. Glycogen and starch are compact, which means they do not occupy a large amount of space. —> Both occur in dense granules within cells
2. Polysaccharides hold glucose molecules in chains, so they can easily be snipped off from the end of the chains by hydrolysis when required for respiration
3. Some chains are unbranched (amylose) and some are branched (amylopectin and glyocgen)
   - Branched chains = More compact but also offer the chance for lots of glucose molecules to be snipped off by hydrolysis at the same time, when lots of energy is required quickly.
4. Polysaccharides are less soluble in water than monosaccharides. —> Due to size but also regions which could hydrogen bonds are hidden away in the molecule. Sometimes the amylose molecule may form a double helix which presents a hydrophobic external surface in contact with surrounding solution

Question 17

What is enzyme Amylase responsible for?

Answer 17

Hydrolysing 1-4 glycosidic linkages

Question 18

What is the enzyme glycosidic responsible for?

Answer 18

Hydrolysing 1-6 glycosidic linkages

Question 19

What is Cellulose?

Answer 19

1. Found in plants
2. Forms the cell walls
3. It is a Tough, insoluble and fibrous substance

Question 20

Why doesn’t Cellulose spiral like chains of α-glucose?

Answer 20

1. Hydrogen and hydroxyl groups on Carbon 1 are inverted in β-glucose. —> Every β-glucose molecule in the chain is rotated by 180°. —> Prevents from spiralling
2. Hydrogen bonding between the rotated β-glucose gives the chain additional strength, and stops it spiralling
3. Hydroxyl group on carbon 2 sticks out, enabling hydrogen to be formed between chains –> proves additional whole structure strength

Question 21

How many cellulose chains are required to form microfibrils?

Answer 21

60-70 which then bundle together to make macrofibrils containing up to 400 microfibrils

Question 22

Why is cellulose an excellent material for plant cell walls?

Answer 22

1. Microfibrils and macrofibrils have very high tensile strength, both because of the strength of glycosidic bonds and hydrogen bonding between chains. —> Macrofibrils are stronger than steel wire of the same diameter
2. Macrofibrils run in all directions, criss-crossing the wall for extra strength
3. It is difficult to digest cellulose because glycosidic bonds between the glucose molecules are less easy to break.

Question 23

Why do features of the Cellulose help the plant cell wall do its job?

Answer 23

1. Because plants do not have a rigid skeleton, each cell needs to have strength to support the whole plant
2. There is a space between macrofibrils for water and mineral ions to pass on their way into and out of the cell. This makes the cell wall fully permeable.
3. High tensile strength —> Prevent cell from bursting
4. Macrofibril structure can be reinforced with other substances for extra support or to make walls water proof. E.g Lignin

Question 24

Where else can Cellulose be used?

Answer 24

Cotton is 90% cellulose

Cellophane and clelluloid are also derived from cellulose

Question 25

What is the bacteria cell wall made out of?

Answer 25

Peptidoglycan —> cross linked by short peptide chains (amino acids)

Question 26

What are exoskeletons of insects made out of?

Answer 26

Chitin, differs from cellulose as it has a acetylamino group rather than a hydroxyl group on carbon 2.