1.2 Carbohydrates

Question 1

What is a carbohydrate

Answer 1

• Molecules which consist only of carbon, hydrogen and oxygen.
• They are long chains of sugar units called saccharides.

Question 2

What are a single monomer and pair of monomers of saccharides called

Answer 2

• A single monomer is a monosaccharide.
• A pair of monomers is a disaccharide.

Question 3

What bond joins monosaccharides together in a polysaccharide and what type of reaction is this

Answer 3

They are joined together with a glycosidic bond formed in a condensation reaction.

Question 4

Name 3 monosaccharides

Answer 4

• Glucose
• Fructose
• Galactose

Question 5

What is the general formula of a monosaccharide and what does the n represent

Answer 5

• (CH2O)n
• the n can be any number between 3 and 7

Question 6

What is glucose

Answer 6

• A monosaccharide containing 6 carbon atoms.
• It is the main substrate for respiration.
• It has 2 isomers - alpha glucose and beta glucose.

Question 7

Describe the difference between alpha and beta glucose

Answer 7

Alpha glucose has the hydroxyl group below carbon 1 wheres beta glucose had the hydroxyl group above carbon 1.

Question 8

How are disaccharides formed

Answer 8

Two monosaccharides can join together in a condensation reaction to form a disaccharide. This produces a molecule of water and can be reversed by a hydrolysis reaction.

Question 9

How is Maltose formed

Answer 9

By condensation of 2 glucose molecules.

Question 10

How is Sucrose formed

Answer 10

By condensation of a glucose molecule and a fructose molecule.

Question 11

How is Lactose formed

Answer 11

By condensation of a glucose molecule and a galactose molecule

Question 12

How are polysaccharides formed

Answer 12

From many glucose units joined together.

Question 13

How are glycogen and starch formed

Answer 13

By the condensation of alpha glucose.

Question 14

Describe the structure and function of glycogen

Answer 14

• Main energy storage molecule in animals
• Formed from many alpha glucose molecules joined together by 1-4 and 1-6 glycosidic bonds.
• Has a large number of side branches meaning energy can be released quickly as enzymes can act simultaneously on these branches.
• It is a relatively large but compact molecule which maximises the amount of energy it can store.
• It is insoluble which means it will not affect the water potential of cells and cannot diffuse out of cells.

Question 15

How is cellulose formed

Answer 15

By the condensation of beta glucose

Question 16

Describe the structure and function of cellulose

Answer 16

• A component of cell walls in plants and is composed of long, unbranched chains of beta glucose which are joined by glycosidic bonds.
• It is important in stopping the cell wall from bursting under osmotic pressure.
• This is because it exerts inward pressure that stops the influx of water
• This allows cells to stay turgid and rigid, helping maximise the surface area of plants for photosynthesis.

Question 17

Which two polysaccharides form starch

Answer 17

• Amylose
• Amylopectin

Question 18

Describe the structure of amylose

Answer 18

• An unbranched chain of glucose molecules joined by 1-4 glycosidic bonds.
• This causes amylose to be coiled, so is a very compact molecule storing a lot of energy.

Question 19

Describe the structure of amylopectin

Answer 19

• A branched chain made of glucose molecules joined by 1-4 and 1-6 glycosidic bonds.
• Side branches can be acted on simultaneously by many enzymes, so can be broken down to release energy.

Question 20

How are the structures of glycogen and amylopectin different and why is this necessary

Answer 20

• Glycogen is more branched, making it more compact and has more terminal glucose molecules.
• This means glycogen can be hydrolysed and converted back into glucose at a quicker rate than amylopectin.
• This is necessary animals are more mobile, so they have a higher rate of respiration as they need energy quicker.

Question 21

What are some key properties of starch

Answer 21

• It’s insoluble so will not affect cell water potential.
• It’s compact so a lot of energy can be stored in a small space
• When it is hydrolysed the released alpha glucose can be transported easily.

Question 22

What is a reducing sugar + some examples

Answer 22

• Sugars that can donate an electron to the Benedict’s reagent (an alkaline solution of copper II sulfate)
• All monosaccharides are reducing sugars and some disaccharides (maltose and galactose)

Question 23

What is the biochemical test for reducing sugars

Answer 23

1. Add 2cm3 of the food sample to be tested to a test tube.
2. Add 2cm3 of Benedict’s reagent.
3. Heat the mixture gently in a water bath for 5 minutes. If the solution turns yellow/green/brick red then a reducing sugar is present and it is a positive result.

Question 24

What is the biochemical test for non-reducing sugars

Answer 24

1. 2cm3 of food sample is added to 2cm3 of benedict’s reagent.
2. If the colour does not change from blue to yellow/green/brick red then a reducing sugar is not present.
3. Another 2cm3 of the same food sample is added to 2cm3 of dilute hydrochloric acid. The test tube is then placed in a water bath for 5 minutes. The dilute HCl will hyrdrolyse the disaccharides and polysaccharides into their constituent monosaccharides.
4. After, sodium hydrogencarbonate is added to neutralise the test tube (benedict’s wont work in acidic conditions) and pH paper is used to check the solution is neutralised.
5. The solution can now be tested again as a reducing sugar.

Question 25

What is the biochemical test for starch

Answer 25

1. Add iodine to a food sample. 2. If starch is present, the solution will turn from orange/brown to blue/black.