3.1.2 Carbohydrates

Question 1

What are monosaccharides?

Answer 1

Monomers from which larger carbohydrates are made. They are soluble

Question 2

What are three examples of monosaccharides?

Answer 2

Glucose, galactose, fructose

Question 3

How is a glycosidic bond formed?

Answer 3

Through a condensation reaction between two monosaccharides

Question 4

What are disaccharides?

Answer 4

Formed by the condensation of two monosaccharides

Question 5

What is maltose?

Answer 5

A disaccharide formed by condensation of two glucose molccules

Question 6

What is sucrose?

Answer 6

A disaccharide formed by condensation of a glucose molecule and a fructose molecule

Question 7

What is lactose?

Answer 7

A disaccharide formed by condensation of a glucose molecule and a galactose molecule

Question 8

What are the two glucose isomers?

Answer 8

α-glucose, β-glucose

Question 9

How does α-glucose and β-glucose differ?

Answer 9

α-glucose - H above, OH below
β-glucose - H below, OH above

Question 10

What are polysaccharides?

Answer 10

Formed by the condensation of many monosaccharide units. They are insoluble

Question 11

Which polysaccharides are formed by the condensation of α-glucose?

Answer 11

Glycogen, starch

Question 12

Which polysaccharides are formed by the condensation of β-glucose

Answer 12

Cellulose

Question 13

Why is dilute hydrochloric acid added into food sample of non-reducing sugar?

Answer 13

It will hydrolyse any disaccharide present into its constituent monosaccharides by breaking its glycosidic bonds

Question 14

Why is sodium hydrogen carbonate solution (NaHCO3) added to hydrochloric acid (HCl)?

Answer 14

Benedict’s reagent will not work in acidic conditions - needs to be neutralised

Question 15

What is the test for reducing sugar?

Answer 15

1.Combine equal quantities of test solution and Benedict’s reagent in a test tube
2. Heat in water bath at 80°C
3. Observe change in colour and opacity of solution - indicates concentration of reducing sugar: Green(least), Yellow, Orange, Brick Red(most)

Question 16

Why is non-reducing sugar not detected by Benedict’s reagent?

Answer 16

They do not change colour of Benedict’s reagent as they have a lack of reducing parts - used in making glycosidic bond

Question 17

How to detect non-reducing sugars?

Answer 17

It must be hydrolysed into its monosaccharide components (reducing sugar) by hydrolysis as they do not change the colour of Benedict’s reagent

Question 18

When do you do a non-reducing sugar test?

Answer 18

When results for Benedict’s reagent is negative

Question 19

What is the test for non-reducing sugar?

Answer 19

1. Hydrolyse the glycosidic bond with warm dilute hydrochloric acid
2. Neutralise with sodium hydrogen carbonate
3. Perform a Benedict’s test again

Question 20

What is the structure of amylose?

Answer 20

A straight chain, unbranched polymer of glucose units bound by (α1-4) glycosidic bond

Question 21

What are the main two components of starch?

Answer 21

Amylose and amylopectin

Question 22

Which type of glucose is amylose and amylopectin made of?

Answer 22

α-glucose

Question 23

What secondary structure does amylose form?

Answer 23

α-helices

Question 24

What is the structure of amylopectin?

Answer 24

A branched chain polymer with α1-4 glycosidic bonds, and occasional α1-6 bonds in order to create branches

Question 25

Why is the rate of hydrolysis of amylopectin faster?

Answer 25

It has branches which increases surface area

Question 26

What is the test for starch?

Answer 26

1. Add iodine/ potassium iodide 2. Observe change in colour: Yellow to Blue/Black

Question 27

How does iodine indicate the presence of starch?

Answer 27

Iodine passes through the helix structure of amylose, which change the chemical properties of the helix, causing a different colour of light to reflect

Question 28

What is the structure of glycogen?

Answer 28

It is a macro molecule with 1-4 and 1-6 glycosidic bonds from α-glucose

Question 29

What is the difference in structure between starch and glycogen?

Answer 29

Glycogen has more frequent branching than starch - more surface area as demand in energy is more in animals for movement

Question 30

What is the structure of cellulose?

Answer 30

It is a straight chain polymer of β-glucose monomers

Question 31

How does fibres formed from cellulose give plant cell wall its strength

Answer 31

Cellulose molecules join to make microfibres through hydrogen bonds Microfibres group into fibres The arrangement of OH group of β-glucose on either side of chain allows for hydrogen bonds to form between adjacent chains above and below All the hydrogen bonds have collective strength

Question 32

How does cellulose make plant cell walls permeable?

Answer 32

Fibres are laid down in layers which are arranged at 90°to form sheet-like structure - cross-hatches which they are holes between chains