3.1.2 - CARBOHYDRATES

Question 1

raffinose is a trisaccharide of three monosaccharides: galactose, glucose, and fructose. the chemical formulae of these monosaccharides are:

• galactose: C₆H₁₂O₆
• glucose: C₆H₁₂O₆
• fructose: C₆H₁₂O₆

give the number of carbon, hydrogen and oxygen atoms in one molecule of raffinose. (1)

Answer 1

carbon = 18
hydrogen = 32
oxygen = 16

Question 2

describe a biochemical test to show that a solution contains a non-reducing sugar. (3)

Answer 2

1. heat with hydrochloric acid and neutralise
2. then heat with Benedict’s solution
3. the formation of a red precipitate indicates the presence of the non-reducing sugar

Question 3

suggest a method, other than using a colorimeter, to measure the quantity of reducing sugar in a solution. (2)

Answer 3

1. filter and dry the precipitate
2. find the mass

Question 4

using a colorimeter would improve the repeatability of the results. give one reason why. (1)

Answer 4

1. colour change is subjective
2. so using a colorimeter standardises the method

Question 5

lactulose is a disaccharide formed from one molecule of galactose and one molecule of fructose.

other than both being disaccharides, give one similarity and one difference between the structures of lactulose and lactose. (2)

Answer 5

similarity: both contain galactose
difference: lactulose contains fructose, whereas lactose contains glucose instead.

Question 6

glycogen and cellulose are both carbohydrates.
describe two differences between their structures. (2)

Answer 6

any two:

1. cellulose is made up of beta-glucose monomers and glycogen is made up of alpha-glucose monomers
2. cellulose molecule is a straight chain and glycogen is branched.
3. cellulose molecule has a straight chain and glycogen is coiled.
4. glycogen has 1, 4 and 1, 6 glycosidic bonds and cellulose has only 1, 4 glycosidic bonds

Question 7

starch is a carbohydrate often stored in plant cells.

describe and explain two features of starch that make it a good storage molecule. (2)

Answer 7

any two:

1. insoluble in water, so doesn’t affect water potential
2. branched/coiled so the molecule is compact
3. polymer of alpha-glucose so it provides glucose for respiration
4. branched so more ends for enzyme action (fast breakdown of glucose)
5. large molecule so can’t cross the cell membrane

Question 8

which test would be used to show the presence of starch (1)

Answer 8

iodine/potassium iodide tewst

Question 9

describe the structure of glycogen (2)

Answer 9

1. polysaccharide made up of alpha-glucose monomers joined by glycosidic bonds.
2. branched structure

Question 10

during early pregnancy, the glycogen in the cells lining the uterus is an important energy source for the embryo.

suggest how glycogen acts as a source of energy. (2)

Answer 10

1. hydrolysed into glucose.
2. glucose used in respiration

Question 11

name the monomers from which a maltose molecule is made (1)

Answer 11

glucose and glucose

Question 12

name the monomers from which a lactose molecule is made (1)

Answer 12

glucose and galactose

Question 13

name the monomers from which a sucrose molecule is made (1)

Answer 13

glucose and fructose

Question 14

name the type of chemical bond that joins the two monomers to form maltose (1)

Answer 14

glycosidic bond

Question 15

explain the difference in the chemical structure of a starch molecule and the cellulose molecule (2)

Answer 15

1. starch is formed from alpha-glucose but cellulose formed from beta-glucose
2. position of the hydrogen and hydroxyl groups on carbon atom 1 are inverted

Question 16

explain one way in which starch molecules are adapted for their function in plant cells (2)

Answer 16

any one:

1. insoluble in water, so doesn’t affect water potential
2. helical structure, so it is compact
3. large molecule, so cannot leave cell

Question 17

explain how cellulose molecules are adapted for their function in plant cells (3)

Answer 17

1. long and straight chains
2. become linked together by many hydrogen bonds to forms fibrils
3. provide strength to the cell wall

Question 18

give two ways the structure of starch is similar to cellulose (2)

Answer 18

1. both are polysaccharides made up of monosaccharides
2. both contain glucose
3. contain glycosidic bonds (1, 4)
4. hydrogen bonding withing each of their structures

Question 19

give two ways the structure of starch is different to cellulose (2)

Answer 19

1. starch contains alpha-glucose, not beta-glucose
2. starch is branched, not straight
3. also contain 1, 6 glycosidic bonds
4. no hydrogen bonds between the molcules
5. no fibrils