Topic 1.2 - Carbohydrates

Question 1

What are monosaccharides?

Answer 1

Monomers from which larger carbohydrates are made

Question 2

Give 3 common examples of monosaccharides

Answer 2

• Glucose
• Fructose
• Galactose

Question 3

What are disaccharides and how are they formed? (2)

Answer 3

• 2 monosaccharides joined together with a glycosidic bond
• Formed by a condensation reaction, releasing a water molecule

Question 4

Give 3 common examples of disaccharides

Answer 4

• Maltose
• Sucrose
• Lactose

Question 5

How is maltose formed?

Answer 5

Glucose + glucose

Question 6

How is sucrose formed?

Answer 6

Glucose + fructose

Question 7

How is lactose formed?

Answer 7

Glucose + galactose

Question 8

What are polysaccharides and how are they formed? (2)

Answer 8

1) Many monosaccharides joined together with glycosidic bonds

2) Formed by many condensation reactions, releasing water molecules

Question 9

What is the function of starch?

Answer 9

Energy store in plant cells

Question 10

What is the structure of starch? (3)

Answer 10

1) Polysaccharide of a-glucose

2) Amylose - 1,4-glycosidic bonds -> unbranched

3) Amylopectin - 1,4- and 1,6-glycosidic bonds -> branched

Question 11

What is the function of glycogen?

Answer 11

Energy store in animal cells

Question 12

What is the structure of glycogen? (2)

Answer 12

1) Polysaccharide made of a-glucose

2) 1,4- and 1,6- glycosidic bonds -> branched

Question 13

How does the structure of starch relate to it’s function? (3)

Answer 13

1) Helical -> compact / fit more molecules in a small area

2) Large, insoluble polysaccharide molecule

3) Insoluble in water -> water potential of cell not affected

Question 14

How does the structure of glycogen relate to its function? (4)

Answer 14

1) Branched -> compact / fit more molecules in a small area

2) Branched -> more ends for faster hydrolysis -> release glucose for respiration to make ATP for energy release

3) Large, insoluble polysaccharide molecule -> can’t leave cell / cross cell membrane

4) Insoluble in water -> water potential of cell not affected

Question 15

What is the basic function of cellulose?

Answer 15

Provides strength and structural support to plant / algal cell wall

Question 16

What is the structure of cellulose? (3)

Answer 16

1) Polysaccharide of B-glucose

2) 1,4-glycosidic bond -> straight, unbranched chains

3) Chains linked in parallel by hydrogen bonds forming microfibrils

Question 17

How does the structure of cellulose relate to its function? (4)

Answer 17

1) Every other B-glucose is inverted in a long, straight, unbranched chain

2) Many hydrogen bonds link parallel strands to form microfibrils

3) Hydrogen bonds are strong in high numbers

4) So provides strength to plant cell walls

Question 18

Give examples of reducing sugars (3)

Answer 18

1) Monosaccharides

2) Maltose

3) Lactose

Question 19

Describe the test for reducing sugars (3)

Answer 19

1) Add Benedict’s solution (blue) to sample

2) Heat in a boiling water bath

3) Positive result = green / yellow / orange / red precipitate

Question 20

Give an example of a non-reducing sugar

Answer 20

Sucrose

Question 21

Describe the test for non-reducing sugars (5)

Answer 21

1) Do Benedict’s test and stays blue / negative

2) Heat in a boiling water bath with acid

3) Neutralise with alkali

4) Heat in a boiling water bath with Benedict’s solution

5) Positive result = green / yellow / orange / red precipitate

Question 22

Suggest a method to measure the quantity of sugar in a solution (2)

Answer 22

1) Carry out Benedict’s test, then filter and dry precipitate

2) Find mass / weight

Question 23

Suggest another method to measure the quantity of sugar in a solution (6)

Answer 23

1) Make sugar solutions of known concentrations

2) Heat a set volume of each sample with a set volume of Benedict’s solution for same time

3) Use colorimeter to measure absorbance of each known concentration

4) Plot calibration curve - concentration on x axis, absorbance on y axis and draw line of best fit

5) Repeat Benedict’s test with unknown sample and measure absorbance

6) Read off calibration curve to find concentration associated with unknown sample’s absorbance

Question 24

Describe the biochemical test for starch (2)

Answer 24

1) Add iodine dissolved in potassium iodide and shake / stir

2) Positive result = blue / black