1.2 Carbohydrates

Question 1

What are carbohydrates?

Answer 1

Carbohydrates are molecules which consist of only carbon, hydrogen and oygen. They are long chains of sugar units called saccharides

Question 2

What are monosaccharides?

Answer 2

Monosaccharides are the monomers from which larger carbohydrates are made

Question 3

Name the 3 hexose monosaccharides

Answer 3

Glucose
Fructose
Galactose

Question 4

Name the type of bond that forms when monosaccharides react

Answer 4

(1,4 or 1,6) glycosidic bond

Question 5

What are disaccharides?

Answer 5

Disaccharides are formed when a condensation reaction forms glycosidic bond between 2 monosaccharides

Question 6

Name the 3 disaccharides

Answer 6

Maltose: glucose + glucose
Sucrose: glucose + fructose
lactose: glucose + galactose

Question 7

Describe alpha glucose

Answer 7

Alpha glucose has the H molecule above

Question 8

Describe beta glucose

Answer 8

Beta glucose has the H molecule below

Question 9

What are polysaccharides?

Answer 9

Combining many monosaccharides results in the formation of a polysaccharide

Question 10

What polysaccharides are formed from alpha glucose?

Answer 10

Glycogen and starch

Question 11

What polysaccharide is formed from beta glucose?

Answer 11

Cellulose

Question 12

Describe the function of glycogen

Answer 12

Glycogen is the main storage polymer of alpha glucose in animal cells and is formed from many molecules of a-glucose joined together by 1,4 and 1,6 glycosidic bonds

Question 13

Describe the structure of glycogen

Answer 13

• Large number of side branches = terminal ends for hydrolysis
• Insoluble = no osmotic effect and does not diffuse out of cells
• Compact = maximising the amount of energy it can store

Question 14

Describe the function of starch?

Answer 14

Starch stores energy in plants and is a mixture of two polysaccharides: amylose and amylopectin

Question 15

Describe the structure of starch

Answer 15

• Insoluble = no osmotic effect on cells
• Large = does not diffuse out of cells

Question 16

Describe the structure of amylose

Answer 16

• Unbranched chain of glucose molecules joined by 1,4 glycosidic bonds
• Coiled, therefore very compact = storing lots of eneryg

Question 17

Describe the structure of amylopectin

Answer 17

• Made up of 1,4 and 1,6 glycosidic bonds
• Branched = many terminal ends for hydrolysis into glucose

Question 18

Describe the function of cellulose

Answer 18

Cellulose is a polymer of beta glucose which gives rigidity to plant cell walls (prevents bursting under turgor pressure, holds stems up)

Question 19

Describe the structure of cellulose

Answer 19

• Made from 1,4 glycosidic bonds
• Straight-chain, unbranched molecule
• H-bond crosslinks between parallel strands form microfibrils = high tensile strength

Question 20

Describe the Benedict’s test for reducing sugars

Answer 20

1. Add an equal volume of Benedict’s reagent to a food sample
2. Heat the mixture in an electric water bath at 100 degrees Celsius for 5 minutes
3. Positive result: colour change from blue to orange and brick-red precipitate forms

Question 21

Describe the Benedict’s test for non-reducing sugars

Answer 21

1. Negative results: Benedict’s reagent remains blue
2. Hydrolyse non-reducing sugars e.g. sucrose into their monomers by adding 1cm3 of HCl. Heat in a boiling water bath for 5 minutes
3. Neutralise the mixture using sodium carbonate solution
4. Proceed with the Benedict’s test as usual

Question 22

Describe the test for starch

Answer 22

1. Add iodine solution
2. Positive result: colour change from orange to blue-black

Question 23

Outline how colorimetry could be used to give qualitative results for the presence of sugars and starch

Answer 23

1. Make standard solutions with known concentrations. Record absorbance or % transmission values
2. Plot calibration curve: concentration (x-axis)
3. Record absorbance or % transmission values of unknown samples. Use calibration curve to read off concentration