1.2 Carbohydrates

Question 1

What are monosaccharides?

Answer 1

• Monomers from which larger carbohydrates are made
• Glucose, fructose, galactose

Question 2

What is the difference between a-glucose and B-glucose?

Answer 2

• Isomers (same molecular formula but differently arranged atoms)
• OH group is below carbon 1 in a-glucose but above carbon 1 in B-glucose

Question 3

What are disaccharides?

Answer 3

• Two monosaccharides joined with a glucosidic bond
• Formed by condensation reaction, releasing a molecule of water
• Maltose, sucrose, lactose

Question 4

What are polysaccharides?

Answer 4

• Many monosaccharides joined with glycosidic bonds
• Formed by many condensation reactions, releasing many molecules of water
• Starch, glycogen, cellulose

Question 5

Starch

Answer 5

Energy store in plants
Polysaccharide of a-glucose
Coiled so compact, branched so more ends for faster hydrolysis, large so can’t cross cell membrane, insoluble in water so doesn’t affect water potential by osmosis

Question 6

Glycogen

Answer 6

Energy store in animal cells
Polysaccharide of a-glucose
Highly branched so more ends for faster hydrolysis, large so can’t cross cell membrane, insoluble in water so doesn’t affect water potential by osmosis

Question 7

Cellulose

Answer 7

Provides strength and structural support to plant/algal cell walls
Polysaccharide of B-glucose
Every other B-glucose inverted so long straight unbranched chain, many hydrogen bonds to form strong microfibrils, insoluble in water so doesn’t affect water potential by osmosis

Question 8

How do you test for reducing sugars?

Answer 8

• Add Benedict’s reagent
• Heat in boiling water bath
• Brick red precipitate if positive

Question 9

How do you test for non-reducing sugars?

Answer 9

• Benedict’s test stays blue
• Heat in boiling water bath with acid
• Neutralise with alkali
• Heat in boiling water bath with Benedict’s reagent
• Brick red precipitate if positive

Question 10

How can you measure quantity of sugar in a solution?

Answer 10

• Carry out Benedict’s test, filter and dry precipitate
• Find mass
OR
• Make dilution series of sugar solutions of known concentrations
• Heat set volume of each sample with set volume of Benedict’s reagent for same time
• Use colorimeter to measure absorbance
• Plot calibration curve with concentration on x axis and absorbance on y axis
• Repeat with unknown sample and read off calibration curve

Question 11

How do you test for starch?

Answer 11

• Add iodine dissolved in potassium iodide and shake
• Blue/black if positive