Biological Molecules: Carbohydrates

Question 1

Structure & Role of Carbohydrates

Answer 1

• Carbohydrates are organic molecules made from carbon, hydrogen and oxygen
• Commonly used as respiratory substrates and form structural components in plasma membranes and cell walls

Question 2

Monosaccharide

Answer 2

• The sugar monomers from which larger carbohydrates are made
• E.g glucose, fructose, galactose

Question 3

Glycosidic Bond

Answer 3

• A condensation reaction between two monosaccharides forms a glycosidic bond

Question 4

Disaccharides

Answer 4

• Molecules formed by the condensation of two monosaccharides
• E.g sucrose, maltose, lactose

Question 5

Molecular Structure of Glucose Isomers

Answer 5

• α-glucose
• OH on C-1 is below ring
• Down, up, down, down (OH)
• β-glucose
• OH on C-1 is above ring
• Down, up, down, up (OH)

Question 6

Polysaccharides: Examples & Monosaccharides Involved

Answer 6

• Molecules formed by the condensation of many monosaccharides
• Glycogen and starch are formed by the condensation of α-glucose
• Cellulose is formed by the condensation of β-glucose

Question 7

Sucrose

Answer 7

• Disaccharide made from glucose and fructose

- Found in sugar beet and sugar cane

Question 8

Maltose

Answer 8

• Disaccharide made from two alpha glucose molecules

- Found in germinating seeds

Question 9

Lactose

Answer 9

• Disaccharide made from galactose and glucose

- Found in milk and dairy

Question 10

Biochemical Test for Reducing Sugars

Answer 10

• Benedict’s reagent can be used to test for the presence of reducing sugars
• All monosaccharides and some disaccharides are reducing sugars
• These are therefore sugars that can donate an electron to the Benedict’s Reagent
• Gather food sample (liquid form)
• Add Benedict’s Reagent
• Heat the mixture gently in a water bath for five minutes
• If a reducing sugar is present, the solution should turn blue to brick red

Question 11

Biochemical Test for Non-Reducing Sugars

Answer 11

• Some disaccharides and all polysaccharides are non-reducing sugars
• Benedict’s Test must be altered to test for these
• Add food sample (liquid form) to Benedict’s reagent
• Place in water bath and gently warm for 5 minutes
• If the colour does not change from blue to brick red then a reducing sugar is not present
• Add part of same food sample and boil in hydrochloric acid. The dilute HCl will hydrolyse the disaccharides and polysaccharides into their constituent monosaccharides
• Add sodium hydrogencarbonate to neutralise the test tube as the Benedict’s Reagent will not work in acidic conditions. Add until solution stops fizzing
• The solution can now be retested by adding Benedict’s Reagent to the solution and placing in a water bath for 5 minutes
• If non reducing sugar is present then Benedict’s Reagent will turn blue to brick red

Question 12

Chemical Test for Starch

Answer 12

• Iodine/potassium iodide is used to test for starch
• Add to sample and mix
• If present, iodine solution will turn from orange/brown to blue/black