Biological Molecules - Carbohydrates

Question 1

What are monomers?

Answer 1

• Small units which are the components of larger molecules
• They are joined by a chemical bond in a condensation reaction whereby a water molecule is eliminated
• E.g. monosaccharides such as glucose, amino acids and nucleotides

Question 2

What are polymers?

Answer 2

• Molecules made from many monomers joined together
• Polymers are hydrolysed to form monomers when water is added

Question 3

What are carbohydrates?

Answer 3

• Molecules which consist onlt of carbon, hydrogen and oxygen
• They are long chains of sugar units called saccharides

Question 4

What are monosaccharides, disaccharides and monosaccharides?

Answer 4

• A single monomer, e.g. glucose, galactose and fructose
• A pair of monomers is called a disaccharide
• Combining many monosaccharides results in the formation of polysaccharides
• These are all joined together with a glycosidic bond formed in a condensation reaction

Question 5

What features do monosaccharide have?

Answer 5

• Typically sweet tasting
• Soluble
• Have a general formula of (CH2O)n, n= 3-7

Question 6

What monomers are maltose made from?

Answer 6

• Maltose is a disaccharide formed by condensation of two glucose molecules

Question 7

What monomers are sucrose made from?

Answer 7

• Sucrose is a disaccharide formed by condensation of glucose and fructose

Question 8

What monomers are lactose made from?

Answer 8

• Lactose is a disaccharide formed by condensation of glucose and galactose

Question 9

What are polysaccharides?

Answer 9

• Formed from many glucose units joined together
• Glycogen and starch are both formed by the condensation of α-glucose
• Cellulose formed by the condensation of β-glucose

Question 10

What is glycogen?

Answer 10

• Main energy storage molecule in animals
• Formed from many molecules of α-glucose joined together by 1,4 and 1,6 glycosidic bonds
• Has a large number of side branches meaning that energy can be released quickly as enzymes can act simultaneously on these branches
• Large but compact thus maximising the amount of energy it can store
• Insoluble so doesn’t affect the water potential of cells and cannot diffuse out

Question 11

What are the roles and features of starch?

Answer 11

• Stores energy in plants and is a mixture of two polysaccharides called amylose and amylopectin
• It is insoluble so will not affect cell water potential
• It is helical so compact and can store lots of energy in a small space
• When it is hydrolysed the released α-glucose can be transported easily

Question 12

What is amylose and amylopectin?

Answer 12

• Amylose is an unbranched chain of glucose molecules joined by 1,4 glycosidic bonds, and as a result is coiled and thus compact
• Amylopectin is branched and is made up of glucose molecules joined by 1,4 and 1,6 glycosidic bonds

Question 13

What are the roles and features of cellulose?

Answer 13

• It is a component of cell walls in plants and is composed of long, unbranched chains of β-glucose which are joined by glycosidic bonds
• Microfibrils are strong threads which are made of long cellulose chains running parallel to one another that are joined by hydrogen bonds forming strong cross linkages
• Stops the cell wall from bursting under osmotic pressure, so can stay turgid and rigid

Question 14

State and describe the biochemical test for reducing sugars (all monosaccharides and some disaccharides)

Answer 14

• Benedict’s test
• Add 2cm3 of the food sample to be tested (needs to be in liquid form)
• Add 2cm3 of Benedict’s Reagent
• Heat the mixture gently in a water bath for 5 minutes
• If the solution turns brick red then a reducing sugar is present and it is a positive result

Question 15

State and explain the biochemical test for non-reducing sugars (some disaccharides and polysaccharides)

Answer 15

• Carry out test for reducing sugars
• Add another 2cm3 of the same food sample and 2cm3 of dilute hydrochloric acid
• The test tube is then placed in a water bath for 5 minutes
• The dilute HCl will hydrolyse the disaccharides and polysaccharides into their constituent monosaccharides
• Some sodium hydrocarbonate is added to neutralise the test tube as Benedict’s Reagent will not work in acidic conditions
• Add 2cm3 of Benedict’s Reagent to the solution and place in a water bath for 5 minutes
• If a non-reducing sugar is present in the original sample then a colour change from the blue Benedict’s Reagent to brick red will be observed