1.1 carbohydrates

Question 1

explain why water is considered a polar molecule.

Answer 1

• a molecule with an uneven distribution of charge is described as ‘polar’.
• water is considered a polar molecule because the positively charged hydrogen atoms within a water molecule attract the negatively charged oxygen atoms, forming a hydrogen bond and weak electrostatic attraction.

Question 2

what is a condensation reaction?

Answer 2

a condensation reaction refers to a type of chemical reaction in which two monomers combine to form a polymer, producing water as a by product.

Question 3

what is a hydrolysis reaction?

Answer 3

• a hydrolysis reaction refers to a type of chemical reaction where the covalent bonds in a polymer are broken due to the addition of a water molecule.
• hydrolysis reactions break down polymers into their monomer components.

Question 4

polynucleotides, such as nucleic acids, are composed of which monomers?

Answer 4

polynucleotides are composed of mononucleotide subunits.

Question 5

polysaccharides, such as carbohydrates, are composed of which monomers? provide an example.

Answer 5

polysaccharides are formed from monosaccharides of simple sugars, such as glucose.

Question 6

lipids are composed of which two monomer subunits?

Answer 6

fatty acids and glycerol.

Question 7

polypeptides, such as proteins, are composed of which monomers?

Answer 7

polypeptides are composed of monomers of amino acids.

Question 8

which enzymes break down the following dissacharides?

• sucrose.
• maltose.
• lactose.

Answer 8

sucrose - broken down by sucrase.

maltose - broken down by amylase.

lactose - broken down by lactase.

Question 9

give the chemical formula for glucose.

Answer 9

C₆H₁₂O₆

Question 10

what is an isomer?

Answer 10

isomers refer to structures with the same molecular formula but different atomic arrangements.

Question 11

explain what a reducing sugar is, and give an example of a reducing sugar.

Answer 11

• a reducing sugar is a sugar that can donate electrons to (reduce) another chemical.
• glucose is an example of a reducing sugar.

Question 12

give the test used to test for the presence of a reducing sugar.

Answer 12

Benedict’s test.

Question 13

a student carried out Benedict’s test to test for a reducing sugar.

the student observed that the colour of the solution containing the reducing sugar changed from blue to red after being removed from the water bath. explain why this colour change occurred.

Answer 13

• the student obtained a positive result, meaning that a reducing sugar was present.
• when a reducing sugar is heated with Benedict’s reagent, it forms an insoluble red precipitate of copper (I) oxide.
• the colour of the solution depends on the concentration of the sugar so the deeper the shade of red, the higher the concentration of sugar present.

Question 14

another student carried out the same test but found that the solution remained blue. explain why no colour change had occurred.

Answer 14

the student obtained a negative result, meaning that no reducing sugars were present.

Question 15

give the type of bond that holds monosaccharides together.

Answer 15

glycosidic bond.

Question 16

explain why dilute hydrochloric acid and sodium hydrogencarbonate are used when testing for non-reducing sugars.

Answer 16

dilute hydrochloric acid - used to hydrolyse any disaccharide present into its constiuent monosaccharides.

sodium hydrogencarbonate - used to neutralise the hydrochloric acid and ensure the solution is alkaline.

Question 17

potassium iodine is a yellow solution used to test for starch. give the colour of the potassium iodine solution if starch is not present.

Answer 17

the potassium iodine solution would remain yellow, showing a negative result.

Question 18

starch is composed of which monosaccharides?

Answer 18

starch is made up of chains of alpha-glucose monosaccharides that are linked by glycosidic bonds.

Question 19

describe and explain three ways in which the structure of starch is suited to its role of energy storage in plants.

Answer 19

insoluble and does not affect water potential - water is not drawn into the cells by osmosis.

compact - lots can be stored in a small space.

forms alpha-glucose when hydrolysed - easily transported and can be used in respiration.

Question 20

where is glycogen found in animals?

Answer 20

in animals, glycogen is stored as small granules, mainly in the muscles and liver.

Question 21

describe and explain three ways in which the structure of glycogen is suited to its role of energy storage in animals.

Answer 21

insoluble - does not diffuse out of cells and does not tend to draw water into the cells by osmosis.

compact - lots can be stored in a small space.

highly branched ends - can be acted on simultaneously by enzymes, so more rapidly broken down to form glucose monomers that can be used in respiration.

Question 22

describe and explain three ways in which the structure of cellulose is suited to its function of providing support and rigidity in plant cell walls.

Answer 22

• cellulose molecules are made up of beta-glucose so form long, straight, unbranched chains.
• these cellulose molecular chains run parallel to each other and are cross linked by hydrogen bonds, which add collective strength.
• these molecules are grouped to form microfibrils, which are grouped to form fibres, providing more strength to the cell wall.