carbohydrates (mono and disaccharides)

Question 1

what elements do carbohydrates contain?

Answer 1

carbon, hydrogen and oxygen

Question 2

what is the ratio of hydrogen to oxygen atoms in carbohydrates?

Answer 2

2 hydrogen : 1 oxygen

Question 3

what three groups can carbohydrates be classified into?

Answer 3

monosaccharides, disaccharides and polysaccharides

Question 4

what are four properties of monosaccharides?

Answer 4

small soluble molecules that are easy to transport and sweet to taste

Question 5

monosaccharides are ______ sugars

Answer 5

monosaccharides are single sugars

Question 6

what are monosaccharides?

Answer 6

monosaccharides are the basic molecular units (monomers) of carbohydrates

Question 7

what are monosaccharides mainly used in?

Answer 7

• respiration to provide energy
• growth during the formation of larger carbohydrates

Question 8

what are three examples of monosaccharides?

Answer 8

glucose, fructose and galactose

Question 9

what is the formula of these monosaccharides and how do they differ?

Answer 9

all have the same formula C6H12O6 (hexoses) but different structure

Question 10

are monosaccharides reducing or non reducing sugars? therefore, what result do they give in the benedict’s test?

Answer 10

monosaccharides are reducing sugars so give a positive benedict’s test result

Question 11

disaccharides are ______ sugars

Answer 11

disaccharides are double sugars

Question 12

how is a disaccharide formed?

Answer 12

a disaccharide is formed when two monosaccharides are joined together by a glycosidic bond in a condensation reaction

Question 13

what is a condensation reaction?

Answer 13

a condensation reaction is a reaction in which smaller molecules join to form a larger molecule with the release of water and the formation of a bond

Question 14

give the three formulas for the formation of three disaccharides

and the other two general formulas for the formation of disaccharides

Answer 14

glucose + glucose -> maltose + water
glucose + fructose -> sucrose + water
glucose + galactose -> lactose + water

monosaccharide + monosaccharide -> disaccharide + water
C6H12O6 + C6H12O6 -> C12H22O11 + H2O

Question 15

how can a disaccharide be broken down?

Answer 15

a disaccharide can be broken down into its monosaccharides during a hydrolysis reaction

Question 16

what is a hydrolysis reaction?

Answer 16

a hydrolysis reaction is the breakdown (lysis) of a larger molecule into smaller molecules by the addition of water (hydro) and breaking of a bond

Question 17

how can disaccharides be hydrolysed?

Answer 17

disaccharides can be hydrolysed by boiling with acid eg dilute HCl or by heating with an enzyme at an optimum temperature

Question 18

what enzyme hydrolyses sucrose?

Answer 18

sucrase hydrolyses sucrose

Question 19

what monosaccharides and disaccharides give a positive result in the benedict’s test?

Answer 19

all monosaccharides and disaccharides, except for sucrose, give a positive result in the benedict’s test

Question 20

describe how to carry out benedict’s test

Answer 20

• place a small amount of the sample in a test tube with 2cm^3 of benedict’s solution
• heat in a water bath for 5 minutes
• a brick red colour is a positive result and indicates a reducing sugar is present
• if the benedict’s solution remains blue, no reducing sugar is present

Question 21

what causes the brick red colour produced by a positive result in the benedict’s test?

Answer 21

the brick red colour is produced by copper 1 oxide

Question 22

what can the benedict’s test be used to do?

Answer 22

the benedict’s test can be used to compare the amount of reducing sugar in different solutions

Question 23

how do you standardise the benedict’s test?

Answer 23

• use the same volume of benedict’s solution
• use the same volume of the test solutions
• heat the samples for the same amount of time

Question 24

which colour indicates the most and least positive result?

Answer 24

(most) brick red - orange - yellow - green (least)

Question 25

what do you use a colorimeter for and how do you use it (5)?

Answer 25

a colorimeter can be used to accurately compare the colour intensity produced - measure the light absorbances of five glucose solutions at different known concentrations - plot these absorbances against the glucose concentrations - zero the colorimeter using a sample of distilled water - use the colorimeter on the test sample to measure the light absorbance of the solution - use the calibration curve to interpret the result of the test sample

Question 26

how to test for non reducing sugar?

Answer 26

- carry out benedict's test on a small sample to confirm a negative result - hydrolyse another sample by boiling it with a dilute acid eg HCl - when cooled, add dilute sodium hydroxide solution (NaOH) to neutralise the acid - add benedict's solution and heat in a water bath for 5 minutes - a positive brick red colour indicates a non reducing sugar was originally present