carbohydrates ( mono and disaccharides )

Question 1

what elements do carbohydrates contain?

Answer 1

carbon, hydrogen and oxygen
2 hydrogen : 1 oxygen

Question 2

what 3 groups can carbohydrates be classifies into?

Answer 2

monosaccharides, disaccharides and polysaccharides

Question 3

what are some properties of monosaccharides and disaccharides?

Answer 3

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

Question 3

what are monosaccharides?

Answer 3

single sugars that are the basic molecular units (monomers) of carbohydrates

Question 4

what are monosaccharides mainly used in?

Answer 4

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

Question 5

what are examples of monosaccharides?

Answer 5

glucose, fructose and galactose which all have the same formula C6H12O6 (hexoses) but different structure

Question 6

are monosaccharides reducing sugars?

Answer 6

yes - so they give a positive result for benedict’s test

Question 7

draw the structure of alpha glucose

Answer 7

see diagram

Question 8

how is a disaccharide formed?

Answer 8

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

Question 9

what is a condensation reaction?

Answer 9

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

Question 10

what are examples of disaccharides?

Answer 10

sucrose, maltose, lactose

Question 11

how are these disaccharides formed?

Answer 11

glucose + glucose → maltose + water
glucose + fructose → sucrose + water
glucose + galactose → lactose + water
C6H12O6 + C6H12O6 → C12H22O11 + H2O

Question 12

how is a disaccharide broken down?

Answer 12

a disaccharide can be broken down into it’s monosaccharides during a hydrolysis reaction

Question 13

what is a hydrolysis reaction?

Answer 13

hydrolysis is the breakdown (lysis) of a larger molecule into smaller molecules by the addition of water

Question 14

how are disaccharides hydrolysed?

Answer 14

by boiling with acid eg dilute HCl or by heating with an enzyme at optimum temperature

Question 15

are disaccharides reducing sugars?

Answer 15

yes - except sucrose

Question 16

describe how you test for reducing sugars

Answer 16

heat in a water bath with benedict’s solution. a brick red colour indicates a positive result. if the benedict’s solution remains blue, so reducing sugar is present.

LONG ANSWER : benedict’s test. add a small amount of the sample to a test tube with 2cm^3 of benedict’s solution. heat in a water bath for 5 minutes. a brick red or orange colour (produced by copper (I) oxide) is a positive result. if the benedict’s solution remains blue, no reducing sugar is present.

Question 17

how do you standardise the benedict’s test?

Answer 17

use the SAME VOLUME of benedict’s solution, the SAME VOLUME of the test solutions and the samples must be heated for the SAME AMOUNT OF TIME

Question 18

how do you visually tell how much reducing sugar is present and what is the issue with this?

Answer 18

an intense brick red colour indicates a large amount of reducing sugar was present whereas a yellow or green colour shows a small amount was present. the issue with this is that intensity of colour is subjective.

Question 19

what is a better way to measure how much reducing sugar is present?

Answer 19

use a colorimeter - ??

Question 20

how do you test for non reducing sugars?

Answer 20

carry out benedict’s test on a small sample to confirm a negative result. hydrolyse another sample by boiling it with 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.