Carbohydrates

Question 1

What is a monomer?

Answer 1

Monomers are individual molecules which combine to form polymers.

Question 2

What are polymers?

Answer 2

Chains of repeating monomer units.

Question 3

Name the only three elements that carbohydrates contain:

Answer 3

1) Carbon
2) Hydrogen
3) Oxygen

Question 4

What is the chemical formula for glucose?

Answer 4

C6 H12 O6

Question 5

In carbohydrates what is the basic monomer unit known as?

Answer 5

Monosaccharides

Question 6

In carbohydrates what is the polymer called?

Answer 6

Polysaccharides

Question 7

What is a disaccharide?

Answer 7

Two monosaccharides joined together.

Question 8

Give two facts about monosaccharides:

Answer 8

They are sweet tasting and are soluble.

Question 9

Which sugars are reducing sugars?

Answer 9

All monosaccharides and some disaccharides (e.g. maltose).

Question 10

What is reduction?

Answer 10

Reduction is a chemical reaction involving the gain of electrons. (Reducing sugars can donate electrons)

Question 11

What is the test for reducing sugars?

Answer 11

The Benedict’s Test.

Question 12

What is Benedict’s reagent?

Answer 12

It is an alkaline solution of cooper (2) sulphate.

Question 13

When a reducing sugar is heated with Benedict’s reagent what happens?

Answer 13

A red precipitate of copper (1) oxide is formed.

Question 14

Describe how The Benedict’s test is carried out.

Answer 14

1) Add 2cm3 of the food sample to a test tube. If the sample is not already in liquid form, you must grind it up in water first.
2) Add an equal volume of Benedict’s reagent to the test tube.
3) Heat the mixture in a gently boiling water bath for five minutes.

Question 15

What colour will the solution turn if a reducing sugar is present?

Answer 15

Solution will turn orange/brown.

Question 16

What does the term ‘semi quantitative’ mean in terms of The Benedict’s test?

Answer 16

The differences in colour allow us to estimate the approximate amount of reducing sugar in a sample.

Question 17

Name three monosaccharides:

Answer 17

1) glucose
2) fructose
3) galactose

Question 18

Name three disaccharides:

Answer 18

1) maltose
2) sucrose
3) lactose

Question 19

Which two monosaccharides join together to make the disaccharide maltose?

Answer 19

Glucose+Glucose=Maltose

Question 20

Which two monosaccharides join together to make the disaccharide sucrose?

Answer 20

Glucose+Fructose=Sucrose

Question 21

Which two monosaccharides join together to make the disaccharide lactose?

Answer 21

Glucose+Galactose=Lactose

Question 22

What type of the reaction is the joining of monosaccharides?

Answer 22

A condensation reaction. Because a molecule of water is removed.

Question 23

What is the bond that joins two monosaccharides together called?

Answer 23

A glycosidic bond.

Question 24

How do you make monosaccharides from disaccharides?

Answer 24

Under suitable conditions if water is added to a disaccharide, a hydrolysis reaction will occur, breaking the glycosidic bond and releasing the constituent monosaccharides.

Question 25

Which sugars are non-reducing sugars?

Answer 25

All disaccharides apart from maltose.

Question 26

What happens when you heat a non-reducing sugar with Benedict's reagent?

Answer 26

Nothing, there will be no colour change.

Question 27

What must happen before we can detect a disaccharide?

Answer 27

It must be broken down into its monosaccharide constituents.

Question 28

Describe the test for non-reducing sugars:

Answer 28

1) If the sample is not already in liquid form, it must first be ground up in water. 2) Add 2cm3 of the food sample to 2cm3 of Benedict's reagent in a test tube. 3) Place the test tube in a gently boiled water bath for 5 minutes. If the Benedict's reagent does not change colour (the solution remains blue), then a reducing sugar is not present. 4) Add another 2cm3 of the food sample to 2cm3 of dilute hydrochloric acid in a test tube and place the test tube in a gently boiling water bath for 5 minutes. The dilute hydrochloric acid will hydrolyse any disaccharide present into its constituent monosaccharides. 5) Slowly add some hydrogencarbonate solution to the test tube in order to neutralise the hydrochloric acid. (Benedict's reagent will not work in acidic conditions.) Test with pH paper to check the solution is alkaline. 6) Re-test the resulting solution by heating it with 2cm3 of Benedict's reagent in a gently boiling water bath for 5 minutes. 7) If a non-reducing sugar is present in the original sample, the Benedict's reagent will now turn orange-brown. This is due to the reducing sugars that were produced from the hydrolysis of the non-reducing sugar.

Question 29

Name three substances/solutions which have to be added to the food sample during the test for a non-reducing sugar:

Answer 29

1) Benedict's reagent 2) Dilute hydrochloric acid 3) Sodium hydrgogencarbonate

Question 30

What volume of the food sample is added to the test tube in both the tests for reducing and the non-reducing sugars?

Answer 30

2cm3 of the food sample

Question 31

What volume of the Benedict's reagent is added to the test tube in both the tests for reducing and the non-reducing sugars?

Answer 31

2cm3 of Benedict's reagent

Question 32

During the test for a non-reducing sugar, how long do you leave the test tube in the gently boiling water bath?

Answer 32

5 minutes

Question 33

During the test for a non-reducing sugar, which do you add first dilute hydrochloric acid or sodium hydrogencarbonate?

Answer 33

Dilute hydrochloric acid.

Question 34

What does the dilute hydrochloric acid do? (test for non-reducing sugars).

Answer 34

It hydrolyses any disaccharide present into its constituent monosaccharides. (hydrolysis reaction).

Question 35

What does the sodium hydrogencarbonate do?

Answer 35

It neutralises the hydrochloric acid because Benedict's reagent cannot work in acidic conditions.

Question 36

Why is pH paper need in the test for non-reducing sugars?

Answer 36

To ensure the solution is alkaline after sodium hydrogencarbonate has been added as Benedict's reagent cannot work in acidic conditions.

Question 37

What colour will the final solution turn if a non-reducing sugar was present in the original sample?

Answer 37

Orange-brown solution.

Question 38

Why does the final solution turn orange-brown if a non-reducing sugar was present in the original sample?

Answer 38

Because of the reducing sugars which were produced from the hydrolysis of the non-reducing sugars.

Question 39

Are polysaccharides soluble or insoluble?

Answer 39

Insoluble.

Question 40

Why are polysaccharides insoluble?

Answer 40

Because they are very large molecules, made up of many monosaccharides joined together.

Question 41

What feature of polysaccharides makes them suitable for storage?

Answer 41

They are insoluble.

Question 42

Name a condition for the test for starch:

Answer 42

It is carried out at room temperature.

Question 43

Describe the test for starch:

Answer 43

1) Place 2cm3 of the sample being tested into a test tube. 2) Add two drops of iodine solution and shake or stir. 3) The presence of starch is indicated by a blue or black coloration.

Question 44

What colour will the solution turn if starch is present?

Answer 44

Blue/black

Question 45

What causes large volumes of gas to build up? - lactose intolerance

Answer 45

When the undigested lactose reaches the large intestine, microorganisms break it down.

Question 46

Give four effects of lactose intolerance on the body:

Answer 46

1) bloating 2) nausea 3) diarrhoea 4) cramps

Question 47

Give two ways of dealing with lactose intolerance:

Answer 47

1) limiting the amount of lactose in your diet (less dairy products) 2) adding the lactase enzyme to your milk before drinking it

Question 48

Give one issue with lactose intolerance

Answer 48

People may not get enough calcium in their diets.