Carbohydrates

Question 1

What is a carbohydrate?

Answer 1

Carbon molecules (carbo-) combined with water (-hydrate).

Question 2

Why are living organisms based on carbon?

Answer 2

Carbon atoms very readily form bonds with other carbon atoms, this allows a sequence of carbon atoms of various lengths to be built up. These forma ‘backbone’ along which other atoms can be attached.

Question 3

In carbohydrates what is the basic unit/monomer?

Answer 3

The basic monomer unit is sugar, otherwise known as saccharide.

Question 4

What are monosaccharides?

Answer 4

A single monomer, they are sweet-tasting, soluble substances that have the general formula (CH’2’O)n. For example, glucose.

Question 5

What are organic molecules?

Answer 5

Molecules that contain carbon.

Question 6

What is reduction?

Answer 6

Reduction is a chemical reaction involving the gain of electrons or hydrogen.

Question 7

What is a reducing sugar?

Answer 7

A sugar that can donate electrons to another chemical, in order to reduce them.

Question 8

What is the test for reducing sugar called?

Answer 8

Benedict’s test, as Benedict’s reagent is used in the test.

Question 9

Which chemicals are reducing sugars?

Answer 9

All monosaccharides and some disaccharides.

Question 10

What is Benedict’s reagent?

Answer 10

An alkaline solution of copper (II) sulfate.

Question 11

Why is Benedict’s reagent used to test for reducing sugars?

Answer 11

When a reducing sugar is heated with Benedict’s reagent it forms an insoluble red precipitate of copper (I) oxide.

Question 12

Describe the method for the test for reducing sugars.

Answer 12

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

Question 13

When combined in pairs, monosaccharides create…

Answer 13

Disaccharides. For example, glucose joined to glucose to form maltose.

Question 14

What happens when monosaccharides join?

Answer 14

A molecule of water is removed and the reaction is therefore called a condensation reaction. The bond that is formed is called a glycosidic bond.

Question 15

What happens when water is added to a disaccharide?

Answer 15

It breaks down the glycosidic bond releasing the constituent monosaccharides (hydrolysis)

Question 16

How do we detect a non-reducing sugar?

Answer 16

First it is hydrolysed into its monosaccharide components.

1) If not in liquid form, ground up into water.
2) Add 2cm3 of the food sample being tested to 2cm3 of Benedict’s reagent in a test tube and filter.
3) Place the test tube in a gently boiling water bath for five minutes, if it changes blue there is no reducing sugar present.
4) Add the same quantity of food sample again and also an additional 2cm3 of hydrochloric acid and place in the water bath for 5 minutes to hydrolyse.
5) Make the solution neutral so the reagent works, by adding some sodium hydrogencarbonate solution.
6) Re-test the resulting solution by heating it with 2cm3 of reagent in gently boiling water for five minutes.
7) If a non-reducing sugar was present in the original sample the Benedict’s reagent will turn orange-brown.

Question 17

Why will the Benedict’s reagent turn orange-brown if a non-reducing sugar was present originally?

Answer 17

This is due to the reducing sugars that were produced from the hydrolysis of the non-reducing sugar.

Question 18

What are polysaccharides?

Answer 18

Polymers formed by combining together many monosaccharide molecules. They are joined together by glycosidic bonds that were formed by condensation reactions.

Question 19

Why are polysaccharides insoluble?

Answer 19

They are very large molecules, this feature makes them suitable for storage.

Question 20

What happens when polysaccharides are hydrolysed?

Answer 20

They break down into disaccharides and monosaccharides.

Question 21

What is the test for starch?

Answer 21

Starch changes potassium iodide solution from yellow to blue-black.

Question 22

When are carbohydrates formed?

Answer 22

Carbohydrates are molecules created when carbon is combined with water.

Question 23

The general formula of a monosaccharide is…

Answer 23

Monosaccharides have the general formula , where n can be 3-7.

Question 24

State the formula of glucose.

Answer 24

C₆H₁₂O₆.

Question 25

What happens during the test for reducing sugars?

Answer 25

Benedict’s reagent is a solution of copper(II) sulfate. When heated with a reducing sugar, the copper(II) sulfate accepts electrons from the sugar forming an insoluble red precipitate of copper(I) oxide.

Question 26

How can you tell if something is a non-reducing sugar?

Answer 26

A further test shows a non-reducing sugar is present:

Add dilute hydrochloric acid to a separate sample of the substance and heat gently.
The hydrochloric acid will hydrolyse any disaccharides into their component monosaccharides.
Test the solution again with Benedict’s solution. If the colour changes to orange-brown the substance contained a disaccharide.

Question 27

Name the polysaccharides composed of glucose.

Answer 27

Starch, glycogen, and cellulose

Question 28

Adaptations of starch.

Answer 28

Unbranched chains form coils that are very compact.

Starch is a storage molecule. The tightly wound coils mean that it does not take up too much space. Starch is also insoluble meaning that it can not diffuse out of a cell and it does not affect the water potential of a cell.

When hydrolysed starch releases α-glucose which can be used in respiration to generate energy.