Carbohydrates

Question 1

How does a condensation reaction join two monomers?

Answer 1

A molecule of water is removed to make polymers.
The OH group is split leaving the oxygen with a spare electron to bond the 2 monomers together.

Question 2

What is the bond formed by a condensation reaction?

Answer 2

It is a glycosidic bond.

Question 3

How does a hydrolysis reaction break a chemical bond?

Answer 3

Water is added to a disaccharide and breaks the glycosidic bond.

Question 4

What are examples of monosaccharides?

Answer 4

Glucose
Fructose
Galactose

Question 5

What is maltose?

Answer 5

Maltose is a disaccharide formed by the condensation reaction of two α-glucose molecules.

Question 6

What is sucrose?

Answer 6

A disaccharide formed from the condensation reaction between glucose and fructose.

Question 7

What is lactose?

Answer 7

A disaccharide formed from the condensation reaction between glucose and galactose.

Question 8

What is the structure of α-glucose?

Answer 8

H |__ O H
\/ \ /
/ \ \ /
HO \____/ \OH

Question 9

What is the structure of β-glucose?

Answer 9

H |__ O OH
\/ \ /
/ \ \ /
HO \____/ \H

Question 10

What is the function and structure of cellulose?

Answer 10

It provides support and rigidity because:
Cellulose molecules are made up of β-glucose so form long, straight, unbranched chains.
These cellular chains run paralell to each other and are cross linked by hydrogen bonds which add collective strength.
These molecules are grouped to form microfibres which are grouped to form fibres - adding yet more strength.

Question 11

What is the function and structure of glycogen?

Answer 11

It is used for storage, because:
It is insoluble so does not draw water into the cells by osmosis.
It is insoluble so doesn’t diffuse out of cells.
It is compact so a lot can be stored in a small space.
It is more highly branched than starch so more ends can be simultaneously acted on by enzymes and therefore more rapidly broken down to form glucose, used in respiration.
This is important as animals have a higher metabolic and respiratory rate than plants.

Question 12

What is the function and structure of starch?

Answer 12

The main role is energy storage, suited to because:
It is insoluble so it doesn’t affect water potential - water is not drawn into the cells by osmosis.
It’s large and insoluble - doesn’t diffuse out of cells.
It is compact - so a lot can be stored in a small space.
When hydrolysed it forms α-glucose which is easily transported and readily used in respiration.
The branched form has many ends, which can each be acted on by enzymes simultaneously, meaning the glucose monomers are released very rapidly.

Question 13

What is the test for starch?

Answer 13

Place 2cm^3 of the sample to a test tube.
Add two drops of iodine solution and shake or stir.

Question 14

What is the positive result for the starch test?

Answer 14

The solution will turn a blue-black colour.

Question 15

How are glycogen and starch formed?

Answer 15

The condensation of α-glucose.

Question 16

How is cellulose formed?

Answer 16

The condensation of β-glucose.

Question 17

What are examples of polysaccharides?

Answer 17

Starch
Cellulose
Glycogen

Question 18

What are the two polysaccharides starch are a mixture of?

Answer 18

Amylose
Amylopectin - branched.

Question 19

What is the test for non-reducing sugars?

Answer 19

Grind the sample up first if not already in liquid form.
Add 2cm^3 of the sample to 2cm^3 of Benedict’s reagent in a test tube.
Gently boil in a water bath for 5 minutes.
Add another 2cm^3 of the sample to 2cm^3 of hydrochloric acid in a test tube and boil in the water bath for 5 minutes.
Slowly add sodium hydrogencarbonate solution to the test tube to neutralise the acid. Test with pH paper to check the solution is alkaline.
Re-test this solution by heating it with 2cm^3 of Benedict’s in a gently boiling water bath for 5 minutes.

Question 20

What is the negative result for non-reducing sugars?

Answer 20

If, after the mixture is boiled in the water bath, the solution doesn’t change colour (stays blue).

Question 21

What does the hydrochloric acid do in the test for non-reducing sugars?

Answer 21

It will hydrolyse any disaccharide present into its constituent monosaccharides.

Question 22

Why is the hydrochloric acid neutralised?

Answer 22

The sodium hydrogencarbonate neutralises it because Benedict’s reagent does not work in acidic conditions.

Question 23

What is the positive result for non-reducing sugars?

Answer 23

The Benedict’s reagent will now turn orange/red/brown.
This is due to the reducing sugars that were produced by the hydrolysis of the non-reducing sugars.

Question 24

What is the result for the test for reducing sugars?

Answer 24

A positive result forms an insoluble RED precipitate of copper oxide.

Question 25

What is the test for reducing sugars?

Answer 25

Add 2cm^3 of the food sample to a test tube.
Grind it up with water if not in liquid form.
Add an equal volume of Benedict’s reagent.
Heat the mixture in a gently boiling water bath for 5 minutes.