Carbohydrates

Question 1

What elements do carbohydrates contain?

Answer 1

Carbon, oxygen and hydrogen

Question 2

What is the general formula of carbohydrates?

Answer 2

CH20

Question 3

What is a single sugar unit known as?

Answer 3

A monosaccharide- eg glucose, fructose

Question 4

What happens when two monosaccharides link together?

Answer 4

A disaccharide is formed- eg lactose, sucrose

Question 5

What happens when many monosaccharides link together?

Answer 5

A polysaccharide is formed- eg glycogen, cellulose

Question 6

What are the roles of carbohydrates?

Answer 6

Source of energy e.g. glucose is used for energy-release during cellular respiration
Store of energy e.g. glycogen is stored in the muscles and liver of animals
Structurally important e.g. cellulose in the cell walls of plants

Question 7

What is glucose?

Answer 7

A monosaccharide composed of 6 carbons (hexose monosaccharide)

Question 8

What is the difference between alpha and beta glucose?

Answer 8

The OH group on carbon 1 is in opposite positions (below for alpha and above for beta)

Question 9

Why are glucose molecules polar and soluble in water, and why is this important ?

Answer 9

Hydrogen bonds form between the hydroxyl groups and water molecules, meaning glucose is dissolved in the cytosol of cells

Question 10

What is a condensation reaction?

Answer 10

A reaction where a water molecule is formed as one of the products

Question 11

How do two alpha glucose molecules link together?

Answer 11

When the two molecules are side by side, the hydroxyl groups interact forming a covalent bond, called a glycosidic bond. The result is a disaccharide.

Question 12

What is a glycosidic bond?

Answer 12

It is a bond formed between 2 carbohydrate
molecules. The OH group on carbon 1 reacts with the OH group on carbon 4 (therefore it is a 1,4 glycosidic bond). It is formed by a condensation reaction.

Question 13

What is maltose made of?

Answer 13

Alpha glucose+ alpha glucose

Question 14

What is sucrose made of?

Answer 14

Fructose+ glucose

Question 15

What is lactose made of?

Answer 15

Glucose+ galactose

Question 16

What is formed when many alpha glucose molecules are joined by glycosidic bonds?

Answer 16

Starch, which is a chemical energy store of glucose

Question 17

What is starch made up of?

Answer 17

Amylose and amylopectin (both are polymers of alpha glucose)

Question 18

What is the difference between amylose and amylopectin?

Answer 18

Amylose is formed by alpha glucose molecules joined by only 1,4 glycosdic bonds- unbranched structure
Amylopectin is joined by 1,4 and 1,6 glycosidic bonds- it has a branched structure due to the 1,6 glycosidic bonds
Amylose is very compact and insoluble because it forms a helix which is held in place by hydrogen bonds within the glucose molecules

Question 19

What are the properties of starch?

Answer 19

Insoluble in water
Compact
Branched

Question 20

What is the storage molecule in plants and animals?

Answer 20

Plants- starch

Animals- glycogen

Question 21

What are the properties of glycogen?

Answer 21

It has many branches (more than amylopectin) meaning it is more compact, less space needed for it to be stored
Lots of branches means there are a lot of free ends where glucose can be added/ removed- speeds up the process of storing or removing glucose when the cell needs it

Question 22

What is a hydrolysis reaction and why is it important for cells?

Answer 22

Requires the addition of water molecules to break glycosidic bonds and release glucose molecules
Hydrolysis reactions are needed when cells need to release glucose for respiration.

Question 23

What is cellulose made of?

Answer 23

Beta glucose+ beta glucose

Question 24

Why can cellulose not coil or form branches?

Answer 24

Beta glucose molecules cannot join in the same way that alpha glucose molecules do- the OH groups on carbon 1 and 4 are too far to react. Therefore, alternate beta glucose molecules are turned upside down so that the OH groups are close enough to react and form a glycosidic bond

Question 25

Why is cellulose used to make cell walls?

Answer 25

Chains of cellulose molecules make hydrogen bonds with each other- forming fibres. These fibres are strong and insoluble.

Question 26

How can you test for carbohydrates?

Answer 26

Benedicts test for reducing sugars

Question 27

What does it mean to be a reducing sugar?

Answer 27

It means they can donate electrons to another molecule. All monosaccharides and some disaccharides are reducing sugars. Sucrose is not a reducing sugar

Question 28

Method for test?

Answer 28

Place the sample in a boiling tube (liquid form)
Add an equal volume of benedicts reagent
Heat the mixture in a water bath for 5 minutes

Question 29

How do you know if there is a reducing sugar present?

Answer 29

Reducing sugars will react with the Cu2+ ions in benedicts reagent and reduce it to Cu+ (donate an electron)
Positive result- colour change to brick red

Question 30

Why is the reducing sugars test qualitative?

Answer 30

More reducing sugar= more precipitate formed= less blue Cu2+ ions left
The colour depends on the concentration of the reducing sugar present.

Question 31

How do you know if there are no reducing sugars present.

Answer 31

There will be no colour change.

If you boil the sample with HCl then you would get a positive result since the sample would be hydrolysed

Question 32

What is the test for starch?

Answer 32

Add a few drops of iodine. If the solution changes colour from brown to blue/ black, starch is present

Question 33

How can you turn the test into a quantitative test?

Answer 33

Use a colorimeter to measure the absorbance of light. More concentrated solution= more absorption of light