Biological Molecules 2: Carbohydrates

Question 1

Name three common monosaccharides.

Answer 1

Glucose
Fructose
Galactose

Question 2

How many chiral centres does glucose contain?

Answer 2

Four

Question 3

What is an aldose?

Answer 3

A sugar that contains an aldehyde group where the carbonyl is located

Question 4

What is a ketose?

Answer 4

A sugar that contains a ketone group where the carbonyl is located

Question 5

How do Fischer projections represent a sugar molecule?

Answer 5

Defines each chiral centre by arranging the -OH groups so that each horizontal group points outwards from the molecule

Question 6

What are the two types of Fischer projection for glucose?

Answer 6

L and D
Only D-glucose is found in nature

Question 7

What are Haworth projections?

Answer 7

Projections that represent the 3D structures of molecules
Particularly in their ring form

Question 8

Which form of representation best depicts a 3D molecule?

Answer 8

Chair representations

Question 9

How is the pyranose/furanose ring formed in a sugar molecule?

Answer 9

One of the -OH groups must react with the anomeric carbon (the one on the carbonyl group) at the top of the molecule to create a ring
This forms a hemiacetal group

Question 10

If an OH group points to the right on the Fischer projection, where is it located in the Haworth projection?

Answer 10

It will point down

Question 11

Describe the steps involved in identifying a sugar from its representation.

Answer 11

1) Identify the anomeric carbon - this will have two oxygens attached - and this is carbon-1
2) Go around the ring anti-clockwise to determine the position of the -OH groups (if lower than hydrogen, it points down)
3) Convert the Haworth projection into a Fischer projection - break the bond to release the carbonyl carbon
4) CH2OH will be at the bottom then work out which direction the -OH groups point

Question 12

How do you determine if a pyranose ring is alpha or beta?

Answer 12

If the resulting -OH group formed from the carbonyl points up, it is beta,
If it points down, it is alpha

Question 13

How do anomers exist in equilibrium?

Answer 13

They do not exist in equal amounts
For example, in ribose, the straight chain only accounts for about 0.1%, pyranose (alpha and beta together) about 80%, and furanose about 20%

Question 14

What is ribose?

Answer 14

A furanose sugar
Key component of RNA
Deoxyribose is a key component of DNA

Question 15

What monosaccharides make up lactose and which bonds join them?

Answer 15

D-Galactose and D-Glucose
Joined by beta-1,4 glycosidic bonds

Question 16

What monosaccharides make up sucrose and which bonds join them?

Answer 16

D-Glucose and D-Fructose
Joined by alpha-1,2 glycosidic bonds

Question 17

Describe how glycosidic bonds are formed.

Answer 17

One OH group on the ring of one monosaccharide reacts with the anomeric carbon on another monosaccharide, releasing water in a condensation reaction

Question 18

What are the most common glycosidic linkages?

Answer 18

1,2
1,4
1,6

Question 19

Other than carbohydrate polymers, what molecules can be joined by glycosidic bonds?

Answer 19

Glycoproteins
Glycopeptides
Structures of DNA and RNA

Question 20

What is amylose?

Answer 20

A polymer of D-glucose
Joined by alpha-1,4 glycosidic bonds
It is a plant starch
Easily digested into glucose molecules to be used in metabolic processes

Question 21

What is cellulose?

Answer 21

A polymer of D-glucose (in its beta pyranose form)
Joined by beta-1,4 glycosidic bonds
Also a plant starch
But is not digested by humans due to its ability to form strong fibres

Question 22

What is glycogen?

Answer 22

A polymer of D-glucose
Joined by both alpha-1,4 and alpha-1,6 glycosidic bonds (which enables branching)
Glycogen is found in the muscles and liver

Question 23

What is chitin?

Answer 23

A polymer of a glucose-derivative
Joined by beta-1,4 glycosidic bonds
Also contains nitrogen acetylation which makes the polymer more rigid than cellulose

Question 24

Why are D-glucose and D-fructose favoured energetically to sucrose?

Answer 24

Because they have improved biochemical availability