Carbohydrates

Question 1

List some examples of monosaccharides

Answer 1

Glucose, Fructose, Galactose, Ribose, Deoxyribose

Question 2

List some examples of disaccharides

Answer 2

Sucrose, Lactose, Maltose

Question 3

What is the name for the carbohydrate with 3 or more sugars?

List some examples

Answer 3

Oligosaccharides

Glycolipids, glycoproteins

Question 4

List some examples of polysaccharides

Answer 4

Glycogen, Cellulose, Starch

Question 5

What is a hexose?

How are this group of molecules named?

Give some examples

Answer 5

A hexose is a monosaccharide

Monosaccharides are named according to their number of carbons

Hexose: e.g. glucose, fructose, galactose

Pentose: e.g. ribose, deoxyribose

Triose: e.g. glyceraldehyde

Question 6

Which group of sugars can exist as stereoisomers?

What are the conformations? Give an example found in the body

Answer 6

Monosaccharides can exist as stereoisomers (differ in 3-D shape)

The two conformations are D and L conformation

Glucose in the body and diet exists in D form

Question 7

List the important roles of carbohydrates

Answer 7

1. Source of storable energy e.g. glycogen, starch
2. Cell-cell interactions and immune recognition e.g. glycolipids, glycoproteins
3. Carbon skeletons for molecules e.g. ribose, deoxyribose
4. Structural e.g. cellulose, peptidoglycan, chitin

Question 8

What makes glucose relatively chemically inert?

What are the two forms of glucose and the difference between them?

Answer 8

In aqueous solution at pH 7.4 and 37 deg C, glucose is in a cyclic (hemiacetal) conformation, which makes it relatively chemically inert.

Cyclic glucose can be αor β depending on the hydroxyl position at the anomeric carbon (C1):

α: OH below the plane of the hexose ring

β: OH above the plane of the hexose ring

Question 9

What are the bonds linking sugar molecules to each other and other molecules?

Answer 9

Glycosidic bonds, usually O-glycosidic bonds (sometimes N-glycosidic bonds e.g. in DNA and RNA between sugar and nitrogenous base)

Question 10

How are glycosidic bonds described?

Answer 10

Glycosidic bonds are described by the linked carbon numbers on each group and using the terms terms α or β depending upon the position of –OH group

e.g. O-glycosidic β 1-4

Question 11

What is glycogen?

Answer 11

Glycogen is a polymer (polysaccharide) and storage form of glucose

Question 12

List some different types of polymers of glucose

Describe differences in their properties

Answer 12

Polymers of glucose:

1. Glycogen
2. Amylose (polymer of O-glycosidic α 1-4 bonds)
3. Cellulose (polymer of O-glycosidic β 1-4 bonds)

Amylose (starch): Paste, digestible by humans, soluble in water

Cellulose (fiber): Structural fiber, indigestible by humans (dietary fiber), insoluble in water

Question 13

What is an important structural ingedient of bacterial cell walls?

What is its clinical significance?

Answer 13

Peptidoglycan: repeating disaccharides which make bacterial cell walls

Antibiotics such as penicillin, and Antibacterial enzymes such as lysozyme (found in saliva and tears) target peptidoglycan by breaking the β-(1,4)-glycosidicbonds and thereby destroying bacterial cells.