lecture 16 - introduction to the structure and function of carbohydrates

Question 1

What are the functions of carbohydrates?

Answer 1

As monosaccharides and disaccharides they are…..
–used as energy stores, fuels and metabolic intermediates
–important in the structural backbone of DNA and RNA

As polysaccharides they are…..
–structural elements
cell walls- plants and bacteria
extracellular matrix
connective tissue
lubrication of skeletal joints
–linked to proteins and lipids
cell-cell recognition
cell adhesion
intracellular localisation tags on glycoproteins
determine metabolic fate of glycoconjugates
e.g. Starch, Glycogen, Cellulose

Question 2

What is the general formula for carbohydrates?

Answer 2

CnH2nOn

Question 3

What are aldoses and ketoses?

Answer 3

Monosaccharides that contain an aldehyde group are called aldoses, whereas those containing a
ketone group are ketoses. Monosaccharides are also classified by the number of carbon atoms as
trioses, tetroses, pentoses, hexoses, etc.

Question 4

What kind of isomerism is seen in carbohydrates?

Answer 4

Carbohydrates, along with many other natural substances, exhibit a type of isomerism in which two
isomers are mirror images of each other. The two isomers are called enantiomers. The prefixes Dand
L- are used to distinguish between enantiomers (e.g. D-glyceraldehyde versus L-glyceraldehyde;
D-galactose versus L-galactose). Most natural monosaccharides belong to the D-family

Question 5

How do you determine if a carbohydrate is a D or L isomer?

Answer 5

using penultimate carbon (chiral carbon)

D isomer has OH group on right hand side

Question 6

What are epimers?

Answer 6

Isomers that differ only in the configuration around one chiral carbon atom

Question 7

What are heteropolysaccharides and homopolysaccharides?

Answer 7

Homopolysaccharides - only 1 type of monomer

Heteropolysaccharides - multiple types of monomer

Question 8

Describe the structure of glycogen

Answer 8

1-4 glycosidic bonds between alpha glucose monomers
alpha 1-6 branching every 8-12 residues
stored as granules in liver cell
the non-reducing ends are where glycogen is cleaved
the reducing ends have a free hydroxyl group where glucose can bind

Question 9

Describe the structure of starch

Answer 9

amylose - 1-4 glycosidic bonds between alpha glucose monomers

amylopectin- 1-4 glycosidic bonds between alpha glucose monomers
alpha 1-6 branching every 24-30 residues

major fuel store in plant cells
very long polymers of glucose synthesised in chloroplast
can be stored in cytoplasm or chloroplast

Question 10

What is the difference between alpha and beta isomers?

Answer 10

alpha sugars have the C1 OH group below the plane of the ring
beta sugars have the C1 OH group above the plane of the ring