carbohydrates Flashcards
(10 cards)
Structure of a carbohydrate
A carbonyl group either in the form of a ketone or an aldehyde, with multiple OH groups
Ring structure of glucose
6 carbon ring structure, 5 in a ring with one oxygen, carbon 6 on top (CH2OH)
Anomeric carbon is bonded to two oxygen atoms
Hydroxyl group bonded to anomeric carbon can be below (a-glucose) or above (B-glucose) the plane
OH and H groups bonded to rest of carbon atoms
Why is sucrose not a reducing sugar? Explain with link to the Benedict’s test
In Benedict’s test, blue Cu2+ solution is reduced to red Cu+, a brick red precipitate. This makes use of the free carbonyl group in reducing sugars. Sucrose has no free carbonyl group as its anomeric carbon is involved in the formation of a glycosidic bond.
What is the composition of starch?
Unbranched amylose (10-30%), Branched amylopectin (70-90%), only made up of a-glucose monomers
Describe the structure of amylose and relate this to its storage function in starch
Amylose is an unbranched chain of a-glucose monomers joined by a(1,4) glycosidic bonds in a helical structure that is compact. Its anomeric carbon is involved in glycosidic bond formation, making it unreactive and chemically-stable. It is also acts as a large store of carbon which is a respiratory substrate. It is bulky and insoluble in water, thus it does not affect the water potential of cells.
What is the structure of amylopectin and relate this to its function in starch.
It is linked by a(1,4) and a(1,6) glycosidic bonds, making it branched. Extensive branching makes it highly compact, containing up to twice as many glucose residues as amylose. There are also many branch ends to allow enzymes to act on it and it is easily hydrolysed to release energy.
Compare glycogen to amylopectin.
Glycogen is more extensively branched than amylopectin — a(1,6) glycosidic bonds occur every 8-12 glucose units. Thus it is more compact than amylopectin.
How does the structure of cellulose differ from that of glycogen/amylose/amylopectin?
It is linked by B(1,4 ) glycosidic bonds, made up only of B-glucose monomers. Alternate monomers are inverted and run in chains parallel to each other, with their OH groups projecting outwards from each chain.
What features of cellulose help it to ensure high tensile strength for structural support?
Long unbranched chains, with extensive hydrogen bonds between parallel chains. They are grouped into microfibrils and eventually cluster into macrofibrils.
Why is cellulose more stable than amylase for structural support?
Few organisms produce cellulase enzymes that are able to hydrolyse B(1,4) glycosidic bonds. Amylose can easily be hydrolysed by amylase enzymes.
