2.2.4 The Glycosidic Bond

Question 1

Forming the Glycosidic Bond

Answer 1

To make monosaccharides more suitable for transport, storage and to have less influence on a cells osmolarity, they are bonded together to from disaccharides and polysaccharides
Disaccharides and polysaccharides are formed when two hydroxyl (-OH) groups (on different saccharides) interact to form a strong covalent bond called the glycosidic bond (the oxygen link that holds the 2 molecules together)
Every glycosidic bond results in one water molecule being removed, meaning the glycosidic bonds are formed by condensation
Each glycosidic bond is catalysed by enzymes specific to which OH groups are interacting
As there are many different monosaccharides, this results in different types of glycosidic bonding
- e.g. alpha 1,2 / alpha 1,4

Question 2

Types of Different Glycosidic Bonds in Different Saccharides

Answer 2

Disaccharides
Maltose - alpha 1,4
Sucrose - alpha 1,2
Polysaccharides
Cellulose - beta 1,4
Amylose - alpha 1,4
Amylopectin - alpha 1,4 and alpha 1,6

Question 3

Breaking the Glycosidic Bond

Answer 3

The glycosidic bond is broken when water is added in a hydrolysis reaction
- disaccharides and polysaccharides are broken down in hydrolysis reactions
Hydrolytic reactions are catalysed by enzymes
- these are different to those present in condensation reactions
Examples of hydrolytic reactions include the digestion of food in the alimentary tract and the breakdown of stored carbohydrates in muscle and liver cells for use in cellular respiration
Sucrose is a non-reducing sugar which gives a negative result in a Benedict’s test
When sucrose is heated with hydrolytic acid, this provides the water that hydrolyses the glycosidic bond, resulting in two monosaccharides that will provide a positive Benedict’s test

Question 4

Common Disaccharides

Answer 4

Monosaccharides can join together via condensation reactions to form disaccharides
- a condensation reaction is one in which 2 molecules join together via the formation of a new chemical bond, with a molecule of water being released in the process
- the new chemical bond that forms between 2 monosaccharides is the glycosidic bond
- to calculate the chemical formula of a disaccharide, you can add all the carbons, hydrogen and oxygens in both monomers then subtract 2xH and 1xO (for the water molecule lost)
Common examples of disaccharides include:
- maltose (the sugar formed in the production and breakdown of starch)
- sucrose (the main sugar produces in plants)
- lactose (a sugar only found in milk)
All three of these common examples above have the formula C12H22O11

Question 5

Common Disaccharides and their Monosaccharide Monomers Table

Answer 5

Maltose = alpha glucose + alpha glucose
Sucrose = alpha glucose + fructose
Lactose = alpha glucose + galactose