- To make monosaccharides more suitable for transport, storage and to have less influence on a cell’s osmolarity, they are bonded together to form 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 two molecules together)
- Every glycosidic bond results in one water molecule being removed, thus glycosidic bonds are formed by condensation
The formation of a glycosidic bond by condensation between two monosaccharides (glucose) to form a disaccharide (maltose)
- 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 bonds forming (e.g maltose has a α-1,4 glycosidic bond and sucrose has a α-1,2 glycosidic bond)
The formation of a glycosidic bond by condensation between α-glucose and β-fructose to form a disaccharide (sucrose)
Make sure you can identify where the glycosidic bond is in a carbohydrate.
- The glycosidic bond is broken when water is added in a hydrolysis (meaning ‘hydro’ – with water and ‘lyse’ – to break) 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 hydrochloric acid this provides the water that hydrolyses the glycosidic bond resulting in two monosaccharides that will produce a positive Benedict’s test
A molecule of glucose and a molecule of fructose are formed when one molecule of sucrose is hydrolysed; the addition of water to the glycosidic bond breaks it
Remember that disaccharides hydrolyse to two monosaccharides whereas polysaccharides must undergo many hydrolytic reactions until they form monosaccharides.