2.2.42 · Biological Molecules
The Glycosidic Bond — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions
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Lesson summary
2.2.4 The Glycosidic Bond — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Describe how glycosidic bonds form by condensation and break by hydrolysis.Transport and storage — monosaccharides are bonded into disaccharides and polysaccharides for transport and storage.O…
- Monosaccharide, disaccharide, polysaccharide
- Hydroxyl group
- Triglyceride
- Phospholipid
- Hydrophobic
- Osmosis — larger sugars have less influence on a cell's osmolarity
Section 1 of 7Why sugars are joined
Transport and storage
- Transport and storage monosaccharides are bonded into disaccharides and polysaccharides for transport and storage.
Osmosis
- Osmosis larger sugars have less influence on a cell's osmolarity.
Section 2 of 7Forming a glycosidic bond
Reaction
- Reaction two hydroxyl groups on different saccharides react to form a strong covalent bond.
Condensation
- Condensation one water molecule is removed for every glycosidic bond.
Enzymes
- Enzymes specific enzymes catalyse bond formation depending on which –OH groups interact.
Examples
- Examples two glucose molecules form maltose, and glucose with fructose forms sucrose.
Section 3 of 7Types of glycosidic bond
| Sugar | Bond | Type of molecule |
|---|---|---|
| Maltose | alpha-1,4 | Disaccharide |
| Sucrose | alpha-1,2 | Disaccharide |
| Cellulose | beta-1,4 | Polysaccharide |
| Amylose | alpha-1,4 | Polysaccharide |
| Amylopectin | alpha-1,4 and alpha-1,6 | Polysaccharide |
Section 4 of 7Breaking a glycosidic bond
Hydrolysis
- Hydrolysis water is added to break the bond.
Enzymes
- Enzymes different enzymes catalyse hydrolysis from those used in condensation.
Examples
- Examples digestion of food and breakdown of stored carbohydrate in muscle and liver for respiration.
Sucrose
- Sucrose heated with hydrochloric acid, it hydrolyses to glucose and fructose, which give a positive Benedict's test.
Polysaccharides
- Polysaccharides need many hydrolysis reactions to reach monosaccharides, whereas disaccharides need one.
Section 5 of 7More from the specification
1Starch
- An insoluble polysaccharide of glucose used by plants for energy storage.
2Triglyceride
- A lipid made of glycerol and three fatty acids.
3Phospholipid
- Glycerol, two fatty acids and a phosphate group, which is amphipathic.
4Hydrophobic
- Water-hating; non-polar and unable to dissolve in water.
5Carbon
- Element with four valence electrons that forms the backbone of organic molecules.
6Lipids
- Concentrated energy, insulation, cell membranes.
7Fatty acid
- A hydrocarbon chain with a carboxyl group.
8Non-reducing sugar
- A sugar that cannot reduce Benedict's reagent until hydrolysed.
9Monosaccharide, disaccharide, polysaccharide
- Monosaccharide, disaccharide, polysaccharide carbohydrates made of one, two, or many sugar units, joined by glycosidic bonds in the larger two.
10Polymer
- Polymer a long chain made of many repeating monomers joined by covalent bonds.
Section 6 of 7Key terms
| Term | Meaning |
|---|---|
| Glycosidic bond | A covalent bond between two sugars via an oxygen link. |
| Condensation | A reaction that forms a bond and releases water. |
| Hydrolysis | A reaction that breaks a bond using water. |
| Maltose | A disaccharide of two alpha-glucose units. |
| Sucrose | A disaccharide of glucose and fructose. |
| Hydroxyl group | An –OH functional group. |
Section 7 of 7Exam technique
1
Saying a glycosidic bond forms by hydrolysis — it forms by condensation.
2
Forgetting that one water molecule is released per bond — count one per glycosidic bond.
3
Saying maltose and sucrose have the same bond — maltose is alpha-1,4 and sucrose is alpha-1,2.
4
Saying a polysaccharide needs one hydrolysis to give monosaccharides — many bonds must be broken.
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