#8. Carbohydrates - Monosaccharides, disaccharides

Carbohydrates:
- Sugar polymers
- Molecules contain C, H, O atoms
- H atoms are twice as many as C or O atoms (C₆H₁₂O₆)

Monosaccharides 

• The simplest carbohydrates 
• They are sugar: C = 3 = triose   C = 4 = tetrose  C = 5 = pentose C = 6 = hexose
• Examples of hexose sugars: glucose, fructose, galactose (C₆H₁₂0₆₎
• Molecules often have the form of a ring, made up of some C atoms and one O atom. 

• Glucose molecules has 2 forms: α-glucose and β-glucose.

Disaccharides

•  Different disaccharides can be formed by linking different monosaccharides. The bond that joins them together = glycosidic bond.

 

• Condensation reactions (dehydration): 2 monosaccharides covalently joined; H₂0 is formed.

• Hydrolysis reaction (splitting by water): disaccharides are split into 2 monosaccharides by breaking the glycosidic bond; a molecule of H₂0 is added. 

Functions of monosaccharides and disaccharides 

• Good sources of energy in living organisms, used in respiration for making ATP.
• Transportable through the body because of the solubility: glucose is transported dissolved in blood plasma (animal), sucrose is transported in phloem sap (plant).
• All monosaccharides and some disaccharides are reducing sugars (reduce blue Benedict's solution to produce an orange-red precipitate). Sucrose is a non-reducing sugar.

Syllabus 2015  (b) describe the ring forms of α-glucose and β-glucose (candidates should be familiar with the terms monomer, polymer and macromolecule);

Syllabus 2016  - 2018 Carbohydrates and lipids Carbohydrates and lipids have important roles in the provision and storage of energy and for a variety of other  functions such as providing barriers  around cells: the phospholipid bilayer of all cell membranes and the cellulose cell walls of plant cells. a)   describe the ring forms  of α-glucose and β-glucose b)   define  the terms monomer, polymer,  macromolecule, monosaccharide, disaccharide and polysaccharide