27 July 2014

#8. Carbohydrates - Monosaccharides, disaccharides

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

  • 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.

  •  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

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