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Carbohydrates:

Carbohydrates:

  • They have a general formula: Cx(H2O)n
  • Saccharides are made from sugar molecules that are made from combinations of the elements carbon, hydrogen, and oxygen only.
  • They are used for:
  1. Fuels for respiration (glucose).
  2. Energy storage molecules (starch and glycogen).
  3. Structural molecules (cellulose).
Monosaccharides

(Monomers)

 

 – Single sugar units –  glucose

NB: there are 2 types of glucose: alpha and beta

 – Used in respiration
– Fructose – Found in fruit & honey
– Galactose – Found in lactose
(All the above are hexose sugars: C6H12O6)
Disaccharides – 2 single sugar units joined by a glycosidic bond. – Maltose

(2 glucose molecules)

 – Found in germinating seeds e.g. barley
– Sucrose

(Glucose and fructose)

 – Crystals used in cooking
– Lactose

(Glucose and galactose)

 – Sugar found in milk
Oligosaccharides – 3-10 sugar units. Found in vegetables e.g. leeks, lentils, beans

Polysaccharides (polymers) are long chains of glucose molecules.

  • Glycogen:
  • Found in muscle and liver cells for energy storage.
  • Made of poly alpha glucose linked together.
  • Insoluble because it has 1,4 and some 1,6 links, which form branches in the chain.
  • Very compacted, thus good for storage.
  • Starch:
  • Found in amyloplasts (starch grains) inside plant cells for energy storage.
  • Made of 2 types of molecules: amylose and amylopectin.
  • Amylose molecule is a very long chain of glucose molecules with 1,4 links.
  • Amylopectin is similar to glycogen.
  • Insoluble and very compact.
  • Cellulose:
  • Made from poly beta glucose
  • Main component of cell walls as it is a very strong structural molecule.
  • Cellulose has no branches, so adjacent cellulose chains line up close.
  • Hydrogen bonds between adjacent chains, creating very strong cellulose fibrils.

 

  • Saccharides link together by condensation reactions, producing water. A glycosidic bond forms between the saccharide molecules.
  • The opposite of a condensation reaction is hydrolysis. It requires water.
  • Tests for saccharides:
  • Iodine solution turns brown to blue/black in the presence of starch.
  • Benedict’s solution turns blue to brick red in the presence of a reducing sugar.
  • Non-reducing sugars (disaccharides and polysaccharides) will give a positive result to Benedict’s if heated in acid first.