• sugar polymers
  • made up of carbon, hydrogen and oxygen atoms.
  • They have the general chemical formula of Cx(H2O)y
  • mainly used as an energy source or as energy storage (e.g. starch).
  • They also have structural uses, such as cellulose.



  • The simplest carbohydrates
  • Examples of hexose sugars: glucose, fructose, galactose
  • 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(the hydroxyl OH group of the Carbon 1 is above the plane of the carbon ring) and β-glucose



  • Two monosaccharides join together in a condensation reaction to form a disaccharide.
  • The two molecules are held together by a covalent bond called a glycosidic bond
  • A disaccharide can be split via hydrolysis (addition of water) into its monosaccharides.


Monosaccharides Disaccharide
glucose + glucose maltose
glucose + galactose lactose
glucose + fructose sucrose



  • mainly for storing energy (starch and glycogen) or for forming strong structures (cellulose)


The molecular structure of starch.

  • Is a polymer of alpha glucose.
  • It is an energy store in plant.
  • It has two components which are amylase and amylopectin.
  • Both are polymers of alpha glucose
  • Amylases have a straight chain and the glucose residues are joined together by 1,4 glycosidic bonds.
  • Amylase forms 20% of starch
  • These bonds cause the chains to coil helically into more compact shape.
  • Amylopectin is also more compact as it has many branches formed by 1,4 and 1,6 glycosidic bonds.
  • Coiled chains (may) contain 1 500 monomers with branches every ten (10) units
  • Forms 80% of Starch
  • As suspension of amylase in water gives a blue black colour with iodine (potassium iodide) and amylopectin give a red violet colour .



  • act as storage polysaccharides in animals and fungi
  • Made of α-glucose molecules linked together by glycosidic bonds
  • Most of the bonds are α 1-4 links (C1 on one glucose + C4 on the next)
  • There are some 1-6 links, which form branches in the chain.
  • The bonds can be hydrolyzed by carbohydrase enzymes to form monosaccharides, used in respiration.
  • The branches increase the rate of hydrolysis



  • Made of many β glucose molecules, linked by β 1-4 links.
  • Adjacent glucose molecules in the chain are upside-down to one another.
  • The chain is straight (not spiraling).
  • there are hydrogen bonds between chains forming very strong microfibrils
  • Microfibrils then bond together to form microfibrils which are woven in layers to form structures, such as cell walls
  • as a result, cell wall will not break easily if the plant cell absorbs water; difficult to digest (few organisms have enzyme that can break the β 1-4 bonds).
  • Its tensile strength helps plant cells in osmosis


Summary of differences between starch, glycogen and cellulose


Starch Glycogen Cellulose
Name of monomer alpha glucose alpha glucose beta glucose
bond between monomers 1,4 glycosidic
1,4 + 1,6 glycosidic
1,4 + 1,6 glycosidic 1,4 glycosidic
Characteristics of chains Coiled unbranched + long branched Short branched chains Straight long chains

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