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Have two main functions in living things:

  • Energy - source of energy in animals, are a store of energy in plants
  • Structural - froms cellulose cell walls in plants, animals can't digest it


  • General formula - CH2O 
  • Name is determined by number of carbons e.g 3 carbons is a triose, 5 carbons is a pentose, 6 carbons is a hexose
  • Glucose is a hexose sugar and exists as two isomers; alpha glucose and beta glucose. They have considerable biological differences.


  • 2 monocaccerides undergo a condensation reaction to form a disacceride.
  • The condensation reaction eliminates water and forms a glycosidic bond


Is an example of a polysacceride. Polysaccerides are formed when a large number of monosaccheride units are linked together by glycosidic bonds.

Starch is used for storage in plant cells, it is ideal for storing energy because it is compact and is non-soluble so does not affect the water potential of a cell. When needed starch is hydrolysed to alpha glucose before being transported to where it is needed.

  • It is made up of many alpha glucose molecules
  • Each molecule is connected to the next one by a 1,4 glycosidic bond
  • It forms a globular carbohydrate.
  • Is made of two polymers: amylopectin (branched) and amylose( unbranched and coils into helix)


Is a structural polysaccheride found in cell walls. 

  • Is made up of long parallel chains of beta glucose which are linked by hydrogen bonds.
  • Due to the different position of the -H and -OH group on the carbon the beta glucose must be rotated 180 degrees compared to the next one to form a 1,4 glycosidic bond
  • It is a fibrous molecule


Their biological importance:

  • protection of delicate organs
  • insulation and buoyancy
  • they are a good source of energy, can store twice the amount as carbohydrates. This is why seeds are so small - contain a lot of energy for their size
  • can be oxidised to produce water e.g camel
  • waterproofing as they are insoluble in water and having a waxy cuticle cuts down on water loss. This occurs when a fatty acid room is replaced by an alcohol group.

Unsaturated fatty acids have at least one double bond so have fewer hydrogen atoms.They are liquids at room temperature.

Saturated fatty acids have no double bonds and are solid…


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