As Biology - Biological Molecules - Carbohydrates

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  • Most carbohydrates are large, complex molecules composed of long chains of monosaccharides (e.g. starch is a large carbohydrate composed of long chains of glucose)
  • Single monosaccharides are also called carbohydrates though
  • Glucose is a hexose sugar - a monosaccharide with six carbon atoms in each molecule
  • There are two types of glucose - alpha and beta
  • Glucose's structure is related to its function as the main energy source in animals and plants. Its structure makes it soluble so it can be easily transported. Its chemical bonds contain lots of energy.


  • Monosaccharides are joined together by glycosidic bonds in a condensation reaction
  • A hydrogen on one monosaccharide bonds to a hydroxyl (OH) group on the other, releasing a molecule of water.
  • The reverse of this is a hydrolysis reaction - a molecule of water reacts with the glycosidic bond, breaking it apart
  • When two monosaccharides join together, they form a disaccharide


  • A polysaccharide is formed when more than two monosaccharides join together.
  • Polysaccharides form in exactly the same way as disaccharides - monosaccharides join together in a condensation reaction, releasing a molecule of water.

Three polysaccharides

  • (1) Starch - the main energy storage material in plants


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