carbohydrates

• • energy source - respiration (glucose)
  • energy store - eg starch
  • structure - eg cellulose
  • contains C, H and O
• there are 3 main groups of carbs:

  • monosaccharide- simple sugars, water soluble
  • disaccharide - double sugars made from 2 monosaccharide
  • polysaccharide - these are large molecules formed from many monosaccharide
• glucose is an example of a monosaccharide containing 6 carbons , hexose

  • major energy source for most cells
  • highly soluble
  • main form in which carbohydrates are transported around the body of animals
• glucose exists in different forms called structural isomers

  • alpha - OH group away from the O
  • beta - OH group next to the group
• maltose

  • formed from 2 alpha glucose molecules joined by an alpha 1,4 glycosidic bond
• sucrose

  formed from alpha glucose and fructose joined by an alpha 1,4 glycosidic bonds
• lactose 


  formed from beta galactose and alpha glucose joined by beta 1,4 glycosidic bonds
• examples of disaccharides

  • maltose
  • sucrose
  • lactose
• respiration


  animals and plants have specific enzyme that can break down alpha glucose only in respiration

  • beta glucose cannot be broken down by animals and plants due to its different arrangement of H and OH at carbon 1
• homopolysaccharide

  polysaccharide made form 1 type of monosaccharide eg starch
• heteropolysaccharide


  polysaccharide made up from more than 1 type of monosaccharide eg hyaluronic acid
• polysaccharide are mainly used as an energy store and as structural components of cells
• the major polysaccharide are:

  • starch and cellulose in plants
  • glycogen in animals
• 3 types of polysaccharides:

  • starch
  • glycogen
  • cellulose
• starch

  • plants store excess glucose as starch
  • amylose and amylopectin
  • starch is also insoluble in water , makes it good for storage
• amylose

  • long and unbranched chain of alpha glucose, giving it a coiled structure
  • making it compact so it is good storage
  • 1,4 glycosidic bonds
• amylopectin

  • long, branched chain of alpha glucose
  • side branches allow the enzyme that breaks down the molecule to get to the glycosidic bonds easily meaning glucose can be released quickly
  • 1,4 and and has branches formed by 1,6 glycosidic bonds
• glycogen

  • main storage in animals
  • store excess glucose as glycogen, another polysaccharide of alpha glucose
  • similar structure to amylopectin except that it has loads more side branches
  • compact molecule
  • 1,4 and and has branches formed by 1,6 glycosidic bonds
• cellulose

  • long unbranched beta glucose
  • when beta glucose molecules bond, they form straight cellulose chains
  • linked by H bonds to form strong fibres called microfibrils - provides structural support for cells
  • found in cell walls
• structure and function of plant cell walls

  • microfibrils and macrofibrils have high tensile strength because of the glycosidic bonds and also H bonds between chains
  • macrofibrils run in all direction criss-crossing the wall for extra strength
  • difficult to digest cellulose
  • because plants do not have a rigid skeleton, each cell needs to have strength to support the whole plant
  • space between macrofibrils for water and mineral ions = making it permeable
  • high tensile strength to prevent it from bursting when turgid