1.2 2

• Monosaccharides joining to form polysaccharides

  1. Condensation reactions

  2. Forming glycosidic bonds

  3. Hydrolysis reactions splitting glycosidic bonds
• Oligosaccharides


  Molecules with 3-10 sugar units
• Polysaccharides


  Molecules containing 11 or more monosaccharides
• Polysaccharides do not have the sweet taste of many mono- and disaccharides
• Structure of polysaccharides


  • They can form very compact molecules, so large numbers can be stored in a cell
  • The glycosidic bonds are easily broken, allowing rapid release of monosaccharide units for cellular respiration
  • They are not very soluble in water, so have little effect on water potential within a cell and cause no osmotic water movements
• Hydrolysis of glycosidic bonds

  1. Water is added to the bond

  2. Polysaccharides are gradually broken down into shorter and shorter chains

  3. Eventually single sugars are left

  4. Disaccharides break down to form two monosaccharides

  5. Hydrolysis takes place during digestion in the gut, and also in the muscle and liver cells when the carbohydrate stores are broken down to release sugars for use in cellular respiration
• Starch


  • Particularly important as an energy store in plants
  • Sugars produced by photosynthesis are rapidly converted into starch, which is insoluble and compact but can be broken down rapidly to release glucose when it is needed
• Glycogen


  • Sometimes referred to as 'animal starch'
  • The only carbohydrate energy store found in animals
  • An important storage carbohydrate in fungi
  • Very similar to the amylopectin molecules in starch, made up of many a-glucose units
  • More 1,6-glycosidic bonds, giving it many side branches
  • Can be broken down very rapidly, making it an ideal source of glucose for active tissues with a constantly high rate of cellular respiration, such as muscle and liver tissue
• Starch


  • Made up of long chains of a-glucose
  • Amylose: an unbranched polymer made up of between 200 and 5000 glucose molecules
  • Amylopectin: a branched polymer of glucose molecules with many terminal glucose molecules that can be broken off rapidly when energy is needed
• Amylose is made up purely of a-glucose molecules joined by 1.4-glycosidic bonds
• In amylopectin many of the glucose molecules are joined by 1.4-glycosidic bonds, but there are also a few 1.6-glycosidic bonds
• Starch has a combination of straight chain amylose and branched chain amylopectin molecules
• The combination of amylose and amylopectin explains why carbohydrate foods like pasta are so good for you when you are doing sport
• Amylopectin releases glucose for cellular respiration rapidly when needed, while amylose releases glucose more slowly over a longer period, keeping you going longer
• Cellulose


  • An important structural material in plants
  • The cell wall is made up largely of insoluble cellulose, giving plants strength and support
  • Consists of long chains of glucose joined by glycosidic bonds, but the monomer units are B-glucose rather than a-glucose
• The linking of B-glucose molecules in cellulose means the hydroxyl (-OH) groups stick out on both sides of the molecule, allowing hydrogen bonds to form between them
• This difference in structure between starch and cellulose gives them very different properties and functions
• Starch is an important source of energy in the diet for many animals, but most animals do not possess the enzymes needed to break the 1,4-glycosidic bonds between the molecules of B-glucose and so they cannot digest cellulose
• Ruminants such as cows and sheep, have bacteria, fungi and protozoa living in their gut which produce cellulose-digesting enzymes
• Cellulose in plant food acts as roughage or fibre in the human diet, an important part of a healthy diet even though it cannot be digested