Carbohydrates

• CARBOHYDRATES:

  • Monomer: glucose
  • Covalent bond: glycosidic
  • Polymer made: starch
  • Polymer examples: cellulose, starch, glycogen
• α-glucose is an isomer of glucose which is less stable and more reactive.
• β-glucose is an isomer of glucose which is more stable and less reactive.
• Cellulose is a complex carbohydrate consisting of C, H and O atoms which is structurally important in cell walls.
• A disaccharide is a double sugar formed when two monosaccharides are joined by glycosidic linkage.
• A monosaccharide is the simplest form of sugar and the most basic units from which carbohydrates are built.
• Glycogen is a stored form of glucose consisting of many joined glucose molecules.
• A hexose sugar is a monosaccharide with six carbon atoms, for example glucose (C₆H₁₂O₆).
• A non-reducing sugar cannot donate electrons and cannot be oxidised (most disaccharides).
• A polysaccharide is a long-chain carbohydrate polymer composed of monosaccharides joined by glycosidic linkage.
• A reducing sugar can donate electrons and act as a reducing agent in chemical reactions.
• In α-glucose the H group is situated above the OH group.

  In β-glucose the H group is situated below the OH group.
• Starch is a polysaccharide with a spiral shape held together by hydrogen bonds. Its monosaccharide is glucose and consists of two types of polymers: amylose and amylopectin. Amylose is a straight-chain polymer and amylopectin is a branched-chain polymer. Starch can be used as an energy source and store of glucose in plants.
• Glycogen is a polysaccharide made of alpha glucose molecules joined by glycosidic linkage. Its structure consists of many side branches whose terminal ends can be acted on by enzymes to release energy. Its function is as a store of glucose that can maintain blood glucose levels in humans and animals.
• Cellulose is a polysaccharide made of β-glucose units joined by glycosidic bonds. Its structure is unbranched chains of linear glucose molecules. Alternate molecules are inverted and the chains are parallel. Cellulose is responsible for the physical strength of the cell wall.
• Chitin is a polysaccharide found in exoskeletons, shells, and fungal cell walls. It has a similar structure to cellulose. Chitin provides structural support and protection against predators.
• Maltose is a disaccharide made from joining glucose and glucose.
• Lactose is a disaccharide made from joining glucose and galactose.
• Sucrose is a disaccharide made from joining glucose and fructose.
• Amylopectin is an a-glucose branched structure with mostly short chains and 1,6 glycosidic bonds.
• Amylose is made of a long, branched chain of a-glucose molecules joined together through 1,4 glycosidic bonds.
• Starch consists mainly of amylose (20%) and amylopectin (80%).
• Glycogen is a highly branched polysaccharide consisting of alpha glucose units linked by 1,4 and 1,6 glycosidic bonds.
• Cellulose properties:

  • microfibrils group to form fibres, which have high tensile strength
  • can be stretched to keep cells turgid by withstanding turgor pressure
  • cellulose fibres are freely permeable so water and solutes can reach the cell surface membrane
  • only a few organisms have the enzyme to hydrolyse cellulose (cellulase)
• Cellulose structure:

  • made up of β-glucose, every other unit is inverted 180° so the OH group can form H bonds
  • β 1,4 glycosidic bonds
  • straight unbranched chains running parallel to each other and linked by H bonds between the OH group of adjacent chains
• Glycogen Properties:

  • insoluble (water doesn't enter by osmosis)
  • insoluble (water doesn't diffuse out)
  • compact
  • highly branched (many terminal ends can be hydrolysed faster to release more glucose for a higher rate of metabolism in animals)
• Glycogen Structure:

  • frequent branching
  • α 1,6 glycosidic bonds
  • branches come off the main chain every four glucose molecules
• Amylose:

  • a straight chain of glucose
  • coils into a helix
  • forms starch grains inside cells
  • OH forms H bonds
  • forms α 1,4 glycosidic bonds
  • stains deep blue with iodine
  • compact
  • stable so used for storage
  • 10-30% of starch
• Amylopectin:

  • more terminal ends
  • α 1,4 glycosidic bonds
  • α 1,6 glycosidic bonds every 30 monomers
  • 70-90% of starch
• What term is used to describe the different structures of a-glucose and b-glucose?

  Isomers
• Amylase hydrolyses starch into maltose.
• Lactase breaks lactose into one glucose and one galactose molecule.
• Sucrase breaks down sucrose into one glucose and one fructose molecule.
• Maltase breaks down maltose into two glucose molecules.
• Compare and contrast the structure of starch and the structure of cellulose.

  1. Both are glucose polymers;
  2. Both contain glycosidic bonds between monomers;
  3. Both contain C, H and O;
  4. Starch has α-glucose and cellulose has β-glucose;
  5. Starch is coiled and cellulose is straight;
  6. Starch is branched and cellulose is unbranched;
  7. Cellulose has microfibrils and starch does not;
• Describe the structure of glycogen.

  1. Polysaccharide of α-glucose;
  2. Joined by glycosidic bonds;
  3. Branched structure;
• Suggest how glycogen acts as a source of energy. Do not include transport across membranes in your answer.

  1. Hydrolysed to glucose;
  2. Glucose used in respiration;