1 Carbohydrates (DIY)

• Physical Properties of Carbohydrates (Monosaccharides)

  • Sweet with crystalline structures
  • Soluble in water due to its small size and numerous -OH groups that can form H-bonds with water
  • Pentose and Hexose can exist as rings which are more stable as building blocks for the synthesis of disaccharides and polysaccharides 
• Chemical Properties of Carbohydrates (Monosaccharides)

  • Disaccharides can be hydrolysed to monosaccharides under acid hydrolysis or via enzymatic reactions
  • All reducing sugars, except sucrose
• Maltose (glucose+glucose)

  • Product formed during starch digestion by amylase
  • Occurs commonly in animals and germinating seeds 
• Lactose (glucose+galactose)

  • Exclusively found in milk (milk sugar)
  • A significant energy source for young mammals
• Sucrose (glucose+fructose)

  • NON-REDUCING SUGAR
  • Most plentiful disaccharide in nature 
  • Large quantities transported through phloem
  • Good transport sugar due to its high solubility in water and can be transported efficiently in high concentrations
  • Tends not to go into general metabolism on its way from one destination to another (fairly unreactive)
  • Extracted from sugarcane for use, in cooking and food production
• Glycosidic Bond

  • Covalent bond formed through condensation reaction which involves the loss of 1 water molecule each 
• Polysaccharides properties

  • straight/ helical chains!
  • Does not taste sweet
  • Large in size 
  • Insoluble in water (will not affect water potential of the cell)
• Starch (FUNCTION)

  • energy storage molecule in plants
  • Stored as starch granules within cellular structures such as chloroplasts
• Starch (STRUCTURE)

  • polymer made of alpha-glucose 
  • With amylose helices being entangled in the branches of amylopectin molecules
• Starch - Amylose (STRUCTURE)

  • alpha-glucose monomers joined by alpha-1,4 glycosidic bonds 
  • Folded into a compact helix stabilised by hydrogen bonding (stores many glucose residues in a limited space of the plant cell)
  • Hydroxyl group (-OH) on C2 of each chain projects into the helix (formed H-bonds with each other, stabilising the helix shape)
  • UNBRANCHED
• Starch - Amylopectin (STRUCTURE)

  • alpha-glucose monomers joined by alpha-1,4 glycosidic bonds and alpha-1,6 glycosidic bonds at branch points
  • Branching occurs every 20-30 glucose residues (provides many ends which are accessible to hydrolytic enzymes, allowing quick hydrolysis of amylopectin to release glucose molecules)
  • BRANCHED (increased branching —> increased branch end point —> greater accessibility to amylase —> faster release)
• Glycogen (FUNCTION)

  • energy storage polysaccharide and the main energy storage compound in animals
  • Excess glucose is converted into this molecules for storage under the action of insulin —> accumulated in liver and muscle cells
  • Converted when blood glucose level is low, breakdown to form glucose is controlled by the hormone glucagon
• Glycogen (STRUCTURE)

  • Polymers of alpha-glucose monomers where each monomer is linked by alpha-1,4 glycosidic bonds (helical structures that make the molecule more compact, storing more glucose within a limited space of the storage cell)
  • Branched with alpha-1,6 glycosidic bonds at the branch points, allowing for multiple branch ends to be available for enzymes to hydrolyse the molecules at the same time to release glucose quickly 
  • MORE EXTENSIVELY BRANCHED than amylopectin (branched every 10-20 residues)
• Cellulose (FUNCTION)

  • a structural component of all plant cell walls
• Cellulose (STRUCTURE)

  • each molecule is made up of about 10 000 beta-glucose residues joined together in a long and straight chain
  • In each cellulose chain, adjacent beta-glucose molecules must be rotated 180deg about horizontal axis in alternate
  • This brings the hydroxyl groups of C1 of 1 glucose to the C4 of the other on the same plane
  • UNBRANCHED
  • beta-1,4 glycosidic bonds make the cellulose chain straight —> allows cellulose chains to run parallel to each other (COMPARED to starch’s helical shape)
• Organisation of Cellulose

  • Hydroxyl groups (-OH) project outwards from each cellulose chain in both directions —> forms H-bonds with neighbouring chains —> extensive inter-chain hydrogen binding —> forms a cellulose microfibril
  • Several ,microfibrils are arranged in larger bundles, macrofibrils, later assembling to form cellulose fibres
  • A meshwork of criss-crossing cellulose fibres forms a layer, with successive layers being interwoven and embedded and tethered by other cell wall polysaccharides in a gel-like matrix
• Cellulose (PROPERTIES)

  • high tensile strength + insolubility
  • Organisation of cellulose chains into microfibrils and macrofibrils confer high tensile strength —>  allows cell wall to withstand mechanical stress and prevents the cell wall from bursting when water enters by osmosis
  • Most hydroxyl groups of each beta-gluscose residue are involved in inter-chain hydrogen bond formation, resulting in very few hydroxyl groups that are available to hydrogen bond with water —> insoluble in water and is an ideal structural material
  • Cellulose is porous due to mesh like structure of the cell wall
• Formation of Cell Wall

  • Cellulose is synthesised by cellulose synthase enzyme
  • As part of a large enzyme complex, these enzymes are found on the cell surface membrane of plant cells
  • Cellulose molecules are synthesised by the enzyme complex and simultaneously associated into microfibrils that are laid down on the outside of the cell