2) Biological Molecules

• Water molecule structure

  Oxygen atom gets slightly more than its fair share of electrons, has a small negative charge (𝜹-), hydrogen atoms get slightly less than their fair share, have a small positive charge (𝜹+), creating a dipole
• Hydrogen bonding in water is much weaker than a covalent bond but has a significant effect
• Water acts as a medium for reactions inside cells and a transport medium
• Water's high surface tension and cohesion are due to hydrogen bonds, allowing water to move in long columns and certain organisms to exploit its surface
• Water has a high specific heat capacity due to hydrogen bonds, making it resistant to temperature changes
• Changes in water density with temperature cause currents that help maintain nutrient circulation in the ocean
• Water is a good solvent due to its charged nature, dissolving polar molecules (hydrophilic) but not non-polar molecules (hydrophobic)
• Hydrogen bonds occur wherever there is an –OH, –NH, or SH group, not just in water
• Ice is less dense than liquid water, floating on it, which helps in insulating and maintaining life in cold conditions
• Water's high latent heat of vaporization requires a lot of energy to change state from liquid to gas
• Hydrogen bond in water

  Attraction between negatively charged oxygen of one molecule and positively charged hydrogen of another
• Water acts as a reagent in some chemical reactions like photosynthesis and hydrolysis
• Monomers join together

  By condensation reactions (Polymerization) where a water molecule is formed
• Hydrogen bonds occur not only in water but wherever there is an –OH, –NH, or SH group
• Polymers


  Molecules made up of many repeating subunits that are similar or identical to each other
• Carbohydrates contain only the elements carbon, hydrogen, and oxygen
• Monosaccharide

  • (CH2O)n
• Polysaccharide


  • (C 6 H 10 O 5 )n
• Sugars having long chain structure can close up on itself to form a ring structure which is a more stable arrangement
• Maltose is made by the condensation reaction between 2 α Glucose molecules forming α 1 – 4 Glycosidic bond
• Starch, glycogen, cellulose, chitin, and agar are all examples of polysaccharides
• A disaccharide can be broken down by hydrolysis reaction
• Disaccharide


  • C 12 H 22 O 11
• Monomers


  Subunits that are joined together like beads on a string to form polymers
• Polymers breakdown

  By hydrolysis reactions where a water molecule is used to break the bonds holding the monomers together
• Electron currents help to maintain the circulation of nutrients in the ocean
• Sucrose is made by the condensation reaction between α Glucose & β Fructose molecules forming α 1 – 2 β Glycosidic bond
• Polysaccharides are polymers formed by the joining of many monosaccharide molecules by condensation held by glycosidic bonds
• Starch is an energy store used by plants
• Polysaccharides


  Polymers formed by the joining of many monosaccharide molecules by condensation held by glycosidic bonds
• Amylopectin


  • Long branched chain of α Glucose linked by 𝛂 1 – 4 Gycosidic bond
  • Branches start with 𝛂 1 – 6 Glycosidic bond
  • Branches occur every 24 – 30 glucose units
  • Can be broken down more quickly than amylose by amylase enzyme
• Starch Test: Add sample, add Iodine in Potassium iodide solution, presence of starch turns the solution blue-black
• Starch

  • Mixture of amylose (20%) and amylopectin (80%)
  • Compact for efficient storage
  • Insoluble to prevent diffusion out of cells and avoid osmotic effects
  • Inert to avoid reactions with other molecules in the cell
• Cellulose


  • Polymer of 𝜷 glucose molecules forming long unbranched straight chains
  • Molecules form a straight chain due to flipping of alternate glucose molecules 180o to allow bonding of the hydroxyl groups forming 𝜷 1 – 4 glycosidic bond
  • Cellulose microfibrils are formed by many cellulose molecules held together by hydrogen bonds, providing tensile strength & permeability to the cell wall
• Amylose

  • Long unbranched chain of thousands of 𝛼 Glucose linked together by 𝜶 1 – 4 Gycosidic bond
  • Forms a helix maintained by hydrogen bonds making the molecule more compact
• Polysaccharides


  • Final molecule may be several thousand monosaccharide units long, forming a macromolecule
  • Examples include Starch, glycogen, cellulose, chitin, and agar
  • They differ primarily in the three-dimensional pattern in which the monomers are bonded to each other
  • They are all large, insoluble, non-sweet & non-reducing
• Starch breakdown


  Starch can be broken down back into glucose molecules, which can be used in respiration to release energy
• Glycogen


  • Energy store for animals, especially in muscle & liver cells
  • Similar to amylopectin but branches occur every 8 – 12 glucose units, making it more compact
• Reducing Sugar Test (Benedict Test): Heating reducing sugars with alkaline copper sulphate changes solution color from blue to orange-red
• Observing concentration

  1. Colour intensity by eye or colorimeter (the darker the colour the more the concentration)

  2. Time taken for colour to change (the shorter the time the more the concentration)

  3. Precipitate weight (the heavier the ppt the higher the concentration)