3.1 Biological Molecules

• Monomers


  Small units which are the components of larger molecules
• Polymers


  Molecules made from many monomers joined together
• Monomer joining


  1. Condensation reaction

  2. Water molecule eliminated

  
• Polymer breaking down


  1. Hydrolysis

  2. Water added to break chemical bond
• Carbohydrates


  Molecules consisting of carbon, hydrogen and oxygen, long chains of sugar units called saccharides
• Monosaccharide


  Single sugar unit
• Disaccharide


  Two monosaccharides joined together
• Polysaccharide


  Many monosaccharides joined together
• Glucose


  • Six carbon atoms in each molecule
  • Main substrate for respiration
  • Has alpha and beta isomers

  
• Common monosaccharides


  • Glucose
  • Galactose
  • Fructose

  
• Disaccharide formation


  1. Condensation reaction

  2. Water molecule produced

  3. 1,4 glycosidic bond between two alpha glucose molecules to form maltose

  
• Polysaccharides


  Formed from many glucose units joined together
• Polysaccharides


  • Glycogen
  • Starch
  • Cellulose
• Glycogen


  • Main energy storage molecule in animals
  • Formed from alpha glucose
  • 1,4 and 1,6 glycosidic bonds
  • Branched structure
  • Compact and insoluble

  
• Starch


  • Energy storage in plants
  • Amylose (unbranched, 1,4 bonds)
  • Amylopectin (branched, 1,4 and 1,6 bonds)
  • Insoluble, compact, easily hydrolysed
• Cellulose


  • Component of plant cell walls
  • Unbranched chains of beta glucose
  • Microfibrils of parallel cellulose chains
  • Provides structural support and prevents cell bursting
• Benedict's test for reducing sugars


  1. Add sample + Benedict's reagent

  2. Heat in water bath

  3. Red precipitate = positive
• Benedict's test for non-reducing sugars


  1. Add sample + Benedict's reagent, heat

  2. No colour change

  3. Add dilute HCl, heat to hydrolyse

  4. Neutralise with NaHCO3

  5. Retest with Benedict's, colour change = positive

  
• Iodine test for starch


  Blue/black colour = starch present
• Lipids


  Biological molecules made of carbon, hydrogen and oxygen, only soluble in organic solvents

  
• Triglycerides


  Lipids made of one glycerol and three fatty acids joined by ester bonds

  
• Saturated lipids


  No carbon-carbon double bonds

  
• Unsaturated lipids


  Contain carbon-carbon double bonds, can bend

  
• Triglyceride properties


  • High energy density
  • Low mass to energy ratio
  • Insoluble in water
  • Release water when oxidised
• Phospholipids


  One fatty acid replaced by phosphate-containing group, polar molecule
• Phospholipid properties


  • Form bilayers in aqueous environments
  • Hydrophilic heads at cell surface
  • Can form glycolipids for cell recognition

  
• Emulsion test for lipids

  1. Add sample + ethanol, shake

  2. Add water, shake

  3. Cloudy-white colour = lipids present

  
• Amino acids


  Monomers that make up proteins, contain amino, carboxyl and R groups
• There are 20 different amino acids
• Peptide bond formation


  Condensation reaction, water molecule formed

  
• Dipeptide


  Two amino acids joined
• Polypeptide


  Three or more amino acids joined
• Protein structure


  • Primary (sequence of amino acids)
  • Secondary (alpha helix, beta pleated sheet)
  • Tertiary (3D shape, disulfide bridges, ionic bonds)

  
• Primary structure of a protein

  The order and number of amino acids in a protein. This primary structure contains the initial sequence of amino acids and will therefore determine the proteins function in the end.
• Secondary structure of a protein

  The shape that the chain of amino acids makes – either alpha helix or beta pleated sheet. The hydrogen in the -NH has a slight positive charge whilst the oxygen in the -C=O has a slight negative charge. As a result weak hydrogen bonds can form leading to alpha helices or beta pleated sheets.

  
• Tertiary structure of proteins


  The 3D shape of the protein and is formed from further twisting and folding. A number of different bonds maintain the structure, these are: Disulfide bridges, Ionic bonds, Hydrogen bonds.
• Proteins


  • Can be globular or fibrous. Globular proteins such as enzymes are compact whereas fibrous proteins such as keratin are long and thus can be used to form fibres.
• Biuret Test


  1. Place the sample to be tested in a test tube and add an equal volume of sodium hydroxide at room temperature

  2. Add a few drops of very dilute (0.05%) copper (II) sulfate soliton and mix gently

  3. A purple colouration indicates the presence of a peptide bond and hence a protein. A negative result would mean the solution remains blue.
• Enzymes


  Increase rate of reaction by lowering the activation energy of the reaction they catalyse. They are 3D tertiary structured globular proteins whose shape is determined by the primary sequence of amino acids.
• Active site


  An area of the enzyme that is made up of only a few amino acids and forms a small depression in the overall enzyme. The molecule that the enzyme acts upon is called the substrate.