Biological molecules

• How hydrogen bonds form between water molecules

  1. Water is polar: O more electronegative than H, so attracts electron density in covalent bond more strongly. Forms O 𝛿- (slightly negative) & H 𝛿+ (slightly positive)

  2. There are intermolecular forces of attraction between a lone pair on O 𝛿- of one molecule & H 𝛿+ on an adjacent molecule
• 7 biologically important properties of water

  • Reaches maximum density at 4℃
  • High surface tension
  • Incompressible
  • Metabolite/ solvent for chemical reactions in the body
  • High specific heat capacity
  • High latent heat of vaporisation
  • Cohesion between molecules
• Incompressible nature of water

  • Provides turgidity to plant cells
  • Provides hydrostatic skeleton for some small animals e.g. earthworms
• Why ice floats on water

  1. Ice is less dense than water because H-bonds hold molecules in fixed positions further away from each other

  2. Insulates water in arctic climates so aquatic organisms can survive. Water acts as a habitat
• High surface tension of water

  • Slows water loss due to transpiration in plants
  • Water rises unusually high in narrow tubes, lowering demand on root pressure
  • Some insects can 'skim' across the surface of water
• Water as a solvent for organisms

  Polar universal solvent dissolves & transports charged particles involved in intra & extracellular reactions e.g. PO4

  1. for DNA synthesis
• High specific heat capacity and latent of vapourisation of water

  • Acts as a temperature buffer which enables endotherms to resist fluctuations in core temperature to maintain optimum enzyme activity
  • Cooling effect when water evaporates from skin surface as sweat/ from mouth when panting
• Monomer


  Smaller units that join together to form larger molecules
• Polymer


  Molecules formed when many monomers join together
• Monomers


  • Monosaccharides (glucose, fructose, galactose, ribose)
  • Amino acids
  • Nucleotides
• Polymers


  • Polysaccharides
  • Proteins
  • DNA/ RNA
• Condensation and hydrolysis reactions

  1. Condensation: chemical bond forms between 2 molecules & a molecule of water is produced

  2. Hydrolysis: a water molecule is used to break a chemical bond between 2 molecules e.g. peptide bonds in proteins, ester bonds between fatty acids & glycerol in lipids
• Elements found in


  • Carbohydrates & lipids: C, H, O
  • Proteins: C, H, O, N, S
  • Nucleic acids: C, H, O, N, P
• α-glucose

  Hexose monosaccharide (6C) with ring structure, cis isomer
• β-glucose

  Hexose monosaccharide (6C) with ring structure, trans isomer
• Properties of α-glucose


  • Small & water soluble = easily transported in bloodstream
  • Complementary shape to antiport for co-transport for absorption in gut
  • Complementary shape to enzymes for glycolysis = respiratory substrate
• Ribose


  Pentose monosaccharide (5C) with ring structure
• Glycosidic bond


  • Bond that forms when monosaccharides react
  • 2 monomers = 1 chemical bond = disaccharide
  • Multiple monomers = many chemical bonds = polysaccharide
• Starch

  • Storage polymer of α-glucose in plant cells
  • Insoluble = no osmotic effect on cells
  • Large = does not diffuse out of cells
  • Made from amylose: 1,4 glycosidic bonds, helix with intermolecular H-bonds = compact
  • And amylopectin: 1,4 & 1,6 glycosidic bonds, branched = many terminal ends for hydrolysis into glucose
• Glycogen


  • Main storage polymer of α-glucose in animal cells (but also found in plant cells)
  • 1,4 & 1,6 glycosidic bonds, branched = many terminal ends for hydrolysis
  • Insoluble = no osmotic effect & does not diffuse out of cells
  • Compact
• Cellulose


  • Polymer of β-glucose gives rigidity to plant cell walls (prevents bursting under turgor pressure, holds stem up)
  • 1,4 glycosidic bonds, straight-chain, unbranched molecule
  • Alternate glucose molecules are rotated 180°
  • H-bond crosslinks between parallel strands form microfibrils = high tensile strength
• How triglycerides form


  Condensation reaction between 1 molecule of glycerol & 3 fatty acids which forms ester bonds
• Saturated fatty acids


  Contain only single bonds, straight-chain molecules have many contact points, higher melting point = solid at room temperature, found in animal fats
• Unsaturated fatty acids


  Contain C=C double bonds, 'kinked' molecules have fewer contact points, lower melting point = liquid at room temperature, found in plant oils
• Functions of triglycerides


  • High energy:mass ratio = high calorific value from oxidation (energy storage)
  • Insoluble hydrocarbon chain = no effect on water potential of cells & used for waterproofing
  • Slow conductor of heat = thermal insulation e.g. adipose tissue
  • Less dense than water = buoyancy of aquatic animals
• Phospholipids


  • Amphipathic: glycerol backbone attached to 2 hydrophobic fatty acid tails & 1 hydrophilic polar phosphate head
  • Forms phospholipid bilayer in water = component of membranes
  • Tails can splay outwards = waterproofing e.g. for skin
• Phospholipids and triglycerides are not polymers
• Cholesterol


  • Steroid structure of 4 hydrocarbon rings, hydrocarbon tail on one side, hydroxyl group (-OH) on the other side
  • Adds stability to cell surface phospholipid bilayer by connecting molecules & reducing fluidity
• Amino acid structure


  • -COOH carboxyl / carboxylic acid group
  • -R variable side group consists of carbon chain & may include other functional groups e.g. benzene ring or -OH (alcohol)
  • -NH2 amine/ amino group
• How polypeptides form


  1. Condensation reactions between amino acids form peptide bonds (-CONH-)

  2. There are 4 levels of protein structure
• Primary structure of a protein

  Sequence, number
• Amino acid


  • -COOH carboxyl / carboxylic acid group
  • -R variable side group consists of carbon chain & may include other functional groups e.g. benzene ring or -OH (alcohol)
  • -NH2 amine/ amino group
• Polypeptide formation


  Condensation reactions between amino acids form peptide bonds (-CONH-)
• Protein structure


  4 levels
• Primary structure


  Sequence, number & type of amino acids in the polypeptide, determined by sequence of codons on mRNA
• Secondary structure


  Hydrogen bonds form between O δ- attached to ‒C=O & H δ+ attached to ‒NH
• Types of secondary protein structure

  • α-helix
  • β-pleated sheet
• Tertiary structure


  • 3D structure formed by further folding
  • Disulfide bridges: strong covalent S-S bonds between molecules of the amino acid cysteine
  • Ionic bonds: relatively strong bonds between charged R groups (pH changes cause these bonds to break)
  • Hydrogen bonds: numerous & easily broken
• Quaternary structure


  • Functional proteins may consist of more than one polypeptide
  • Precise 3D structure held together by the same types of bond as tertiary structure
  • May involve addition of prosthetic groups e.g metal ions or phosphate groups
• Globular proteins


  • Spherical & compact
  • Hydrophilic R groups face outwards & hydrophobic R groups face inwards = usually water-soluble
  • Involved in metabolic processes e.g. enzymes such as amylase, insulin (2 polypeptide chains linked by 2 disulfide bonds), haemoglobin