biological molecules

• how do hydrogen bonds form between $H_2O$ molecules 


  • water is polar - O is more electronegative and attracts electron pairs more in covalent bond more strongly forming O $^{\delta^-}$ and H $^{\delta^+}$
  • intermolecular forces of attraction between lone pair on O of one molecule and H of adjacent molecule
• importance of waters incompressible nature

  • provides turgidity to plant cells
  • provides hydrostatic skeleton for some small animal, eg. earthworms
• why does ice float? + its importance

  • ice is less dense than water because hydrogen bonds hold molecules in fixed positions further away from each other
  • insulates water in arctic climates so aquatic organisms can survive
• importance of high surface tension

  • slows water loss during transpiration in plants
  • some insects can skim across water surface
• why is water an important solvent?

  • polar universal solvent that dissolves + transports charged particles involved in intra + extracellular reactions e.g $PO_4^{3-}$ for DNA synthesis
  • solubility depends if water molecules can interact with a substance - attracted to the slightly charged part of solute molecules, keeping them apart
• latent heat vaporisation 


  • to change states, a large amount of thermal energy must be absorbed by water to break hydrogen bonds
  • only a little amount of water is require to evaporate for organisms to lose a large amount of heat - providing a cooling effect - evaporation of water in sweat on skin
• high specific heat capacity

  • the amount of thermal energy needed to raise the temp of 1kg of a substance by 1°C
  • high SHC due to many hydrogen bonds which takes a lot of energy to break/build so water's temp does not fluctuate greatly - biological processes in humans are stable at non-fluctuating temperatures, optimum enzyme activity
• monomer


  smaller units that join together to form larger molecules, e.g:

  • monosachharides (glucose, fructose)
  • amino acids
  • nucleotides
• polymer


  molecules formed when many monomers join together, e.g:

  • polysachharides
  • proteins
  • DNA/RNA
• condensation reaction


  chemical bond forms between 2 molecules and a molecule of $H_2O$ is produced
• hydrolysis reaction


  a $H_2O$ molecule is used to break a chemical bond between 2 molecules, e.g peptide bonds in proteins
• elements found in carbohydrates + lipids

  • carbon
  • hydrogen
  • oxygen
• elements found in proteins

  • carbon
  • hydrogen
  • oxygen
  • nitrogen
  • sulfur
• elements found in nucleic acids

  • carbon
  • hydrogen
  • oxygen
  • nitrogen
  • phosporous
• $\alpha$-glucose

  • small + water soluble = easily transported in bloodstream
  • complementary shape to enzymes for glycolysis = respiratory substrate
• cohesion of water

  • water molecules stick together, creating surface tension
  • makes thin, long water columns which are strong + hard to break
  • the xylem relies on cohesion to pull up water molecules
• transportation of water

  • movement of materials requires liquid transport medium
  • hydrogen bonding means it's a liquid at biologically important temperatures
• freezing of water

  • ice is less dense than water, so it floats - creates habitat
  • water beneath is insulated so aquatic organisms can survive
• metabolic reactions of water

  • water is a reactant in some processes
  • used in hydrolysis and photosynthesis
• thermal stability of water

  • water has a high SHC due to many hydrogen bonds
  • oceans provide a relatively stable environment in terms of temp.
  • organisms use evaporation as a cooling mechanism e.g sweating
• describe ways in which the properties of water allow organisms to survive over a range of temperatures

  • freezing → ice is less dense than water, so floats - creates habitat for polar bears + water below is insulated so aquatic organisms can survive
  • high SHC → 4200 Kg/°C due to large amount of hydrogen bonds so water remains liquid at biologically important temperatures and oceans are relatively stable environments
  • high LHV → only a little amount of water is needed to change states and evaporate to provide a cooling effect through sweating
  • transparent → allows for underwater photosynthesis
  • cohesion → causes surface tension, which creates a habitat for insects
• testing for proteins

  • biuret solution
  • alkali + copper (II) sulfate
  • blue -> lilac
• primary structure of proteins

  • sequence of amino acids in a polypeptide chain
• testing for lipids

  • emulsion test
  • dissolve sample in ethanol
  • pour on top of distilled water
  • white emulsion forms
• testing for starch

  • iodine solution
  • orange -> blue/black
• testing for non-reducing sugars


  • negative benedicts test
  • add HCl and boil - splits disaccharides
  • cool solution, add NaOH to neutralise
  • add Benedicts solution + heat at 80°C for 5 mins
  • blue -> orange/brick-red (more sugar content)
• testing for reducing sugars

  • Benedict's solution
  • heat at 80°C for 5 mins
  • blue -> green/yellow/brick-red
  • the more red, the higher the concentration of reducing sugars
• polypeptide vs protein

  polypeptide - linear sequence of amino acids joined by peptide bonds

  protein - functional macromolecule formed from 1 or more polypeptide chains with a 3D structure (2°, 3°, 4°)
• cholesterol

  • 4 C rings with -OH group at end
  • hydrophobic + hydrophilic region
  • reduces fluidity at high temp and vice versa
• secondary structure

  • hydrogen bonding between C=O of one and H of another amino acid causes folding
  • forms alpha helix + beta pleated sheet
• tertiary structure

  • 2° folded further to form a 3D shape
  • held by:
  • hydrophilic/phobic interactions
  • (weak) hydrogen bonds
  • (stronger) ionic bonds
  • disulfide bonds if 2 R-groups contain sulfur
• quaternary structure

  • protein has more than one polypeptide chain
  • e.g haemoglobin:
  • 4 polypeptide chains, conjugated - each prosthetic group contains Fe2+
• collagen

  • in tendons, ligaments, cartilage, skin
  • 3 polypeptide chains, held by H-bonds + covalent crosslinks
  • flexible but not stretchy
• elastin

  • in elastic fibres around blood vessels/alveoli
  • allows them to stretch + recoil
• keratin

  • in skin, nails, hair - which all protect the body
  • insoluble
• insulin

  • hormone, lowers blood glucose conc.
  • specific 3D shape complimentary to receptors on cell surface membrane of target cells, e.g muscle/liver
• colorimetry

  1. set filter in colorimeter
  2. calibrate to 0 with distilled water
  3. insert sample
  4. measure percentage transmission of light
  5. draw calibration curve using results from known conc. of gluclsoe
• glycosidic bonds

  join two monosaccharides in carbohydrates
• disaccharides

  form when 2 simple sugar units are joined by a condensation reaction

  • sucrose - glucose + fructose (1,2)
  • maltose - $\alpha$-glucose + $\alpha$-glucose (1,4)
  • lactose - glucose + galactose (1,4)
• peptide bond

  link amino acids

  • between amine group of one amino acid and carboxyl group of another
  • condensation reaction