Biological Molecules

• Triglycerides

  Have one molecule of glycerol with three fatty acids attached to it. Fatty acid molecules have long tails made of hydrocarbons. The tails are hydrophobic (they repel water molecules). These tails make lipids insoluble in water.
• Fatty acids

  All fatty acids consist of the same basic structure, but the hydrocarbon tail varies. There are two kinds - saturated and unsaturated. The difference is in their hydrocarbon tails (R groups).
• Saturated fatty acids


  • Don't have any double bonds between their carbon atoms. The fatty acid is 'saturated' with hydrogen.
• Unsaturated fatty acids


  • Do have double bonds between carbon atoms, which cause the chain to kink.
• Triglycerides


  Formed by condensation reaction between fatty acids and glycerol
• Triglyceride formation

  1. Fatty acid joins glycerol

  2. Ester bond forms

  3. Water molecule released
• Phospholipids

  • Similar to triglycerides but one fatty acid replaced by phosphate group
  • Phosphate group is hydrophilic
  • Fatty acid tails are hydrophobic
• Cell membrane

  • Phospholipids form the cell membrane
  • Phospholipids have hydrophilic and hydrophobic regions
• Lipids are insoluble in water
• Triglycerides

  • Used as energy storage molecules
  • Contain lots of chemical energy in hydrocarbon tails
  • Insoluble in water so don't affect water potential of cell
• Triglycerides


  • Bundle together as insoluble droplets in cells
• Lipids contain about twice as much energy per gram as carbohydrates
• Phospholipids


  Make up the bilayer of cell membranes
• Phospholipid bilayer formation

  1. Heads facing out towards the water on either side

  2. Centre of the bilayer is hydrophobic
• Water-soluble substances can't easily pass through the hydrophobic centre of the phospholipid bilayer
• Emulsion test for lipids


  1. Shake test substance with ethanol

  2. Pour solution into water

  3. Milky emulsion indicates presence of lipid
• Dipeptide formation


  Two amino acids join together
• Polypeptide formation


  More than two amino acids join together
• Proteins


  Made up of one or more polypeptides
• Protein building blocks


  • Monomer (one amino acid)
  • Dipeptide (two amino acids)
  • Polypeptide (more than two amino acids)
• Amino acid structure

  • Carboxyl group (-COOH)
  • Amine or amino group (-NH)
  • R group (variable side group) attached to a carbon atom
• The only difference between amino acids is what makes up their R group
• Dipeptide and polypeptide formation


  1. Amino acids are linked together by condensation reactions

  2. A molecule of water is released during the reaction

  3. The bonds formed between amino acids are called peptide bonds

  4. The reverse reaction (hydrolysis) happens when dipeptides and polypeptides are broken down
• Hydrogen bond


  Relatively weak bonds formed between atoms of hydrogen and atoms of oxygen
• Primary structure


  Sequence of amino acids in the polypeptide chain
• Secondary structure


  Polypeptide chain coils into alpha helix or folds into beta pleated sheet due to hydrogen bonds
• Tertiary structure


  Coiled or folded chain of amino acids with more bonds including hydrogen, ionic and disulfide bridges
• Quaternary structure


  Assembly of multiple polypeptide chains held together by bonds
• If a protein has one long polypeptide chain, its tertiary structure is its final 3D structure
• If a protein has multiple polypeptide chains, its quaternary structure is its final 3D structure
• No protein is present

  Solution stays blue
• Enzymes


  Biological catalysts that speed up chemical reactions
• Enzymes


  • Proteins
  • Have an active site with a specific shape
  • Bind to substrate molecules
  • Highly specific due to tertiary structure
• Lowering activation energy


  Speeds up the rate of reaction
• Enzyme specificity

  • Determined by tertiary structure and active site
  • Allows enzymes to bind to specific substrates
• When a substrate fits into the enzyme's active site it forms an enzyme-substrate complex
• Enzyme-substrate complex


  Lowers the activation energy
• Enzyme-substrate complex

  • Holds substrate molecules close together, reducing repulsion
  • Puts strain on bonds in substrate, making it break up more easily
• Enzyme action


  1. Substrate binds to active site

  2. Active site changes shape slightly

  3. Reaction occurs

  4. Enzyme is unchanged after reaction
• Lock and key model


  • Enzymes only work with substrates that fit their active site
  • Substrate fits into enzyme active site like a key fits into a lock