A-level biology

• Triglyceride formation


  Condensation reaction between glycerol and fatty acids, forming ester bonds and releasing water
• Phospholipids


  • Similar to triglycerides but one fatty acid replaced by a hydrophilic phosphate group
  • Phosphate group and hydrophobic fatty acid tails allow phospholipids to form a bilayer in cell membranes
• Properties of lipids


  • Triglycerides good for energy storage due to high energy content of fatty acid tails
  • Lipids insoluble in water so don't affect water potential of cells
• Phosphate group


  Hydrophilic (attracts water)
• Fatty acid tails


  Hydrophobic (repel water)
• The phosphate group and fatty acid tails are important in the cell membrane
• Emulsion test for lipids

  1. Shake test substance with ethanol

  2. Pour into water

  3. Milky colour indicates lipid
• Triglycerides


  • Mainly used as energy storage molecules
  • Contain lots of chemical energy in the long hydrocarbon tails
  • Insoluble in water so don't affect water potential of cells
• Phospholipids


  • Make up the bilayer of cell membranes
  • Heads are hydrophilic, tails are hydrophobic
  • Form a double layer with heads facing water, centre is hydrophobic
• Cell membranes control what enters and leaves a cell
• Amino acids


  Monomers that proteins are made from
• Dipeptide


  Two amino acids joined together
• Polypeptide


  More than two amino acids joined together
• Protein


  One or more polypeptides
• Amino acid structure


  • Carboxyl group, amine group, R group
  • R group generally contains carbon, except for glycine which has just hydrogen
• All living things share a bank of only 20 amino acids
• Dipeptide and polypeptide formation


  1. Amino acids linked by condensation reactions

  2. Molecule of water released

  3. Bonds formed are called peptide bonds

  4. Reverse reaction is hydrolysis
• Primary structure


  Sequence of amino acids in the polypeptide chain
• Secondary structure


  Polypeptide chain coils into alpha helix or folds into beta pleated sheet
• Tertiary structure


  Coiled or folded chain with more bonds forming, including hydrogen bonds and ionic bonds
• Quaternary structure


  How multiple polypeptide chains are assembled together
• Proteins and their functions

  • Enzymes - involved in metabolism
  • Antibodies - part of immune response
  • Transport proteins - move molecules across membranes
  • Structural proteins - physically strong
• Biuret test for proteins

  1. Add sodium hydroxide

  2. Add copper(II) sulfate

  3. Purple colour indicates presence of protein
• Enzymes


  Proteins that speed up the rate of chemical reactions
• Enzymes as biological catalysts

  • They catalyse metabolic reactions both at a cellular level and for the organism as a whole (e.g. digestion in mammals)
  • They can affect structures in an organism (e.g. enzymes are involved in the production of collagen, an important protein in the connective tissues of animals) as well as functions (like respiration)
  • Enzyme action can be intracellular-within cells, or extracellular-outside cells
• Enzyme


  A protein with an active site that has a specific shape
• Enzyme specificity


  Enzymes are highly specific due to their tertiary structure
• How enzymes speed up reactions

  1. Enzymes lower the amount of activation energy that's needed, often making reactions happen at a lower temperature than they could without an enzyme

  2. This speeds up the rate of reaction
• Enzyme-substrate complex


  • When a substrate fits into the enzyme's active site it forms an enzyme-substrate complex, which lowers the activation energy
  • If two substrate molecules need to be joined, being attached to the enzyme holds them close together, reducing any repulsion between the molecules so they can bond more easily
  • If the enzyme is catalysing a breakdown reaction, fitting into the active site puts a strain on bonds in the substrate, so the substrate molecule breaks up more easily
• Lock and key model

  The substrate fits into the enzyme's active site in the same way that a key fits into a lock
• Induced fit model


  The enzyme-substrate complex changes shape slightly to complete the fit, locking the substrate even more tightly to the enzyme
• Enzyme properties


  • Enzymes are very specific - they usually only catalyse one reaction
  • The active site's shape is determined by the enzyme's tertiary structure
  • If the tertiary structure of a protein is altered, the shape of the active site will change, meaning the substrate won't fit and the enzyme will no longer function
• The primary structure (amino acid sequence) of a protein is determined by a gene, and if a mutation occurs in that gene, it could change the tertiary structure of the enzyme produced
• Measuring enzyme activity


  • Can be done by measuring how fast the product is made or how fast the substrate is broken down
• As temperature increases


  The rate of enzyme-controlled reaction increases
• Enzyme denaturation


  If the temperature gets too high, the vibration breaks some of the bonds that hold the enzyme in shape, changing the active site shape so the enzyme and substrate no longer fit together
• Every enzyme has an optimum temperature, usually around 37°C for human enzymes, but some enzymes can work well at 60°C
• As pH moves away from the optimum


  The H and OH ions found in acids and alkalis can disrupt the ionic bonds and hydrogen bonds that hold the enzyme's tertiary structure in place, causing denaturation
• As substrate concentration increases


  The rate of the enzyme-controlled reaction increases, up to a saturation point where all the active sites are occupied
• As enzyme concentration increases


  The rate of the enzyme-controlled reaction increases, but if the substrate amount is limited, there comes a point where adding more enzyme has no further effect