Rate of decay (required practical): Ecology: Biology: GCSE (9:1)

• Decomposition


  Animals, plants and waste products are typically broken down by microorganisms, assuming the conditions for decay are available

  
• Decomposer


  An organism responsible for the decay and breakdown of waste products and other dead organisms, so that important nutrients can be recycled

  
• Rate of decay

  The speed or rate at which waste products or dead organic material is broken down by decomposers

  
• Factors affecting the rate of decay

  Temperature, water and oxygen availability
• Temperature

  Decomposing microorganisms need warm temperatures so that enzymes have enough energy to break down waste or dead organic material

  
• Investigating the rate of decay

  Decay is a slow process, so the process can be simulated by using enzymes to break down milk into fatty acids and glycerol

  
• Milk decomposition


  Milk turns sour if left for an extended period of time, this is because lactic acid is produced which is a product of the bacterial decomposition of milk

  
• pH change

  Milk normally becomes more acidic due to lactic acid formation, the pH change associated with milk decomposition is simulated in this investigation by producing fatty acids instead

  
• Enzymes

  Biological catalysts that speed up the rate of reaction for specific substances, bacteria and other microorganisms use enzymes to aid digestion and decomposition

  
• Lipase


  An enzyme used to break lipids down into glycerol and fatty acids

  
• Dependent variable


  The variable that is measured, in this case the rate at which milk is broken down by lipase or the time taken for the milk solution to turn yellow

  
• Independent variable


  The variable that is changed, in this case the temperature of solution to demonstrate the effect of temperature change on the rate of decay

  
• Control variables


  The variables that are kept the same such as the variety of milk used, the volumes of solutions and concentrations of solutions

  
• Cresol red

  An indicator or dye that changes colour from purple in alkaline conditions to yellow in acidic conditions

  
• Sodium carbonate

  An alkaline compound that is added to milk at the start of the investigation, causing the cresol red dye to become purple

  
• Completed reaction


  When cresol red has changed colour completely from purple to yellow, suggesting milk has been decomposed into fatty acids and glycerol by lipase

  
• Expected results

  Milk will decay quickest in warm temperatures where enzymes have enough energy to break down organic material, but will not denature
• Phenolphthalein


  An alternative indicator that can be used in this investigation, will change colour from pink in alkaline conditions to colourless in acidic conditions

  
• Water bath


  A container of water heated to a given temperature, each test needs to occur at different temperatures to measure the rate of decay

  
• Acclimatisation

  To ensure results are valid, it is important to leave the solutions for a period of time in each temperature condition of the water bath before recording results
• Complications

  It can be difficult to visually determine when a reaction is completed based on colour change, the experiment should be repeated and data should be collected by different groups to account for this