practical 9

• Yeast


  A single celled organism that respires both aerobically and anaerobically
• Respiration


  Electrons are transferred to synthesise ATP
• Redox indicator dye


  Such as methylene blue, which accepts these electrons and undergoes a colour change from blue to colourless
• Equipment


  • Yeast and glucose in buffered solution
  • Water bath
  • Thermometer/temperature probe
  • Test tubes
  • Timer
• Investigation method


  1. Set up water bath at 35°C

  2. Add yeast and glucose solution to test tubes, equilibrate for 10 minutes

  3. Add methylene blue, start timer, shake for 10 seconds

  4. Repeat experiment at 40°C, 50°C, 60°C and 70°C

  5. Find mean results and calculate average rate of respiration
• The yeast and glucose solution should be buffered to maintain a constant pH
• Hazards


  • DCPIP - irritant to skin and eyes, may cause staining
  • Biohazard - allergies
  • Broken glass - cuts
  • Hot liquids - scalding
• Safety precautions


  • Wear eye protection, wash from skin/eyes immediately using cold water
  • Wash hands after use, seek assistance
  • Take care when handling glass objects, keep away from edge of desk, elevate cuts, apply pressure, do not remove glass from wound, seek medical assistance
  • Handle with care, use tongs to remove boiling tubes from water bath, wear eye protection, run burn under cold water, seek medical assistance
• Graph


  Plot rate of respiration against temperature
• Yeast has an optimum temperature range for respiration, shown by a peak on the graph
• As temperature moves away from the optimum

  The rate of reaction decreases as enzyme action decreases, and at high temperatures denaturation may occur
• As enzymes are crucial to respiration, as their activity decreases


  The rate of respiration decreases, meaning the methylene blue will take longer to turn colourless when the temperature is further from the optimum