RP8: DCPIP Decolourisation

• Leaf midrib and leaf stalk were not blended because no chloroplasts are present here
• Blender used to break open cell wall and cell-surface membrane
• Isolation medium was cold:

  • Reduces enzyme activity
  • Enzymes released during blending
  • Chloroplasts will not be damaged
• Isolation medium needs to be isotonic:

  • Prevents osmosis
  • Chloroplasts do not burst
• Aluminium foil used to prevent entry of light
• Control experiment shows that light does not decolourise DCPIP and shows chloroplasts are needed
• DCPIP solution added first so it can diffuse into the cells
• DCPIP decolourises:

  • Chlorophyll absorbs light and excites electron so is lost
  • DCPIP reduced by electron so turns colourless
• Ammonium hydroxide slows the electron transport chain so less electrons move down chain and decolorises DCPIP slowly