Required practical 8

• Describe the role of the enzyme dehydrogenase in photosynthesis

  ● Catalyses the reduction of NADP in the light-dependent reaction

  ○ NADP accepts (gains) electrons from photoionisation of chlorophyll / photolysis of water
• Describe how rate of dehydrogenase activity in extracts of chloroplasts can be measured

  • Extract chloroplasts from a leaf sample
  • Set up test tubes:
  • 1 - DCPIP, water and chloroplasts in isolation medium, cover in foil to block light
  • 2 - DCPIP, water and isolation medium, no chloroplasts
  • 3 - water and chloroplasts in isolation medium, no DCPIP
  • 4 - DCPIP, water and chloroplasts in isolation medium
  • Shine light on test tubes and time how long it takes for DCPIP to decolourise in tube D
  • Compare to a colour standard (C) to identify end point
  • Rate of dehydrogenase activity = 1 / time taken
• Give examples of variables that

  could be controlled. (3)

  ● Source of chloroplasts

  ● Volume of chloroplast suspension

  ● Volume / concentration of DCPIP
• Explain the purpose of control 1

  (tube A). (2)

  ● Shows light is required for DCPIP to decolourise

  ● Shows that chloroplasts alone do not cause DCPIP to decolourise
• Explain why DCPIP in control 1

  stays blue. (2)

  ● No light so no photoionisation of chlorophyll

  ● So no electrons released to reduce DCPIP
• Explain the purpose of control 2

  (tube B). (2)

  ● Shows chloroplasts are required for DCPIP to decolourise

  ● Shows that light alone does not cause DCPIP to decolourise
• Explain why DCPIP changes from

  blue to colourless. (2)

  ● DCPIP is a redox indicator / DCPIP gets reduced by electrons

  ● From photoionisation of chlorophyll
• Suggest a limitation with the

  method and how the experiment

  could be modified to overcome

  this. (4)

  ● End point (colour change) is subjective

  ● Use a colorimeter

  ● Measure light absorbance of sample at set time intervals

  ● Zero colorimeter using the colour standard
• A student set up a tube of solution without chloroplasts and with DCPIP in light. Explain the importance of this tube

  • to show that chloroplasts are required
  • to show light does not affect DCPIP