Core Practical 11

• What is dehydrogenase?

  An enzyme found in plant chloroplasts
• What stage of photosynthesis is dehydrogenase crucial for?

  The light dependent stage
• What molecule accepts electrons in the light dependent stage?

  NADP
• Who discovered the reaction involving NADP?

  Robin Hill
• What is the Hill reaction?

  The reaction where NADP accepts electrons
• What happens to DCPIP when it is reduced?

  It turns from blue to colourless
• What equipment is needed for the experiment?

  • Leaf sample
  • Scissors
  • Mortar and pestle (cold)
  • Nylon mesh
  • Filter funnel
  • Centrifuge
  • Centrifuge tubes
  • Ice-water-salt bath
  • Glass rod
  • Measuring cylinder
  • Beaker
  • Pipettes
  • Bench lamp
  • Buffer
  • Isolation medium
  • DCPIP solution
• What are the steps in the method for the experiment?

  1. Remove stalks from leaf samples and grind.
  2. Filter sample using muslin cloth.
  3. Chill sample in an ice water bath.
  4. Centrifuge to separate chloroplasts.
  5. Remove supernatant and add pellet to medium.
  6. Store isolation solution on ice.
  7. Set colorimeter to red filter and zero it.
  8. Add DCPIP and measure absorbance.
  9. Measure absorbance every 2 minutes.

  10. Repeat for different distances from lamp.
• Why should the experiment be done in a darkened room?

  To make results more reliable
• What is the risk associated with DCPIP?

  It is an irritant to skin and eyes
• What should you do if DCPIP contacts skin or eyes?

  Wash immediately with cold water
• What is a safety precaution for using lamps?

  Do not look directly at the lamp
• How does light intensity affect the rate of photosynthesis?

  Lower light intensity decreases the rate
• What happens to the rate of photoionisation with decreased light intensity?

  It slows down
• What does a higher rate of absorbance decrease indicate?

  Higher activity of the dehydrogenase enzyme
• What is the relationship between DCPIP color and absorbance?

  • DCPIP is blue when absorbance is high.
  • DCPIP turns colourless as absorbance decreases.
  • The rate of absorbance decrease indicates enzyme activity.
• What should be plotted on the graph in the experiment?

  • Absorbance against time
  • Different distances from the light source