Energy Transfers in and Between Organisms: Photosynthesis - Required Practical 8, Effect of Light Intensity on Dehydrogenase Activity

Question 1

Which type of photosynthesis (light-independent/dependent) is dehydrogenase involved in?

Answer 1

• Light-dependent stage of photosynthesis

Question 2

What is the role of dehydrogenase in photosynthesis?

Answer 2

• In PSI, NADP acts as an electron acceptor and is reduced

- This reaction is catalysed by a dehydrogenase enzyme

Question 3

What is the purpose of DCPIP in this experiment?

Answer 3

• It’s a redox indicator dye
• Acts as an alternate electron acceptor instead of NADP
• As the dye is reduced, it turns from blue to colourless

Question 4

How can you use DCPIP to measure the rate of dehydrogenase activity?

Answer 4

• The rate of dehydrogenase activity can be measured using the rate at which DCPIP loses its blue colour

Question 5

What is a colorimeter?

Answer 5

• Measures how much light a solution absorbs when a light source is shone directly on it

Question 6

Outline the method of this experiment

Answer 6

• Remove stalks and midribs from leaf samples
• Grind sample, using a pestle and mortar, and place into a chilled isolation solution
• Use a muslin cloth and funnel to filter sample into a beaker. Suspend beaker in an ice water bath
• Transfer liquids into centrifuge tubes and centrifuge at high speed for 10 minutes. The chloroplasts will gather at the bottom of each tube in a ‘pellet’
• Remove supernatant and add pellet to fresh isolation medium. Store isolation solution on ice
• Set colorimeter to red filter. Zero using a cuvette containing chloroplast extract and distilled water
• Place test tube in the rack 30cm from light source and add DCPIP. Immediately take a sample and add to cuvette. Measure absorbance using colorimeter
• Measure absorbance every 2 minutes for 10 minutes
• Repeat for different distances from lamp up to 100cm

Question 7

Why is the plant extract chilled in an ice-water bath?

Answer 7

• To lower the activity of enzymes to prevent them from breaking down the chloroplasts

Question 8

How is light intensity controlled?

Answer 8

• Adjust the distance of the lamp from the set up

- Perform the practical in a dark room so that the only light source is the lamp

Question 9

What is the function of the muslin cloth?

Answer 9

• To filter out debris but allowing chloroplasts to pass through

Question 10

Why are the stalks removed before grinding?

Answer 10

• Stalks don’t contain many chloroplasts