Required practical 8

Question 1

Describe the role of the enzyme dehydrogenase in photosynthesis

Answer 1

● Catalyses the reduction of NADP in the light-dependent reaction
○ NADP accepts (gains) electrons from photoionisation of chlorophyll / photolysis of water

Question 2

Describe how rate of dehydrogenase activity in extracts of chloroplasts can
be measured

Answer 2

1. Extract chloroplasts from a leaf sample using the method in ‘2.1.3 Methods of studying cells’
2. Set up test tubes as follows: A. Control 1 - set volume of DCPIP (redox indicator dye, electron acceptor), water and chloroplasts in isolation medium, covered in foil to block light
   B. Control 2 - set volume of DCPIP, water and isolation medium without chloroplasts
   C. Standard - set volume of water and chloroplasts in isolation medium, without DCPIP
   D. Experiment - set volume of DCPIP, water and chloroplasts in isolation medium
3. Shine light on test tubes and time how long to it takes for DCPIP to turn from blue (oxidised) to colourless (reduced) in tube D (tube A and B should show no change)
   ○ Compare to a colour standard (tube C) to identify end point
4. Rate of dehydrogenase activity (s
   -1) = 1 / time taken

Question 3

Give examples of variables that
could be controlled.

Answer 3

● Source of chloroplasts
● Volume of chloroplast suspension
● Volume / concentration of DCPIP

Question 4

Explain the purpose of control 1
(tube A).

Answer 4

● Shows light is required for DCPIP to decolourise
● Shows that chloroplasts alone do not cause DCPIP to decolourise

Question 5

Explain why DCPIP in control 1 stays
blue.

Answer 5

● No light so no photoionisation of chlorophyll
● So no electrons released to reduce DCPIP

Question 6

Explain the purpose of control 2
(tube B).

Answer 6

● Shows chloroplasts are required for DCPIP to decolourise
● Shows that light alone does not cause DCPIP to decolourise

Question 7

Explain why DCPIP changes from
blue to colourless.

Answer 7

● DCPIP is a redox indicator / DCPIP gets reduced by electrons
● From photoionisation of chlorophyll

Question 8

Suggest a limitation with the
method and how the experiment
could be modified to overcome
this.

Answer 8

● End point (colour change) is subjective
● Use a colorimeter
● Measure light absorbance of sample at set time intervals
● Zero colorimeter using the colour standard