RP8 - Investigation Into The Effect Of A Named Factor On The Rate Of Dehydrgenase Activity In Exctracts Of Chloroplasts

Question 1

Describe the role of the enzyme dehydrogenase in photosynthesis (1+1)

Answer 1

Catalyses the reduction of NADP in the light dependent reaction
- NADP accepts electrons from photoionsiation of chlorophyll / photolysis of water

Question 2

Describe how rate of dehydrogenase activity in extracts of chloroplasts can be measured (4)

Answer 2

1) Extract chloroplasts from a leaf sample using cell fractionation

2) Set up test tubes as follows:
A) Control 1 - set volume of DCPIP, 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 it takes for DCPIP to turn from blue to colourless in tube D

4) Rate of dehydrogenase activity = 1 / time taken

Question 3

Give examples of variables that could be controlled (3)

Answer 3

1) Source of chloroplasts

2) Volume of chloroplast solution

3) Volume / concentration of DCPIP

Question 4

Explain the purpose of control 1 (tube A) (2)

Answer 4

1) Shows light is required for DCPIP to decolourise

2) Shows that chloroplasts alone do not cause DCPIP to decolourise

Question 5

Explain why DCPIP in control 1 stays blue (2)

Answer 5

1) No light so no photoionisation of chlorophyll

2) So no electrons released to reduce DCPIP

Question 6

Explain the purpose of control 2 (tube B) (2)

Answer 6

1) Shows chloroplasts are required for DCPIP to decolourise

2) Shows that light alone does not cause DCPIP to decolourise

Question 7

Explain why DCPIP changes from blue to colourless (2)

Answer 7

1) DCPIP is a redox indicator / DCPIP gets reduced by electrons

2) From photoionisation of chlorophyll

Question 8

Suggest a limitation with the method and how the experiment could be modified to overcome this (4)

Answer 8

1) End point is subjective

2) Use a colorimeter

3) Measure light absorbance of sample at set time intervals

4) Zero calorimeter using the colour standard