RP 8 - The effect of ammonium hydroxide on the time taken for chloroplasts to decolourise DCPIP

Question 1

You were told not to put the leaf midrib or leaf stalk in the blender (Step 2). Suggest why.

Answer 1

Few/no chloroplasts present
OR
Tough material so will not be cut up

Question 2

What was the purpose of using a blender?

Answer 2

Break open cells/cell walls/ allow release of chloroplasts

Question 3

The isolation medium was cold and was also isotonic with the spinach tissue.

(a) Explain why it was important that the isolation medium was cold (3)

Answer 3

Stops/slows enzyme activity
Enzymes released during blending
Chloroplasts not damaged

Question 4

The isolation medium was cold and was also isotonic with the spinach tissue.

(a) Explain why it was important that the isolation medium was isotonic with the spinach tissue (2)

Answer 4

Maintains water potential
Chloroplasts/organelles do not burst

Question 5

There are large pieces of tissue and other organelles in your chloroplast suspension.
Describe how you could isolate the chloroplasts from the other components in the
suspension.

Answer 5

Filter/centrifuge
At low speed then high speed/centrifuge again at higher speed
Collect pellet

Question 5

You were told to wrap tube A (DCPIP solution, water and chloroplast suspension) in aluminium foil (step 8). Explain why.

Answer 5

Prevent entry of light/ stop photosynthesis/ keep in the dark

Question 6

Tube B was a control experiment (DCPIP solution, Water and isolation medium) (step 8). Explain how the results in this tube acted as a control experiment.

Answer 6

Shows that colour change in X / Colour does not change in B

Not due to isolation medium

Due to chloroplasts/chlorophyll

Question 7

In your investigation, the colour change in DCPIP was caused by the transfer of
electrons from chlorophyll. Name the part of a chloroplast where this transfer of
electrons takes place.

Answer 7

Chloroplast membrane

Question 8

DCPIP is decolourised more slowly when ammonium hydroxide is added. Use your
knowledge of the light-dependent reaction to suggest a reason why.

Answer 8

Ammonium hydroxide accepts electrons
So electrons do not pass alone electron transport chain/to DCPIP

Question 9

Some weed killers are substances that accept electrons during photosynthesis.

12 (a) Suggest how these substances kill weeds.

Answer 9

Stop/slow LDR
Stop/less production of NADPH/ATP

Question 10

Some weed killers are substances that accept electrons during photosynthesis.

12 (b) Suggest one disadvantage of using these weed killers on weeds growing in a crop. Explain your answer (2)

Answer 10

1. Absorbed by crop
   reduce 2. Growth/yield/weed killer not selective
2. Toxic effects on other wild life/when crop eaten