EN: Photosynthesis Experiments

Question 1

How can you investigate the pigments in leaves?

Answer 1

Using chromatography

Question 2

In addition to the photosynthetis pigments, what other pigments can be found in leaves?

Answer 2

Ones that play other essential roles, eg. protecting the leaves from excessive UV radiation.

Question 3

TLC

Answer 3

Thin layer chromatography

Question 4

Mobile phase

What is this in TLC?

Answer 4

When molecules can move.

In TLC, this is the liquid solvent.

Question 5

Stationary phase

What is this in TLC?

Answer 5

Where molecules can’t move.

In TLC, this is a solid (eg. glass) plate with a thin layer of gel (eg. silica gel) on top.

Question 6

Describe how you would use TLC to compare the pigments present in shade-tolerat plants and shade-intolerant plants:

Answer 6

1. Grind up leaves from shade-intolerant plants and add anhydrous sodium sulfate, then a few drops of propanone.
2. Transfer the liquid to a test tube, add petroleum ether and gently shake the tube.
3. Two distinct layers form - top layer is petroleum ether mixed with pigments.
4. Transfer liquid from top layer to test tube containing anhydrous sodium sulfate.
5. Draw horizontal pencil line at bottom of TLC plate.
6. Add concentrated spot of liquid from liquid in step 4 - point of origin.
7. Once dry, put plate into small glass container with prepared solvent - point of origin should be just above solvent.
8. Put a lid on the container and allow plate to develop.
9. After, mark solvent front with pencil and leave to dry.

Question 7

How do you calculate the R_f value?

Answer 7

R_f value = distance travelled by spot / distance travelled by solvent.

Question 8

In photsystem I, during the light-dependent stage of photosynthesis, NADP acts as an electron acceptor and is reduced.

What is this reaction catalysed by?

Answer 8

Dehydrogenase enzyme

Question 9

Describe an experiment used to investigate the effect of light intensity on dehydrogenase activity:

Answer 9

1. Cut leaves into pieces and remove tough stalks.
2. Use a pestle and mortar to grind the leaf pieces with some chilled isolation solution (sucrose, potassium chloride and phosphate buffer at pH 7).
3. Filter liquid into a beaker through a funnel lined with muslin cloth.
4. Transfer liquid to centrifuge tubes and centrifuge at high speed for 10 minutes - chloroplasts with gather at bottom.
5. Dispose of liquid at top of the tube.
6. Re-suspend remaining pellets in fresh, chilled isolation solution - chloroplast extract.
7. Set up colorimeter with red filter and zero using a cuvette containing chloroplast extract and distilled water.
8. Set up test tube rack at set distance from bench lamp and turn lamp on.
9. Put test tube in the rack, add a set volume of chloroplast extract and set volume of DCPIP.
10. Immediately take a sample of mixture from the tube and add to a clean cuvette.
11. Read absorbance.
12. Repeat every 2 minutes for the next 10 mins.
13. Repeat for different distances.

Question 10

What will happen if dehydrogenase activity is taking place?

Answer 10

Absorbance will decrease as DCPIP gets reduced and loses blue colour.