• provide evidence for assessment of Outcome 3
  (For advice on marking Outcome 3 report, please contact SAPS Scotland)
  
• develop knowledge and understanding of compensation points in sun and shade plants
  
• develop problem solving skills and in particular Outcome 2 PC:
  (d) Experimental procedures are planned, designed and evaluated appropriately.

• Leaf discs in 10 cm³ syringes from a sun plant and a shade plant are put in a sodium hydrogen carbonate solution. The air is then sucked out of them so that they sink. They are then allowed to photosynthesise in white light, the O₂ produced making them re-float.
  
• The experiment is then repeated in green light. If the leaf discs can use the green light for photosynthesis, e.g. shade plants, they will eventually rise as before. If the discs cannot use green light, e.g. sun plants, they will stay at the bottom of the container.

SAPS Student Sheet 3 (1990) Investigating the behaviour of leaf discs. Osmosis 1, 4-5.
http://www-saps.plantsci.cam.ac.uk/worksheets/ssheets/ssheet3.htm

• Since the apparatus used in this experiment is not expensive and probably readily available in most schools/colleges, it may be appropriate for students to work individually or at the most in pairs.
  
• Prior to students carrying out the experiment, arrange a maximum time to wait for the leaf discs to rise - say 15 or 20 minutes. Assume they will never float after that time.
  
• For reasons that are not immediately obvious, only certain plants work in this practical so it is worth testing them quickly beforehand.
  
• Timings will be shorter if good illumination is provided (e.g. light bank / warm, sunny windowsill) and if the sodium hydrogen carbonate solution is warm (20 – 30ºC). (If the sodium hydrogen carbonate solution is cold to start with, heat from the lights will probably increase its temperature as the experiment proceeds, influencing the results.
  
• Even when working individually, the experiment should take less than one hour. Working in pairs will reduce this time considerably.

In order to satisfy the core skill in problem solving, students will be required to “identify and obtain resources” required for themselves. It is therefore not appropriate to provide all equipment and materials in e.g. a tray system for each student/group. Normal laboratory apparatus should not be made available in kits but should generally be available in the laboratory. Trays could be provided containing one type of specialist equipment or materials.

• Does the response of the same species vary if one plant has previously been in bright light and one in the shade?
  
• Vary the bathing solution e.g. concentration of sodium hydrogen carbonate or pH.
  
• Response to different colours of light using filters.

2 x 10 cm³ syringes
about 20 cm³ 0.2 M sodium hydrogen carbonate solution
stop clock
2 green syringe covers

Strong source of light
Sheets of glass or perspex can also be provided to prevent heat from the light reaching the syringe.
cork borer no. 3
Shade plant e.g. aspidistra
Sun plant e.g. punnet of cress, ’fast’ plant cotyledons
Other plants are possible but they must be in good health and growing rapidly. It is best to check possible plants beforehand for this experiment.

• 0.2M sodium hydrogen carbonate (20 cm³ per group/student) made up with water that has been allowed to come up to room temperature (20 – 30ºC). 4–5 drops of detergent should be added to a litre of solution. This helps prevent the discs from sticking to the sides of the syringe.
  
• Green syringe covers - Green colour filter sheet from Philip Harris, Catalogue no. Q59854/0. Cut a piece of filter sheet 10.5 x 5.5 cm. Fit this around a 10 cm³ syringe. Use sellotape to seal the seam and to close the end at the tip of the syringe. (Over 20 such covers can be made from one sheet).

It is not appropriate to provide all equipment and materials in e.g. a tray system for each student/group. Equipment and materials should be supplied in a way that students have to identify and obtain resources. Normal laboratory apparatus should not be made available in kits but should generally be available in the laboratory. Trays could be provided containing one type of specialist equipment or materials.

1. What is the aim of the activity?

2. The colour of light is being varied in the activity. How would you describe the colour of each kind of light used?

3. What variables must be kept constant?

4. What measurements are you going to make?

In your groups decide how the activity will be managed by allocating tasks to each member. For Outcome 3 it is important that you play an active part in setting up the experiment and in collecting results.

Prepare a table to record the results. You should use a ruler, correct headings and appropriate units.

1. Why are 3 leaf discs taken from each plant instead of just 1 or 2?

2. If you use more than 3 discs per syringe the discs may overlap when lying at the bottom of the syringe. Why is this a problem and what steps can be used to avoid overlapping even when using 3 discs?

3. How do you think heat from the light source will affect the results? How can this be prevented?

4. What other measurements need to be kept constant throughout the experiment? How will this be achieved?

When leaf discs are immersed in a sodium hydrogen carbonate solution (a source of carbon dioxide) and illuminated, the oxygen produced by photosynthesis causes the leaves to float. The time the leaves take to float can thus be used as an indirect measure of the rate of photosynthesis i.e. the more quickly flotation occurs, the faster the rate of photosynthesis.

Leaf discs from sun and shade plants are illuminated with white light and the times taken to float noted. The experiment is then repeated, this time illuminating the discs with green light.

The experimental results should mimic the conditions in the plant’s natural habitat e.g. the sun plant in the canopy will receive white light and absorb the blue and red light from it in order to photosynthesise. However, the shade plant will receive the light that has already passed through the canopy. In order to photosynthesise it may therefore have to absorb many other wavelengths of light e.g. green

2 x 10 cm³ syringes
20 cm³ sodium hydrogen carbonate solution
stop clock
2 syringe covers made from a green filter sheet
Materials to be shared:
No. 3 cork borer
Shade plant e.g. aspidistra
Sun plant e.g. cress
Strong light source
Sheet of glass or perspex (optional)