The response of leaf discs from sun and shade plants to green light
Student sheet
Preparing for the activity
Read through the Student Activity Guide and consider the following questions.
Analysis of Activity
- What is the aim of the activity?
- The colour of light is being varied in the activity. How would you describe the colour of eachkind of light used?
- What variables must be kept constant?
- What measurements are you going to make?
Getting organised for experimental work
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 incollecting results.
Recording of Data
Prepare a table to record the results. You should use a ruler, correct headings and appropriate units.
Evaluation
- Why are 3 leaf discs taken from each plant instead of just 1 or 2?
- If you use more than 3 discs per syringe the discs may overlap when lying at the bottom ofthe syringe. Why is this a problem and what steps can be used to avoid overlapping evenwhen using 3 discs?
- How do you think heat from the light source will affect the results? How can this be prevented?
- What other measurements need to be kept constant throughout the experiment? How willthis be achieved?
Student activity guide
Background information
When leaf discs are immersed in a sodium hydrogen carbonate solution (a source of carbondioxide) 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 ofphotosynthesis 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 tofloat 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. thesun plant in the canopy will receive white light and absorb the blue and red light from it inorder to photosynthesise. However, the shade plant will receive the light that has alreadypassed through the canopy. In order to photosynthesise it may therefore have to absorbmany other wavelengths of light e.g. green.
Equipment and materials
Materials required by each student\group:
- 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)
Instructions
1. Collect the materials indicated above.
1a. Using the No. 3 cork borer, cut out 3 discs from a shade plant, e.g. Aspidistra.
2. Remove the plunger from a 10 cm3 transparent plastic syringe. Place your finger over the nozzle,add about 5 cm3 0.2 M sodium hydrogen carbonate solution.
3. Carefully put the leaf discs into the solution in the syringe.
4. Carefully replace the plunger and point the syringe upwards.
5. Push out all of the air.
6. Place a finger over the nozzle. Gently pull the plunger down. Many bubbles will appear on the leaf discs.
7. Once the bubble production has slowed down, release your finger from the nozzle and tap thesyringe vigorously so that the air bubbles rise to the top. Repeat steps 5, 6 and 7 until all the leafdiscs sink.
8. Put the syringe close to the strong light source and start a stopwatch. Record the time taken foreach leaf disc to rise.
9. Calculate the average time for the leaf discs to float.
10. Repeat the experiment using 3 discs from a sun plant, e.g. cress cotyledons.
11. Resink the leaf material, cover each syringe with a green filter and again note the time for the leafmaterial to float.
12. Calculate the average time for the leaf discs to float as before. N.B. Discuss with yourteacher/lecturer the maximum time you should wait for the discs to rise. Having waited this length oftime it can be assumed they will never respond.
13. Draw up a table of results using correct headings and appropriate units.
14. Present your results as a graph with suitable scales and axes labelled with quantities and units.
Science & Plants for Schools:
The Response of leaf discs from sun and shade plants to green light p. 1
This document may be photocopied for educational use in any institution taking part in the SAPS programme.
It may not be photocopied for any other purpose. Revised 2012.