A well watered Pelargonium plant shut in a warm dark cupboard for two days loses all the starch from its leaves. After two days the plant and the cells of the leaf are still alive and active. If discs are cut from the leaf with a cork borer, their ability to make starch under different conditions may be investigated. In a decolourised leaf the starch stains blue black with iodine solution.
In this investigation, we suggest that you use a variegated Pelargonium with white leaf patches; you will be varying the conditions of light available to the plant and feeding some of the plants in the dark with sugar.
1. Select from your de-starched Pelargonium plant several leaves in which the white patches are as large as possible.
2. Label four Petri dishes A,B,C and D. Half fill A and B with 5% glucose solution. Half fill C and D with water.
3. With a sharp cork borer, pressing gently against a cork mat, cut 8 white discs from the leaves and 8 green discs from the leaves. Choose good dark green areas for green discs. Be careful not to damage the leaf tissue of the disc.
4. Float two green and two white discs on the liquid in each Petri dish.
5. Place Petri dishes A and C under a lamp, (20 cm from a 40w bulb is ideal). Place Petri dishes B and D in the dark.
6. Leave for 24 hours. If it is necessary to leave them longer, place the dishes in a fridge in the dark and cold until the discs can be tested.
7. Put on to boil a 400 cm3 beaker with 250 cm3 of water in it.
8. Label 8 test tubes A/G, A/W and B/G, B/W and C/G, C/W and D/G and D/W for the leaf discs from dishes A,B,C and D (where G = green, and W = white disc). Carefully put the discs into the correct test tube e.g. from dish A the green disc goes in A/G and the white disc into A/W, etc.
9. Add 5cm3 water to each test tube. Make sure the discs are in the water. Place the test tubes in the boiling water bath. The water must then be free boiling for 5 minutes.
10. After 5 minutes SWITCH OFF THE GAS AND EXTINGUISH THE FLAME. Keep the boiling water in the water bath.
11. Using a test tube holder, pick up each tube in turn and quickly pour off the water. Replace the water with about one third of a test tube of ethanol. Make sure you do not lose your 2 discs in each tube (you can use a spatula or splint to hold them back).
12. Replace the 8 test tubes in the water bath. The ethanol should boil after a few moments. Leave the discs boiling in ethanol for 5 minutes.
13. While you are waiting, divide the white tile into 8 areas, labelled A/G, A/W and B/G, B/W and C/G, C/W and D/G and D/W.
14. Using the test tube holder, again pour off the spirit from the test tubes. Replace the water with warm water from the bath. Then, when the discs have been softened for a moment, tip them out onto the white tile. Make sure they are in the right areas!
15. Add three drops of iodine solution to each disc. Leave it for 3 minutes before draining away the excess stain.
Make a table to express your results.
1. Where has starch been formed?
2. What biological process causes the starch to be lost from the leaves of a plant in the dark?
3. What is the chemical relationship between glucose and starch?
4. What organelle contains the green pigment chlorophyll?
5. Were dishes A and C only different from B and D in the amount of light they received? Was this a fair test?
6. State the advantage of having more than one disc per tube in this experiment?
7. Is light essential for starch formation in a leaf?
8. Can starch be made by photosynthesis in a leaf without chlorophyll?
9. What can you state about the ability of the white areas to make starch?
10. Make a hypothesis to explain the results of this experiment.
A number of ideas are given below. Chose one, formulate a hypothesis based on your scientific knowledge, devise an investigation and predict what outcome you would expect.
Does it make any difference if the leaf discs are the right way up or upside down in this investigation?
Do all leaves behave the same way?
What happens if the leaf discs are exposed to short pulses of light during the dark period?
Do different wavelengths affect photosynthesis and starch formation in different ways?
Science & Plants for Schools: www.saps.org.uk
Can Leaf Discs Make Starch In The Dark: p.
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 2010.