Step-by-Step Method
- Destarch the plant: place it in complete darkness for 48 hours. This ensures any starch already in the leaves is used up by respiration, so you start with starch-free leaves.
- Expose the leaf to the test condition: e.g. Place in light (to test if light is needed) or cover part of the leaf with foil.
- Remove a leaf and kill it: dip the leaf in boiling water for 30 seconds. This kills the cells (stops further reactions) and breaks down the cell membrane (makes the cell permeable for iodine to enter).
- Decolourise the leaf: place the leaf in a tube of hot ethanol (in a water bath, NOT over a flame: ethanol is flammable). The ethanol dissolves the chlorophyll, turning the leaf white/pale so colour changes with iodine are visible.
- Soften the leaf: dip the leaf briefly in warm water. Ethanol makes the leaf brittle; warm water softens it.
- Test with iodine: spread the leaf on a white tile and add drops of iodine solution.
- Blue-black = starch present → photosynthesis occurred.
- Yellow-brown = no starch → photosynthesis did not occur.
Why Each Step Is Necessary
| Step | Purpose |
|---|---|
| Destarching | Removes existing starch so any starch found later must be newly produced by photosynthesis during the experiment |
| Boiling water | Kills cells; denatures enzymes; makes cell membrane permeable to iodine |
| Hot ethanol | Dissolves green chlorophyll so the iodine colour change can be clearly seen |
| Warm water | Softens the leaf (ethanol makes it brittle) and removes residual ethanol |
| Iodine test | Detects the presence of starch: the product of photosynthesis stored in the leaf |
Variations of the Starch Test
The starch test can be adapted to investigate different factors:
- Is light needed? Cover part of a leaf with opaque foil. Only the uncovered part turns blue-black → light is needed.
- Is chlorophyll needed? Use a variegated leaf (green and white parts). Only the green parts turn blue-black → chlorophyll is needed.
- Is CO₂ needed? Enclose a destarched plant in a container with soda lime (absorbs CO₂). The leaf stays yellow-brown → CO₂ is needed.
Why do we test for starch, not glucose? Glucose is soluble and would dissolve in the boiling water and ethanol. Starch is insoluble and stays in the leaf cells, making it detectable with iodine.