IGCSE Biology 0610: Destarching a Leaf and Testing for Starch

A student wants to show that a pelargonium (geranium) plant needs light to make starch in its leaves.

Two days before the investigation, the student places the whole plant in a dark cupboard, where it stays for 48 hours with no light at all.

The student then attaches a small square of aluminium foil to one leaf, covering both the upper and lower surfaces of a patch in the middle of the leaf, while leaving the rest of the same leaf uncovered. The plant is then placed on a sunny windowsill for six hours.

After six hours, the student removes the leaf, takes off the foil, and tests the whole leaf for starch using iodine solution.

(a) Explain why the student places the plant in the dark for 48 hours before starting the investigation. [2]

(b) Describe how the student should prepare the leaf for the iodine test, from removing it from the plant up to the point of adding iodine solution. Your answer should refer to the use of ethanol. [4]

(c) State the colour the iodine solution would turn on (i) the patch of leaf that was covered by foil and (ii) the rest of the leaf that was not covered, and explain what these two results together show. [2]

Part (a): Why destarch the plant first

If the leaf already contained starch before the investigation began, the student would not be able to tell whether any starch found afterwards was made freshly during the test or was simply left over from before. Placing the whole plant in the dark for 48 hours means the plant cannot photosynthesise, so it uses up its existing starch reserves (respiring them or converting them for transport) without making any more. This destarches the leaf, so any starch detected at the end of the investigation must have been made during the six hours of light exposure.

Part (b): Preparing the leaf for the iodine test

Dip the leaf in boiling water for about a minute. This kills the leaf cells and softens the leaf’s structure.
Place the leaf in a test tube of ethanol, and stand this tube in a beaker of boiling water (a water bath), rather than heating the ethanol directly over a flame, because ethanol vapour is flammable. Leave the leaf in the hot ethanol until it has lost its green colour. The ethanol dissolves out the chlorophyll, turning pale/white so that any colour change with iodine will be easy to see.
Dip the decolourised leaf back into hot water briefly, since the ethanol will have made the leaf brittle, and this softens it again.
Spread the softened leaf out flat on a white tile, ready to add iodine solution and observe the colour change.

Part (c): Interpreting the two results

(i) The foil-covered patch: iodine solution stays orange-brown. This patch was kept in the dark by the foil throughout the six hours, so it could not photosynthesise and made no starch.
(ii) The uncovered part of the leaf: iodine solution turns blue-black. This part received light, so it could photosynthesise and did make starch.

Together, these two results on the same leaf, tested under the same conditions except for light, show that light is needed for a leaf to make starch. Direct evidence that light is required for photosynthesis.

Final answers

(a) Destarching uses up existing starch reserves, so any starch found afterwards must have been made freshly during the investigation.
(b) Boil in water, then decolourise in ethanol using a water bath (not a direct flame), then soften in hot water, then spread on a white tile.
(c) (i) Orange-brown (no starch, no light). (ii) Blue-black (starch present, received light). This contrast shows light is needed for starch to be made.