IGCSE Biology 0610: Variegated Leaves and the Starch Test

A student is investigating whether chlorophyll is needed for a leaf to make starch. They use a variegated leaf from a coleus plant, which has patches of green tissue and patches of cream-white tissue scattered across the same leaf. The plant has already been left in bright light for several hours before the leaf is removed and tested for starch with iodine solution.

(a) The student did not destarch the plant in the dark beforehand, and did not cover any part of this leaf with foil. State one variable that is the same across the whole leaf, and one variable that is different across the leaf, that allow this single variegated leaf to act as its own comparison for this investigation. [2]

(b) Predict which parts of the leaf will turn blue-black with iodine solution, and which will stay orange-brown. [2]

(c) Explain, in terms of chlorophyll, why the cream-white patches do not turn blue-black even though they were exposed to exactly the same light as the green patches. [2]

Part (a): Why the variegated leaf provides its own comparison

Because the green and cream-white patches are parts of the same leaf, they automatically share several variables: they receive the same light (same intensity, same duration, since they are literally next to each other), they are the same age, and they are attached to the same plant with the same water and mineral supply. The one variable that genuinely differs between the patches is the amount of chlorophyll present. This means the student does not need a separate control leaf or a foil cover, the leaf provides a built-in comparison between chlorophyll-containing and chlorophyll-lacking tissue under otherwise identical conditions.

Part (b): Predicting the iodine test result

Green patches (contain chlorophyll): these can photosynthesise, so they will have made and stored starch. Iodine solution added here will turn blue-black.
Cream-white patches (no chlorophyll): these cannot photosynthesise, so no starch is made or stored. Iodine solution added here will stay orange-brown.

Part (c): Linking the result to the role of chlorophyll

Chlorophyll is the pigment inside chloroplasts that absorbs light energy and transfers it into the chemical energy needed to drive photosynthesis. The cream-white patches of the leaf are living cells, but they contain no chlorophyll, so they have nothing to absorb the light energy that is falling on them. Without this energy capture, the reaction that converts carbon dioxide and water into glucose cannot proceed, so no glucose is made and no starch is stored in these patches. Even though they were bathed in exactly the same light as the green patches next to them.

Final answers

(a) Same: light received by the whole leaf. Different: amount of chlorophyll present (green vs cream-white patches).
(b) Green patches turn blue-black (starch present); cream-white patches stay orange-brown (no starch).
(c) The white patches have no chlorophyll to absorb light energy, so they cannot photosynthesise and cannot make/store starch.