Two Claims About Light and Photosynthesis
A Class 10 group is asked to prove that light is necessary for photosynthesis, using the iodine test for starch. Team A takes a fresh Coleus leaf that has green and white patches, keeps the whole plant in bright sunlight for six hours, boils the leaf in water, then in alcohol over a water bath, washes it and adds iodine. The green patches turn blue-black; the white patches stay brown. Team B keeps a fully green potted plant in a dark cupboard for 48 hours, then exposes it to sunlight for six hours with one half of a leaf folded inside a strip of black paper. After the same iodine procedure, the exposed half turns blue-black and the covered half stays brown. Both teams claim their result proves that light is needed. The teacher says only one claim survives scrutiny, and that Team A's leaf still has one useful lesson to teach.
1. Which team's conclusion is supported, and what does the other team's result actually establish? Explain in terms of variables.
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Team B's conclusion is supported; Team A's is not.
In Team B, the two halves of the same leaf differ in exactly one respect - whether black paper excluded light. Everything else (same plant, same leaf, same age, same air, same temperature, same destarching treatment, same iodine procedure) is identical. The covered half stays brown, so no starch was made there; the exposed half turns blue-black, so starch was made. Light is therefore the variable that decides starch formation.
In Team A, light was not the variable being changed. Both the green and the white patches sat in the same bright sunlight for the same six hours. The only difference between the two regions is that the white patches lack chlorophyll. So Team A's result demonstrates that chlorophyll is necessary for photosynthesis, not that light is. It is a perfectly good experiment - it just answers a different question. The colour pattern on a variegated leaf tracks chlorophyll, so blue-black appears only where chloroplasts are present.
The general principle: a valid conclusion may only be drawn about the single variable that was actually varied between the two sets of data.
2. Why must the plant be kept in the dark for 24–48 hours before such a test? What exactly would a student see if that step were skipped, and why must the alcohol be heated in a water bath?
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Purpose of the dark period (destarching). A leaf normally carries starch left over from photosynthesis earlier in the day. If that starch is already present, the iodine test will be blue-black everywhere, whether or not the leaf photosynthesises during the experiment. Keeping the plant in darkness for 24–48 hours forces it to use up or export its stored starch, so any starch found afterwards must have been made during the experimental light period.
If skipped: both the covered and the uncovered halves - and both green and white patches could still differ only by chlorophyll - may give a blue-black result. The covered half would then wrongly appear to have made starch in the dark, and the experiment would be inconclusive rather than simply wrong.
Water bath: boiling the leaf in alcohol removes chlorophyll so that the blue-black iodine colour can be seen against a pale background. Alcohol is highly flammable and its vapour ignites easily, so heating it directly over a burner is a fire hazard. A water bath heats it indirectly and safely; the room must also be flame-free and the beaker covered. The leaf is first boiled in water to kill cells and make membranes permeable so the alcohol can act.
3. Design a control experiment to show that carbon dioxide is also required for photosynthesis, and state what result in the control tube would make you distrust the whole experiment.
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Design. Take two healthy potted plants of the same species and destarch both by keeping them in the dark for 48 hours. Enclose one leaf of plant 1 inside a transparent conical flask containing a small dish of potassium hydroxide solution (which absorbs CO2 from the air inside). Enclose one leaf of plant 2 in an identical flask containing a dish of water instead. Seal both flask mouths around the leaf stalk with split corks so the enclosed air is isolated. Keep both plants in bright sunlight for about six hours, then detach both leaves, boil each in water, then in alcohol over a water bath, wash them in warm water and add iodine.
Expected results: the leaf with KOH shows no blue-black colour (no CO2, so no starch); the leaf in the water-only flask turns blue-black. Together these show CO2 is necessary.
Control that could invalidate the result: if the water-only flask leaf also stays brown, something other than CO2 is suppressing photosynthesis - the seal was too tight, the leaf was damaged or the plant was unhealthy - and the experiment must be discarded and repeated. KOH is corrosive, so it is handled with gloves and kept away from eyes.