Laboratory work 5: formation of organic matter in a plant leaf
The visible product of photosynthesis is starch, which accumulates in leaves. If part of a leaf is shielded from light, no starch should form there. Experiment: black paper or foil of the same shape is fixed with a clip on both sides of a leaf of a well-lit houseplant (for example geranium), and the plant stays in light for 2–3 hours (for a more precise result the plant is first kept in the dark for a day). Then the leaf is picked and the paper removed; the steps of holding it in boiling water for a few minutes, decolourising chlorophyll in hot alcohol and placing it in iodine solution are done only by the teacher: boiling water scalds, and alcohol is flammable and must never be heated near an open flame. With iodine the lit areas turn blue-violet (starch present) and the covered areas yellowish-brown (no starch). Conclusion: light is necessary for photosynthesis.
“Experiment design”. Groups plan how to test another factor (CO₂, water) instead of light: what is the control, what is the changed factor, what result is expected? Discuss the plan with the teacher; do not do the experiment at home with boiling water or alcohol.