The diagram given below represents an experiment to prove the importance of a factor in photosynthesis. Answer the questions that follow: The diagram given below represents an experiment to prove the importance of a factor in photosynthesis. Answer the questions that follow:
What will you observe when the leaf A is tested for starch?
Topic: Starch test result
Answer
Leaf A will NOT turn blue-black — it remains BROWN, the colour of iodine. Because it was deprived of carbon dioxide it could not photosynthesise, so it contains NO STARCH.
Give the OBSERVATION and the REASON. The colour alone is only half the answer.
THE IODINE TEST: blue-black means starch is PRESENT; brown (unchanged) means it is ABSENT.
THE PROOF lies in the comparison: a leaf outside the flask, on the same plant and in the same light, DOES turn blue-black. Since the only difference between them is carbon dioxide, carbon dioxide must be necessary for photosynthesis.
Remember also that the plant must be DESTARCHED beforehand — 24 to 48 hours in the dark — or the result would prove nothing.