The diagram given below represents an experiment to demonstrate a particular aspect of photosynthesis. The letter ‘A’ indicates a certain condition inside the flask. Answer the questions: The diagram given below represents an experiment to demonstrate a particular aspect of photosynthesis. The letter ‘A’ indicates a certain condition inside the flask. Answer the questions:
What is the important step that should be taken before performing this experiment?
Topic: Destarching before the experiment
Try it
Pick an option and check your answer. The full worked answer is below either way.
Answer
(c) The plant should be placed in dark for 24 hours to destarch the leaves..
The plant must be DESTARCHED — kept in the dark for 24 to 48 hours so that the starch already stored in the LEAVES is used up by respiration while photosynthesis is stopped.
WHY IT IS ESSENTIAL: if the leaves already contained starch at the start, a positive iodine test at the end would prove nothing, since the starch might be left over. Emptying them first means any starch found afterwards must have been made DURING the experiment.
Option (a) says "the entire plant", but the starch test is performed on the LEAVES and it is the leaves that must be destarched — the printed key gives (c) for that reason.
Option (b) is wrong on the biology: darkness does not remove chlorophyll. Chlorophyll is removed later, in the starch test itself, by boiling the leaf in ALCOHOL.
This step appears in EVERY photosynthesis experiment, whichever factor is being tested.
The plant should be placed in dark for 24 hours to destarch the leaves..