Question

A healthy croton plant bearing variegated leaves was kept in a dark cupboard to destarch it, after which it was placed in sunlight for a few hours. One of the leaves was then plucked and an outline of the leaf making the green and non-green relations were drawn. The leaf was then tested for starch. Using the above information, answer the following question.
Name the chemical used for testing the presence of starch.

Answer 1

Hint:-Photosynthesis is the process through which energy rich compounds like carbohydrates are synthesized from simple inorganic compounds like carbon dioxide and water in the presence of chlorophyll and sunlight. It also liberates energy.

Complete answer:
Starch is a storage homopolysaccharide of plants. It is present in the potato tubers, corn seeds, cereals like rice, wheat, etc. and fruits like banana, mango, etc. Structurally, it is formed of two glucose polymers: a-amylose which is an unbranched but spiral chain of about 200-2000 alpha glucose molecules interlinked by alpha (1, 4) glycosidic bonds and amylopectin which is a branches glucan of about 2000 to 20000 alpha glucose molecules so forming a helical secondary structure. Glucose molecules are interlinked by alpha (1, 4) glycosidic bonds except the bond between the glucose of the main chain and the first glucose of the branch which is a (1, 6) glycosidic bond. Starch can hold iodine molecules in the helices so gives blue colour with iodine. The plant was tested for starch. A plant is destarched by keeping it in dark for 48 hours and destarching that is removing the present starch is done by wrapping the leaves in black paper or aluminum foil for 2-3 days.
So the chemical used for testing the presence of starch in the given experiment is iodine.

Note:- The green parts of the leaf have chlorophyll and they performed photosynthesis so they turn blue-black and confirm the presence of starch in it while non-green parts turned brown as it has no chlorophyll as no photosynthesis occurred in those leaf parts. The experiment confirms that starch is formed only in parts of the leaf that are exposed to sunlight.