Question

To test the presence of starch in a leaf, which of the following instructions to conduct the experiment is not necessary?
A. Boil the leaf first in water and then in alcohol.
B. Wash the bleached leaf.
C. Heat the leaf on a hot flame.
D. Add a few drops of iodine to the bleached leaf.

Answer 1

In the presence of iodine, the amylose in starch causes a vivid blue hue to develop. Inside the amylose coil, the iodine molecule slides. Iodine - KI Reagent: Because iodine is not particularly soluble in water, the iodine reagent is produced by dissolving iodine in water with potassium iodide. This forms a linear triiodide ion complex, which is slid into the starch's coil to produce a blue colour indicating the presence of starch.

Complete answer:
Option:A – Boiling of leaves is an important step in the starch test for leaves.
Option:B – In starch test for leaf, after alcohol treatment the bleached leaf is washed with cold tap water.
Thus, Option:A and B are not correct as they are necessary steps.
Option:C – The leaf is never heated directly on flame for testing the presence of starch.
Thus, Option:C is correct as it is not a necessary step in testing for starch in leaves.
Option:D – Application of Iodine is a must step for any starch test. Thus it is a necessary step.
So, Option:D is also not correct.

Hence, Option C) Heat the leaf on a hot flame is the correct answer.

Note:
Starch is a white, powdery material made mostly of glucose that is utilised as nourishment by plants. Because the production of starch needs photosynthesis, the existence of starch in leaves is sufficient proof of photosynthesis in leaves. Biochemically starch is a polymer of glucose and exists in two forms –the amylose form and the amylopectin form.