Question

A black strip of paper was clipped on to a destarched leaf in a potted plant to cover a part of
the leaf. The plant was then exposed to sunlight for four hours, the paper strip was removed
and the leaf tested for starch. When iodine solution was added :
A.The entire leaf turned blue - black
B.The covered part of the leaf became blue - black
C.The uncovered part of the leaf became blue - black.
D.The colour of iodine solution remained unchanged

Answer 1

Hint: Plants make their food by the process known as photosynthesis. To synthesise food, plants need $CO_{2}$ water, chlorophyll and light/sunlight. In absence of any of these plants are not able to prepare their food.

Complete answer:
The rate of photosynthesis depends on all three factors i.e.—light, temperature, availability of components, i.e.,— $CO_{2}$ and $H_2O$.
If the intensity of sunshine increases the speed of photosynthesis also increases.
When light falls on plants they show photochemical reactions. during this photochemical reaction the water in leaves undergo photolysis i.e.,—the water splits to create oxygen and hydrogen thanks to photons of sunshine. The oxygen gas is released into the atmosphere but hydrogen is kept by the plant. It's this hydrogen that mixes with $CO_{2}$ to make carbohydrates (reduction reaction). Hence; photosynthesis is an oxidation-reduction reaction.
During photosynthesis plants get ready starch. At the point when the leaf is covered, it isn't permitted to take daylight and consequently, no starch was set up in the leaf. Iodine solution becomes blue-dark in presence of starch. On adding iodine solution for covered leaf no shading change was watched. It shows that no starch was made by this leaf. Though the uncovered leaf got daylight for 6 hours and when iodine solution was added to it, the shading changed to blue-dark.

This demonstrates that daylight is needed for photosynthesis.

Hence, the correct answer is option (C)

Note: The aspect of the destarched leaf, when covered with dark strips and presented to daylight it doesn't deliver any starch since it can't get daylight. The piece of destarched lead secured with transparent strips produces starch since it can get daylight. at the point when the test for starch is played out the leaf part which can create starch goes to blue-dark colour i.e transparent strip and revealed part. At the point when the starch test is played out, The part secured with the dark strip stays dull in light of the fact that it can't deliver starch and the remainder of the leaf turns blue-dark since that segment gets daylight and produces starch.