Question

A destarched leaf and a potted plant is coated with Vaseline on both sides and placed in sunlight. After a few hours, it is tested for the presence of starch. Will the test be positive or negative?

Answer 1

Hint: photosynthesis is a process in which green parts of the plants synthesize or manufacture complex organic food substances (carbohydrates) using carbon dioxide and water in the presence of sunlight and release oxygen as a by-product. In this process, energy from the sun is converted into chemical energy. It is an anabolic, endergonic (requiring energy) and Oxido-reduction process.

Complete answer:
Photosynthesis is required for the green plants. To proceed photosynthesis requires the presence of chlorophyll, light and carbon dioxide.
Now let us do an experiment :-
(•) Take a destarched potted plant having variegated leaves and cover 2-3 leaves with the black paper.
(•) Expose the potted plant to sunlight for 1-2 hours.
(•) Pluck one covered leaf and one exposed leaf and test them for starch.
(•) The covered leaf does not show a positive starch test showing that photosynthesis cannot occur in the absence of light.
(•) The exposed leaf shows blue and yellow parts where the blue colour or positive starch test occurs in the chlorophyll-containing parts.
This shows that when leaves are covered with Vaseline it does not absorb sunlight and hence photosynthesis has not been performed so it gives a negative test to starch.

Note: Sunlight plays a much larger role in our sustenance than we may expect, as all the food we eat and all the fossil fuels we use, the air we breathe, they are all products or by-products of photosynthesis. Photosynthesis converts the radiant energy to forms of energy that can be used by the biological systems.