Question

(i) Pluck the leaf to be experimented
(ii) Cover a portion of leaf on both sides by using a strip of black paper
(iii) Keep the plant in a dark place for three days to destarch its leaves
(iv) Test the experimental leaf the presence of starch
(v) Expose the plant to sunlight for four hours
(vi) Remove the black paper strip from the leaf
Given in the box are steps written randomly for the experiment to show that light is essential for photosynthesis. Which of the following sequence of steps is to be followed by the successful run of the experiment?
(a) (i),(iii),(ii),(iv),(v),(vi)
(b) (ii),(i),(v),(vi),(iii),(iv)
(c) (iii),(ii),(v),(i),(vi),(iv)
(d) (v),(ii),(i),(iii),(iv),(vi)

Answer 1

Hint: Photosynthesis takes place with the help of chlorophyll present inside the leaf, carbon dioxide and sunlight. The photosynthesis experiment starts with the plant place in the dark and ends with the portion covered with a light screen.

Complete answer:
The experiment to demonstrate light is essential for photosynthesis starts by placing the potted plant in dark for about three days to make leaves free from starch. Then fix the light screen, especially Ganong's light screen or strip of black paper to one of the leaves and then place the plant in bright sunlight. The leaf with Ganong's light screen or strip of black paper receives light only in exposed parts and after a few hours detach the leaf and boil it in $70\%$ alcohol till the decolourization of the leaf. Wash the leaf with water and put it in an iodine solution. The exposed part of the leaf turns blue and the part covered with a light screen will not turn blue and remain pale.

Additional Information:
The photosynthesis experiment starts with the experimentation of Joseph Priestly, who discovered oxygen in 1774 and proved that air is essential for the growth of green plants by experimenting with a bell jar, candle and mouse. Later Jan Ingenhousz experimented with aquatic plants in bright light and proved that green plants release oxygen. In 1854 Julius Von Sachs proved that plants produce glucose and starch and store it in the form of food. In 1931, Cornelius van Neil on working with purple and green sulphur bacteria proved that photosynthesis is a light-dependent reaction. Rubin Hill in 1931 isolated the chloroplast of Stellaria media in presence of leaf extract and the absence of carbon dioxide, which leads to evolving oxygen from the chloroplast.

So, the correct answer is '(iii),(ii),(v),(i),(vi),(iv)'.

Note:
The chlorophyll experiment takes place mainly in Coleus or Croton plants with the same step as the sunlight experiment. The only difference is that in this tracing papers are used. In the carbon dioxide experiment, the KOH solution is used and the leaf is placed in an empty bottle. The carbon dioxide experiment is also known as Moll's half leaf experiment.