Question

A student sets up an experiment to study the photosynthesis in plants.
The student destarched a potted plant by keeping it in a dark room for 3
days. Half of the portion of destarched leaf was placed in a bottle
containing caustic potash (absorbs CO2) as shown.
The student then places the plant in light and tests the leaf after 5 hours for
the presence of starch. The portions inside the bottle shows negative
starch test by reflecting no change in colour when react with iodine,
however, other upper portions of the leaf gave positive starch
Test showing blue-black colour with iodine. What can be concluded from
this experiment?

Answer 1

The leaf of the potted plant was covered with a piece of black cloth to destarch it. ... From this observation, we conclude that the photosynthesis to make starch in the leaf does not take place without carbon dioxide.

Answer 2

The conclusion obtained from the experiment is: carbon dioxide is essential for photosynthesis.

• Photosynthesis is the process in which sugars are formed from carbon dioxide and water in the presence of sunlight. Oxygen is released in this process. The sugars formed are stored in the form of starch.
• The photosynthesis did not occur in the portion of leaf that was placed in a bottle because caustic potash absorbed all the carbon dioxide surrounding the leaf. Hence, negative result is obtained when this leaf is treated with iodine.
• The photosynthesis occurred in the upper portion of the leaf and hence, it shows blue-black colour when treated with iodine.