Question

How does carbon dioxide return to the atmosphere? Explain.
A student in order to study the importance of certain factors of photosynthesis took a potted plant and dept it in the dark for over 24 hours. Then in the early hours of the morning the student covered one of the leaves with black paper in the centre only. Student placed the potted plant in the sunlight for a few hours, and then tested the leaf which was covered with black paper for starch.
(i) What aspect of photosynthesis was being investigated?
(ii) Is there any control in this experiment? If so, state the name.
(iii) Why was the plant kept in the dark before the experiment?
(iv) Describe step by step how the candidate proceeded to test the leaf for the presence of starch.

Answer 1

Carbon dioxide returns to the atmosphere through respiration by animals and plants. It is also produced by the process of combustion or burning. Vehicular emissions, industrial fumes and the gases released during the process of cooking are some instances of release of carbon dioxide through combustion.

(i) It is investigated that light is necessary for photosynthesis.

(ii) Yes, there is a control in this experiment. The control here is the part of the leaf that is covered with black paper.

(iii) The plant is kept in dark before the experiment to destarch it.

(iv) For checking the presence of starch, the leaf is first plucked and boiled in water. It is then treated with alcohol to decolour it by removing chlorophyll. Iodine solution is then put on the leaf by a dropper. The change in the colour of iodine solution to blue-black shows the presence of starch.
The part that is not covered with black paper will show the presence of starch. However, the part that was covered with black paper will not show any colour change. This shows that light is necessary for photosynthesis.