Question 5. (a) List the three events that occur during the process of photosynthesis. Explain the role of
stomata in this process.
(b) Describe an experiment to show that “sunlight is essential for photosynthesis”.
Answers
Explanation:
(a) Three most important steps of photosynthesis are:
=》Absorption of light energy by chlorophyll
Conversion of light energy to chemical energy and splitting of water molecules into hydrogen and oxygen
Reduction of carbon dioxide to carbohydrates
Stomata are present in enormous amount over leafs, this tiny pores helps in the exchange of gaseous by photosynthesis. Since large amount of water can also be lost through these stomata, the plant closes these pores when it does not need carbon dioxide for photosynthesis, opening and closing of pours is regulated by guard cells, it swell up when water flows inside them, causing the stomata pore to open. Similarly the pore closes if the guard cells shrink.
(b) Experiment to prove that “sunlight is essential for photosynthes” is explained below:
=》(i) Select two healthy plants.
(ii) Keep them in a dark room for 3 days.
(iii) Now place them in different air tight chambers, one supplied with potassium hydroxide, which is used to absorb carbon dioxide.
(iv) Now keep the plants in sunlight for 2 hours.
(v) Pluck leaf from both plants and check the presence of starch.
(vi) Higher starch content in the jar with potassium hydroxide indicates that in the presence of light the photosynthesis have occurred so higher starch content is found.
Answer:
a. The three events that occur during the process of photosynthesis are: (i) Absorption of light energy by chlorophyll. (ii) Conversion of light energy to chemical energy and splitting of water molecules into hydrogen and oxygen. (iii) Reduction of carbon dioxide to carbohydrates
- stomata help in exchange of gases during photosynthesis
b. The following experiment can be performed to demonstrate that sunlight is essential for photosynthesis.
Experiment steps:
(i) Place an healthy green plotted plant in a dark room for days.
(ii) The previous step is done to confirm that the plant utilises all its reserved food and their is no starch left in the leaves.
(iii) Cover both sides of the green leaf with black paper and fix the cover with two proper clips.
(iv) Expose the plant to bright light.
(v) After few hours remove the leaf and decolourise it with alcohol.
(vi) Put the leaf in iodine solution to check the presence of starch.
If starch is present it will react with iodine to give blue-black colour.
In this experimment, only the uncovered part of the leaves showed colour due to the presence of starch and there was no colour in the covered part.
This proves that light is essential for photosynthesis.