write an activity to show carbon dioxide is necessary for photosnthesis
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Place a healthy green potted plant in a dark room for 1-2 days. This is done to ensure that the plant consumes all its reserve food and the leaves do not contain any starch. Then, cover a portion of a leaf of this plant on both sides with two uniform pieces of black paper, fixed in position with two paper clips.
Now, expose this plant to bright light. After a few hours, remove the leaf, decolourize it with alcohol, and test the presence of food (starch) with iodine solution.
You will observe that the portion of the leaf covered with black paper does not show any presence of starch (food).
Explanation of the activity:
The food prepared by plants (carbohydrates) through the process of photosynthesis is stored as starch. This starch reacts with the iodine solution to change to blue-black colour. Only those portions of the leaf that were exposed to sunlight could photosynthesise and hence, change to blue-black colour when tested with iodine.
Now, expose this plant to bright light. After a few hours, remove the leaf, decolourize it with alcohol, and test the presence of food (starch) with iodine solution.
You will observe that the portion of the leaf covered with black paper does not show any presence of starch (food).
Explanation of the activity:
The food prepared by plants (carbohydrates) through the process of photosynthesis is stored as starch. This starch reacts with the iodine solution to change to blue-black colour. Only those portions of the leaf that were exposed to sunlight could photosynthesise and hence, change to blue-black colour when tested with iodine.
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To prove that CO2 is necessary for photosynthesis the following experiment can be formed:
1. A potted plant is kept in dark for three days so that the leaves become free from starch.
2. A healthy leaf is selected for experiment.
3. a wide mouth bottle is taken and a little KOH (it absorbs CO2) is added to it.
4.Half part of the leaf is kept in the bottle with the help of split cork.
5. The apparatus in kept in bright sunlight for few hours.
6. The leaf is plucked and boil in 70% alcohol. It is stained with iodine solution.
Observation : It was observed that the portion of leaf which was out of the bottle turned blue showing the presence of starch. The portion on leaf which was enclosed within the bottle will not show the presence of starch .

Conclusion : This shows that CO2 is essential for photosynthesis, because the portion inside the bottle did not get the CO2 as it was absorbed by KOH solution. Thus CO2 is necessary for photosynthesis.
1. A potted plant is kept in dark for three days so that the leaves become free from starch.
2. A healthy leaf is selected for experiment.
3. a wide mouth bottle is taken and a little KOH (it absorbs CO2) is added to it.
4.Half part of the leaf is kept in the bottle with the help of split cork.
5. The apparatus in kept in bright sunlight for few hours.
6. The leaf is plucked and boil in 70% alcohol. It is stained with iodine solution.
Observation : It was observed that the portion of leaf which was out of the bottle turned blue showing the presence of starch. The portion on leaf which was enclosed within the bottle will not show the presence of starch .

Conclusion : This shows that CO2 is essential for photosynthesis, because the portion inside the bottle did not get the CO2 as it was absorbed by KOH solution. Thus CO2 is necessary for photosynthesis.
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