experiment to show that sunlight is necessary for photosynthesis

Aditya Raj Anand

Sunday 24 May 2020

Experiment

experiment to show that sunlight is necessary for photosynthesis.

we will now describe some experiments to show that sunlight, chlorophyll, and carbon dioxide is necessary for photosynthesis by green plants. 

It has been found by experiment that sunlight, chlorophyll, and carbon dioxide are necessary for photosynthesis. So, before we do experiment to show that sunlight is necessary for photosynthesis. we have to understand that under what conditions a photosynthesis occur due to sunlight. we can say that : 

These are the conditions necessary for photosynthesis:

1. Sunlight
2. Chlorophyll,
3. Carbon Dioxide,
4. Water

Please note that the same conditions which is necessary for photosynthesis are also the same for  autotrophic nutrition. These experiments also show that leaves finally make 'starch' as food by photosynthesis.

the experiments on photosynthesis depends on the fact that green leaves make starch as food.so before we can use a plant in a photosynthesis experiment the initial starch present in its leaves must be removed. In other words, we should destarch the leaves of a plant before using it in a photosynthesis experiment. The green leaves of a plant are destarched by keeping this plant in a completely dark place in a room for at least 3 days.

Experiment to show that sunlight is necessary for photosynthesis

(1.) Firstly take a potted plant having some green leaves in them, and then placed it in a completely dark room for about three to four days to destarch its leaves. So, in the beginning of the experiment, the leaves do not have any starch in them.

(2.) Now, Take a thin aluminium foil (or black paper) and wrap it in the center of  the one leaf on both sides. So that sunlight may not fall on covered part of the leaf. But sunlight should be fall on the remaining uncovered part of the leaf.

(3.) Keep this potted plant in bright sunlight for three to four days.

(4.) After 3 to 4 days, pluck the leaf which we covered partially with aluminium foils. then, drop this leaf in boiling water for few minutes. This will break down the cell membranes of leaf cells and make the leaf more permeable to iodine solution so that it may reached the starch present inside the leaf cells. This life is now ready to be tested for the presence of starch. But before testing for starch, chlorophyll must be removed from the leaf. 

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(5.) Now put the leaf in a beaker containing some alcohol. Than place the beaker in that beaker which contain some water in it. 
Please note that a water beaker is being used here for heating alcohol because alcohol is a highly inflammable liquid. So if alcohol is heated directly over a flame, then it will catch fire at once. 

(6.) Heat the water beaker. Then the alcohol in the smaller beaker will also get heated and start boiling soon. This boiling alcohol will remove chlorophyll from the green leaf.

(7.) Boil the green leaf in alcohol till all its green chlorophyll is removed. After boiling the leaf will become completely colourless, and alcohol will turn to green.

(8.) Now, remove the colour leaf from alcohol and wash it with hot water to soften it.

(9.) Place the colourless leaf in a glass-dish. Drop some iodine solution over the decolourised leaf with the help of a dropper. Observe the change in color of leaf 

(10.) The middle part of the leaf which was covered with aluminium foils does not turn blue-black on adding iodine solution showing that no starch is present in this middle part of the leaf. This is because sunlight couldn't reach to the middle part of the leaf due to which the covered middle part of leaf could not do photosynthesis to make starch.

(11.) The uncovered part of the leaf which was exposed to sunlight turns black-blue on adding iodine solution showing that starch is present in this part of leaf. This means that the part of leaf which was exposed to sunlight could do photosynthesis to make starch.

(12.) Since the part of leaf which was covered and hidden from sunlight does not contain starch but the part of leaves which was exposed to sunlight contains starch, therefore, we conclude that sunlight is necessary for photosynthesis.

From the above experiment, we actually get two conclusion. That: 

• Sunlight is necessary for the process of photosynthesis. and,
• Leaves make starch as food by photosynthesis.

Experiment to show that chlorophyll is necessary for photosynthesis.

(1.) Take a potted plant like croton whose leaves are partly green and partly white. and we know that the green part of the leaf has chlorophyll but the white part does not have chlorophyll in them.

(2.) Now, place this potted plant in a completely dark room for about 3 to 4 days to destarch its leaves.

(3.) After 3 to 4 days take out the potted plant from the dark room and keep it in bright sunlight for 3 to 4 days.

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(4.) Pluck the variegated leaves from the plant, and boil it in water for a few minutes and then remove it its green colour chlorophyll by boiling it in alcohol. The green part of the leaf get discoloured. 

(5.) Now wash the discoloured leaf with hot water to soften it. 

(6.) Pour iodine solution over the colourless leaf and observe the changes in color of the leaf.

(7.) we will find that the outer part of the leaf that was originally white (without chlorophyll) does not turns blue black on adding iodine solution. This shows that no starch is present in this outer part of the leaf. 

(8.) from this observation we conclude that the photosynthesis to make starch does not take place without chlorophyll.

(9.) the inner part of the leaves which was originally green (contain chlorophyll) turns blue black on adding iodine solution. This shows that starch is present in the inner part of the leaves. 

(10.) from this observation we conclude that photosynthesis to make starch takes place in the presence of chlorophyll in another word, chlorophyll is necessary for the process of photosynthesis to take place.