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Importance of Light in Photosynthesis
Photosynthesis is the process of converting light energy from the sun into chemical energy
through a complex biochemical reaction. The process of photosynthesis takes place in the
chloroplasts, specifically using chlorophyll, the green pigment involved in photosynthesis.
In this process, inorganic compounds such as water and carbon dioxide are converted
into organic materials such as glucose. Molecules of glucose later combine with each other to
form more complex carbohydrates, such as starch and cellulose.
To do the experiment, we require:
A healthy potted plant, boiling tube, iodine, Petri dish, methylated spirit/alcohol, distilled
water, forceps and a hot plate.
Procedure:
Take a healthy potted plant and keep it for 2-3 days in a dark place.
Remove the plant from the dark room. Take two uniform pieces of black paper and
paper clips. Select a healthy de-starched leaf and cover
a portion of the leaf on both sides by using these two uniform pieces of black paper.
Fix the two ends of the black paper using paper clips.
Now, place the potted plant in sunlight for the whole day.
Pluck the covered leaf in the late afternoon. Remove the black paper from the leaf.
Take a beaker containing distilled water and place it over a hot plate and boil it.
Put the experimental leaf into the boiling water and boil for 5-10 minutes, till the
leaf becomes soft. Remove the beaker from the hot plate and allow
it to cool for some time. Take another beaker containing distilled water
and place it over a hot plate and boil it at about 60 °C.
Take the bottle containing alcohol and pour it into a clean boiling tube.
Place the boiling tube into the beaker containing water that is boiling.
Take the leaf from the beaker using a forceps. Place the leaf into the boiling tube containing
alcohol. Keep the boiling tube in the beaker till the
leaf becomes colorless. Take the leaf from the boiling tube using
forceps. Dip the leaf in a beaker containing distilled
water and wash it. Place the leaf in a Petri dish.
Take iodine solution using a dropper. Drop a few drops of iodine solution on the
leaf. After iodine treatment, the colour of the
exposed portion of the leaf turns blue-black and the unexposed portion of the leaf turns
to pale yellow. Starch is one of the end products of photosynthesis,
and turns blue-black on contact with iodine. The exposed portion of the leaf that turned
blue-black shows photosynthetic activity while the unexposed portion shows the reverse. This
indicates that light is essential for photosynthesis.
Precautions:
The experimental leaf should be healthy. Clip the black paper carefully to the leaf,
so that the covered portion does not receive any sunlight.
After boiling the leaf in alcohol, it should be washed in water.