Let us do an activity to prove that sunlight is necessary for photosynthesis-
• We take a potted plant having green leaves and place it in a completely dark place for about three days to destarch its leaves. So, in the beginning of the experiment, the leaves do not have any starch in them.
• Take a thin strip of aluminium foil (or black paper) and wrap it in the centre of one leaf on both the sides (while the leaf is still attached to the plant). The aluminium foil should be fixed tightly to the leaf by using paper clips so that sunlight may not enter it from the sides. The aluminium foil should cover only a small part of the leaf so that the remaining part of the leaf remains uncovered and exposed to sunlight. We have covered the centre part of the leaf with aluminium foil so that sunlight may not fall on this covered part of the leaf.
• Keep this potted plant (with partially covered leaf) in bright sunshine for three to four days.
• Pluck the partially covered leaf from the plant and remove its aluminium foil. Immerse this leaf in boiling water for a 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 reach the starch present inside the leaf cells).This leaf is now to be tested for the presence of starch. But before testing for starch, chlorophyll has to be removed from the leaf. This is because chlorophyll interferes in the test for starch due to its green colour.
• Put the plucked leaf in a beaker containing some alcohol. Place the beaker containing alcohol and leaf in a water bath. A water bath is being used here for heating alcohol because alcohol is a highly inflammable liquid.
• Heat the water in the bigger beaker (or water bath). Now, the alcohol in the smaller beaker will also get heated and start boiling soon. This boiling alcohol will extract (or remove) chlorophyll from the green leaf.
• Boil the green leaf in alcohol till all its green pigment 'chlorophyll' is removed. The leaf will now become almost colourless or pale (and the alcohol will turn green).
• Remove the colourless leaf from alcohol and wash it thoroughly with hot water to soften it and remove any chlorophyll which may be sticking to it.
• Place the colourless leaf in a petri-dish. Drop iodine solution over the decolourised leaf with the help of a dropper. Observe the change in colour of leaf.
• The middle part of leaf which was covered with aluminium foil 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 could not reach the covered 'middle part' of the leaf due to which the covered 'middle part' of leaf could not do photosynthesis to make starch.
• The uncovered part of leaf (on both sides of the aluminium foil) which was exposed to sunlight turns blue-black 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.
Since the part of leaf which was covered and hidden from sunlight does not contain starch but the part of leaf which was exposed to sunlight contains starch, therefore, we conclude that sunlight is necessary for photosynthesis (to make food like starch).
From the above experiment, we actually get two conclusions-
(i) Sunlight is necessary for the process of photosynthesis, and
(ii) Leaves make starch as food by photosynthesis.
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