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| Photosynthesis - Some Activities |
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| Following are some simple activities and experiments related to photosynthesis: |
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| The occurrence of starch in the leaves proves that photosynthesis has taken place. Starch can be tested with the help of iodine solution. This is done in the following manner: |
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 The leaf is first decolourised by treating it in 90% ethanol (alcohol) solution. It is then rinsed in hot water to remove all alcohol and to soften the tissue. |
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The leaf is now colourless. Then iodine solution (brown in colour) is poured over the leaf. |
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The leaf turns blue-black indicating that it contains starch. |
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| Take a potted plant and destarch it by keeping it in darkness for 24 hours. To one of the leaves attach a piece of metal foil or paper on both sides. Leave the plant in sunlight for 48 hours. Then take the leaf and remove its metal foil. Decolourise the leaf as in the above experiment and test for starch. |
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| a. Will the entire leaf be blue-black? |
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| No, the portion covered by the strip of metal foil will not be blue-black. |
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| b. Why? |
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| This is because there is no starch in that area. The area was deprived of light which is needed for photosynthesis. Hence, no photosynthesis took place in that area. |
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| c. How does the plant get destarched on keeping it in darkness for 24 hours? |
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| The plant cannot carry out photosynthesis in the absence of light and all the starch present in the leaves is used up. Thus the leaves are destarched. |
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| Investigating the Need for Carbon Dioxide in Photosynthesis |
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| Take a potted plant. Destarch it. Insert one of its leaves in a bottle containing potassium hydroxide (KOH) solution as shown in the figure. KOH absorbs all carbon dioxide in the bottle. Leave the set up in sunlight for 48 hours. Test the leaf for starch. |
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| a. Which portion of the leaf will test postitive for starch? |
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| The portion of the leaf that was outside the bottle will test positive and turn blue-black. |
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| b. Why? |
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| The portion inside the bottle does not get carbon dioxide and thus, no photosynthesis takes place in that region. |
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| Take a variegated leaf (a leaf that is variously coloured such as Coleus or Croton). Decolourise it. Then test it for starch. The leaf will only give blue-black colour in patches. |
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| Why does the leaf give blue-black colour only in patches? |
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| The variegated leaf contains chlorophyll only in patches and hence the photosynthetic activity is restricted to those patches. Thus, only those areas give blue-black colour. |
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| Experiment to show release of oxygen during photosynthesis |
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| Take a few healthy twigs of Hydrilla, a water plant. Place it in a funnel and invert the funnel in a beaker of water. Invert a test-tube over the stem of the funnel. Leave the set-up in sunlight. After sometime, bubbles can be seen rising in the test-tube. Remove the test-tube carefully and insert a glowing splinter deep into it. The splinter burns brightly. |
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| a. Which gas is collected by the downward displacement of water in the test-tube? |
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| Oxygen. This is because the glowing splinter burns brightly. |
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| b. Why does oxygen evolve? |
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| In sunlight the hydrilla twig carries out photosynthesis. During this process, carbon dioxide is taken in and oxygen is released. Hence, oxygen is liberated. |
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