To show experimentally that light is necessary for photosynthesis - Science Practicals

To show experimentally that light is necessary for photosynthesis

Photosynthesis is the process by which green plants synthesize carbohydrates by using carbon dioxide (CO2 ), water, sunlight and chlorophyll present in the leaves. Light is one of the essential requirements for photosynthesis. The energy contained in solar radiation is absorbed by the photosynthetic pigments and is converted into utilizable chemical energy during photosynthesis.

Materials Required
De-starched potted plant (balsam, Amaranthus, Tecoma or any plant with thin herbaceous broad leaves), thick black paper strips, paper clips, alcohol, iodine solution, a beaker (250 mL), a burner (or a spirit lamp), a tripod stand, a wire gauge, a boiling tube, forceps and petridish.

Step 1: A de-starched plant is taken. Using strips of thick black paper and paper clips, a part of an intact leaf of the plant is covered.
Step 2: The set-up is placed in bright sunlight for about two hours.
Step 3: About 150 mL water is taken in a beaker and boiled.
Step 4: The experimental leaves from the potted plant are plucked and the black paper strips are removed from them. These leaves are kept in boiling water for some time till the leaves become soft. Water is stopped heating and the beaker is removed from the tripod stand. It is allowed to cool to about 60 °C.
Step 5: The leaves are transferred to a boiling tube containing alcohol.
Step 6: The boiling tube is placed (containing experimental leaves in alcohol) in the beaker containing hot water at about 60 °C. The boiling tube is kept in the beaker till the leaves become colourless.
Step 7: Some iodine solution is taken in a petridish.
Step 8: The leaves are washed in water and dipped in iodine solution in petridish.
Step 9: After five minutes, the leaves are removed from iodine solution, washed with water and the colours of the exposed part and unexposed (covered with black paper) part of the leaf are observed.

After iodine treatment, colour of covered portion of leaf is blue-black colour and the colour of the exposed portion of leaf is olourless.

Results and Discussion
Starch is one of the end products of photosynthesis and our observation shows that only those areas of the leaf exposed to sunlight turned blue-black on contact with iodine. Since starch turns blue-black on contact with iodine, the portions of the leaf that turned blue-black indicates photosynthetic activity, while the unexposed portion shows the reverse. This clearly indicates that light is essential for photosynthesis.

1. The epidermal peel should be taken from a freshly-plucked leaf.
2. Take the epidermal layer from the lower surface of a leaf, as it has more stomata.
3. Always use a clean glass slide.

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