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As we have learnt in our previous classes,
The process through which plants prepare their food is called photosynthesis.
In the term photosynthesis, the term 'photo' means light, and 'synthesis' means to build. Hence, photosynthesis means 'building up with the help of light.' During the process of photosynthesis, the light energy is converted into chemical energy. As the green plants prepare their food through photosynthesis, they are said to be in an autotrophic mode of nutrition.
The overall equation of photosynthesis is as given below:
Carbon      Water                      Glucose     Water      Oxygen
From the above reaction, we can say that photosynthesis takes place in the presence of sunlight and pigment chlorophyll to produce glucose as the end product. This glucose is converted into starch and stored in the plant body.
Requirements for photosynthesis
The requirements of photosynthesis can be understood by some activities which can be done at home.
As we know, chlorophyll is the pigment involved in photosynthesis. The importance of chlorophyll and light can be proved by the following experiments.
The importance of chlorophyll in photosynthesis:
  • Pluck a variegated leaf (the leaf that has more than one colour) from the coleus plant and place it in sunlight.
  • Now, de-starch the leaf by keeping it in the dark for 24 hours.
  • Draw the picture of the leaf on paper and mark the patches of chlorophyll on the drawing.
  • Immerse the leaf in boiling water and then in alcohol (chlorophyll content gets removed), and test for starch using iodine solution.
  • Now, compare the leaf with the drawing made.
Observation and conclusion:
Only those areas marked as green turns into blue, indicating that chlorophyll is necessary for photosynthesis.
A video explaining that chlorophyll is required for photosynthesis:
  • Take a potted plant and place it in a dark place for two days.
  • It de-starches the leaves.
  • Cover one of its leaves with a black strip of paper on both sides, and place the plant under sunlight for 4 to 6 hours.
  • Now detach the leaf covered with black paper and immerse it into boiling water for few minutes.
  • Then immerse it in alcohol so that the chlorophyll content is removed.
  • Test the presence of starch on the leaf with the help of iodine solution.
  • We can observe that the covered part of the leaf does not turn blue-black, but the uncovered part turns out blue-black, indicating starch.
From this experiment, we can say that the portion of the leaf that was uncovered receives all the necessary photosynthesis factors. Hence, the reaction of photosynthesis occurred only at that place and not at the covered portion. Due to this, the test for starch was positive in that portion.
A video explaining that light is required for photosynthesis:
From the above-given activites, we can list out the various requirements for photosynthesis as follows:
  • Chlorophyll (the green pigment found in the chloroplasts of leaves).
  • Water
  • Carbon dioxide (from the air)
  • Sunlight
Plants indeed take in carbon dioxide for their living, but plants also need oxygen to carry out cellular respiration.
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