IntroductionPhotosynthesis is an important physiological activity of green plants. For photosynthesis to occur in green plants the factors required are Sunlight, Chlorophyll and Carbon dioxide. To prove the necessity of these factors there are certain specific experiments to be done in the laboratory. So students must know clearly how a particular experiment will be done to prove the importance of particular factor clearly and also must know why a particular step will be done for the success of the experiment with reasons.
Here is an Experiment on Photosynthesis-Oxygen is evolved by green plants during photosynthesis
Aim of the experiment:-To prove Carbon dioxide is necessary for the green plants to carry photosynthesis.
Requirements for the experiment:-A well watered potted green plant, conical flask, a split rubber cork, potassium hydroxide crystals, a clamp stand, beaker, water, Bunsen burner, mentholated spirit or rectified spirit or ethanol, iodine solution, a white porcelain tile.
Procedure for the experimentsBefore starting the experiment to prove carbon dioxide is necessary for photosynthesis an important step has to be carried out is known as De-starching. De-starching is a process in which the starch present in the leaves of the plant will be removed by keeping the plant in dark room or closed cup-board for about 48 hours. This step is done to ensure that previously manufactured food in the leaves get consumed up because photosynthesis doesn't occur at that time as the plant is in dark place.
Now one of the healthy leaf of well watered De-starched green plant is kept in between the split pieces of rubber cork and is fitted in to the mouth of a conical flask. The leaf is arranged in such a way that half of the leaf is inside the conical flask and the other half is outside. Potassium hydroxide crystals were kept inside the flask before the arrangement of the leaf. The whole experimental setup was kept under bright sunlight for 6 hours. After this the experimental leaf is taken out from the flask and is plucked from the plant. Then Iodine test is conducted for this experimental leaf to find out whether the leaf prepared starch through photosynthesis or not.
Steps to be followed for Iodine test
Result of Iodine testIodine is a purple colored reagent used to test the presence or absence of starch in the leaf conducted for photosynthesis process. In the presence of starch iodine solution will turn into blue-black color and in the absence of starch iodine will turn into brown color. In the experimental leaf, the part of the leaf kept inside the conical flask will give a negative test for starch i.e. iodine when added to this part of the leaf will turn into brown color. The reason for this is the part of the experimental leaf arranged inside the flask do not get the factor carbon dioxide as this is absorbed by potassium hydroxide crystals kept inside the flask. So photosynthesis doesn't occur and no starch is synthesized. Hence this part give a negative test for starch with iodine test.
The part of the leaf kept outside the flask will get carbon dioxide and so in this part photosynthesis occur and starch is synthesized. Hence this part give a positive test for starch with iodine test i.e. when iodine is added to this part will turn into blue-black color.
Here is an Experiment on Photosynthesis-Sunlight is necessary for Photosynthesis
Questions and answers related to the experiment
Ans. To prove carbon dioxide is essential for photosynthesis process.
Ans. Potassium hydroxide crystals kept inside the flask will absorb carbon dioxide from the air present inside the flask. Due to this the leaf present inside the flask cannot get Co2 for its photosynthesis.
Ans. Soda lime (CaHNaO2) or Caustic soda (NaOH) also can absorb Co2 from the flask.
Ans. Before starting of this experiment De-starching of the leaves of the plant has to be done. This step will be done to ensure that there would be no starch in the leaves of the experimental plant.
Here is Experiment on Transpiration-More transpiration occurs from lower surface of a dicot leaf than upper
Ans. Half of the experimental leaf kept inside the flask will give a negative result for iodine test because this part of the leaf inside the flask don't get Co2 for photosynthesis and so no starch is formed. KOH kept inside the flask will absorb the Co2 from the air present inside the flask will deprive its availability to the leaf.
The part of the leaf kept outside the conical flask will get Co2 and so this part synthesize starch through photosynthesis. Hence it will give a positive result for starch test with iodine solution.
Here is a video on the experiment to show Co2 is necessary for Photosynthesis:-