How to conduct a Botany experiment on Photosynthesis


Photosynthesis is a physiological process of green plants. One factor that is more important for photosynthesis to occur is Carbon dioxide. In this resource I have explained in detail how an experiment can be conducted to prove the importance of the factor Co2 for this process. Along with this experiment I gave some important questions and answers related to the experiment which will be helpful for those preparing for year end practical or theory exams of various Boards.

Introduction

Photosynthesis 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 experiments

Before 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.

Experiment on Photosynthesis-Carbon dioxide is necessary for photosynthesis

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

  • The experimental leaf is immersed in boiling water in the beaker for few minutes so that the cells of the leaf will be killed and at the same time metabolic activities of the leaf cells will be stopped.
  • Now the boiled leaf is removed from the beaker with the help of a forceps. It is then placed in a test tube containing mentholated spirit or ethanol. Then the leaf is boiled in mentholated spirit or ethanol over a water bath by keeping it in boiling water for about 10 minutes. The leaf looses its chlorophyll by boiling it in mentholated spirit and become pale-white.
  • Next the pale-white leaf is removed from the test tube by forceps and then it is washed in warm water to make it soft.
  • Then this leaf is spread over a white porcelain tile and iodine drops are poured over at different places of the leaf

Result of Iodine test

Iodine 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

  • What is the objective of above experimental setup?
    Ans. To prove carbon dioxide is essential for photosynthesis process.

  • Identify the special condition created in the flask in above experimental setup for this experiment?
    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.
  • Name alternative chemicals that can be used in the flask in place of KOH.
    Ans. Soda lime (CaHNaO2) or Caustic soda (NaOH) also can absorb Co2 from the flask.

  • Would you take any step before starting of this experiment? Explain this step and give its importance.
    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
  • If the experimental leaf is tested for iodine test after the experiment, what result you get? Why?
    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:-

Courtesy:-youtube


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