An experiment to show chlorophyll is necessary for plants to carry out photosynthesis

Photosynthesis is a physiological process of green plants which helps to prepare their own food. In this resource I have explained in detail how an experiment can be conducted to prove the importance of the factor chlorophyll 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 and universities.


Photosynthesis is an important physiological activity of all green plants. For photosynthesis to occur in green plants the factors required are Sunlight, Chlorophyll and Carbon dioxide. One factor that is more important for green plants to prepare their own food is the presence of chlorophyll pigment in their leaves.To prove the necessity of this factor there specific experiment to be done in the laboratory. So students must know clearly how this particular experiment will be done to prove the importance of this factor clearly and also must know why a particular step will be done for the success of this experiment with reasons.

Here is How Oxygen is evolved by green plants in an experiment on Photosynthesis

Aim of the experiment:-

To prove that chlorophyll pigment in green leaves is necessary for the plants to prepare their carbohydrate (starch) food.

Requirements for the experiment:-

A well watered variegated leaf plant, beaker, water, Bunsen burner, mentholated spirit or rectified spirit or ethanol, iodine solution, a white porcelain tile.

Procedure for the experiments:-

A variegated leaf plant is taken for this particular experiment to prove the necessity of chlorophyll for photosynthesis. A variegated leaf is a leaf in which different patches of colors are present in a Croton or Coleus or Geranium plant. The place where green patches are present in the leaf, chlorophyll pigment is present while at the other color patches present in the leaf contain other pigments but not chlorophyll. Before starting the experiment to prove chlorophyll is necessary for photosynthesis an important step has to be carried out is known as De-starching. De-starching is a step in which the starch present in the leaves of the plant will be removed by keeping the variegated leaves plant in a dark room or closed cup-board for about 48 hours. This step is done to ensure that previously synthesized starch in the leaves get consumed up because photosynthesis doesn't occur at that time as the plant is kept in dark place.

After De-starching step the variegated leaf plant is kept in bright sunlight for about 6 hours. After this one of the variegated leaf is plucked out from the experimental plant. Then Iodine test is conducted for this experimental leaf to find out whether the leaf prepared starch through photosynthesis or not.

A variegated leaf plant taken to prove chlorophyll is necessary for photosynthesis

Steps to be followed for Iodine test:-

  • The plucked out 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 taken out from the beaker with the help of a forceps. It is then placed in a test tube containing rectified spirit or ethanol. Then the leaf is boiled in ethanol over a water bath by keeping it in boiling water for about 10 minutes. The leaf dissolves its chlorophyll by boiling it in rectified spirit and then become pale-white.
  • Next the pale-white leaf is removed from the test tube by forceps and then it is washed in under warm water to make it soft.
  • Then this leaf is spread over a white porcelain tile and iodine drops were placed over at different parts 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 added to leaf will turn into brown color.

In this experimental leaf, the part of the variegated leaf which contain green patches will give a positive test for starch i.e. iodine when added to this part of the leaf will turn into blue-black color. The reason for this is, this part of the experimental leaf containing green patches include chlorophyll and thus prepare starch through photosynthesis. Thus iodine added to this region will turn into blue-back color. But colored patches of variegated leaf doesn't include chlorophyll and thus don't prepare starch through photosynthesis. So iodine added in this region will change into brown color. Thus this experiment proves chlorophyll is necessary for leaves to prepare starch through photosynthesis.

Here is an Experiment on Photosynthesis-Sunlight is necessary for Photosynthesis

Questions and answers related to the experiment

  • What is the aim of the above experimental setup?
    Ans. To prove chlorophyll is in leaves is necessary to prepare starch in photosynthesis.

  • What is a variegated leaf?
    Ans. A leaf with different colored patches as in Croton leaf is known as variegated leaf.

  • Give two examples of variegated leaf plants?
    Ans. Coleus, Geranium.

  • Why a variegated leaf plant is selected for this experiment?
    Ans. A variegated leaf include different patches of colors like green, yellow, white, brown etc. A green patch in the leaf include chlorophyll while other colored patches in the leaf include other colored pigments other than chlorophyll. But green patches in the leaf only prepare starch as it contain chlorophyll but the other patches cannot prepare starch since they lack chlorophyll.
  • Before starting of the experiment what important step you carry out? Explain this step and give its importance.
    Ans. Before starting of this experiment De-starching of the leaves of the plant has to carried out. De-starching the leaves of the variegated leaf plant can be done by keeping the plant in a dark room for about 48 hours. This step will be carried out to ensure that there would be no starch in the leaves of the experimental plant.

  • Write the sequential steps you follow to carry out starch test for the variegated leaf of experimental plant?
    Ans. Write steps given under the heading 'Steps to be followed for Iodine test' in the above experiment.

  • After the end of the experiment, if you carry out iodine test for the variegated leaf of experimental plant what result you get? Explain your answer with proper reason?
    Ans. Write the content under 'Result of Iodine Test' in the above experiment.

Here is an experiment to prove Co2 is necessary for photosynthesis

Here is a video on the experiment 'Chlorophyll is necessary for photosynthesis in plants';-


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Guest Author: Shambhavi Sinha29 Mar 2019

What are the precautions required when conducting this experiment? Please clarify.

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