Describe an activity to show that chlorophyll is necessary for photosynthesis.
Aim: To show that chlorophyll is essential for photosynthesis.
- Place a variegated plant (i.e., a plant which has both green and non-green areas, for e.g., croton or money plant) in a dark room for 2 – 3 days. This is done to ensure that all the reserve food (starch) is utilized.
- Place this plant in sunlight for six hours to allow photosynthesis to take place.
- Then, pluck a leaf from this plant and trace the green areas on a sheet of paper.
- Now, decolorize the leaf using alcohol and dip it in a dilute solution of iodine for a few minutes. Wash this leaf with water and compare it with the tracings of the leaf done earlier.
- It will be observed that only the green areas of the leaf could Photosynthesize.
The leaf is treated with alcohol so that it loses its green color (chlorophyll pigment) and blue-black colour (in presence of starch) obtained after treatment with iodine.
The green parts of a variegated leaf contain chlorophyll. Therefore, only these parts could photosynthesize and manufacture food. Thus, the change in color was observed only in these parts.
The leaf is kept in boiling water to kill the leaf and damage the membranes so that it will be easier to observe the pigments inside. The death of the leaf also prevents any further changes that may occur. Boiling water makes the leaf soft and smooth so that it does not crimp when tested with iodine.
The leaf is boiled in alcohol to remove chlorophyll pigments from it. This is done so that the green colour of the chlorophyll does not interfere with the iodine test results while testing for starch.