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Photosynthesis

Introduction to photosynthesis

We know that raw materials are utilized by plants to prepare food. Do plants prepare food at all times? Are there any essential conditions required for photosynthesis?

  1. Sunlight is essential for photosynthesis

Place a healthy green potted plant in a dark room for 1-2 days. This is done to ensure that the plant consumes all its reserve food and the leaves do not contain any starch. Then, cover a portion of a leaf of this plant on both sides with two uniform pieces of black paper, fixed in position with two paper clips.

Now, expose this plant to bright light. After a few hours, remove the leaf, decolourize it with alcohol, and test the presence of food (starch) with iodine solution.

You will observe that the portion of the leaf covered with black paper does not show any presence of starch (food).

Explanation of the activity:

The food prepared by plants (carbohydrates) through the process of photosynthesis is stored as starch. This starch reacts with the iodine solution to change to blue-black colour. Only those portions of the leaf that were exposed to sunlight could photosynthesise and hence, change to blue-black colour when tested with iodine.

2. 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, decolourize 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.

Explanation:

The leaf is treated with alcohol so that it loses its green colour (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 colour was observed only in these parts.

3. CO2 is essential for photosynthesis

Select two healthy potted plants of nearly the same size and label them as A and B. Place them in a dark room for 2-3 days. Then, place two glass plates under both the plants. Place a watch-glass containing potassium hydroxide besides pot A. Cover both the plants by inverting separate bell jars over them. Potassium hydroxide, as we know, is used to absorb CO2. Therefore, CO2 is not available for plant A.

Now, seal the bottom of the jars to the glass plates with the help of Vaseline. This prevents the entry of CO2 into the set up. Then, place the plants under sunlight for 2 – 3 hours. Test one leaf each from both plants for the presence of starch, using alcohol and iodine (as explained in the previous activity). It will be observed that plant B has a higher amount of starch as compared to plant A.

Explanation of the activity:

This happens because potassium hydroxide present besides plant A absorbs all the CO2. Therefore, plant A is not able to photosynthesize and manufacture food. Hence, the amount of starch present in plant B is higher than plant A.

Photosynthesis in a laboratory

Place an aquatic plant (hydrilla) in a beaker filled with water. Cover the plant with a transparent funnel. Then, invert a test tube over the open end of the funnel.

While inverting the test tube, make sure it does not contain any air bubbles. Place this apparatus in sunlight and observe the changes.

It will be observed that after sometime, air bubbles (O2) emerge in the test tube.

We know that raw materials are utilized by plants to prepare food. Do plants prepare food at all times? Are there any essential conditions required for photosynthesis?

  1. Sunlight is essential for photosynthesis

Place a healthy green potted plant in a dark room for 1-2 days. This is done to…

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