Class 7 Science- Chapter 1- Nutrition in Plants- Experiment to Test the Occurrence of Photosynthesis

This experiment helps us understand how sunlight is important for photosynthesis in plants. We use a potted plant with broad leaves, covering parts of the leaves with black paper to block sunlight. After exposing the plant to sunlight for several days, we check the leaves with an iodine test to see if photosynthesis happened in the covered and uncovered parts. This activity shows us the role of sunlight in making food for plants.

Activity Objective

To show how sunlight is necessary for photosynthesis in plants.

Key Concepts of Role of Sunlight in Photosynthesis 


  • Definition- Photosynthesis is the process by which plants convert light energy into chemical energy.
  • Process Details- It involves the absorption of sunlight by chlorophyll in the plant’s leaves, using it to transform water and carbon dioxide into glucose (a sugar that serves as food for the plant) and oxygen.

Role of Sunlight in Photosynthesis-

  • Energy Source- Sunlight provides the energy needed for photosynthesis.
  • Chlorophyll’s Function- Chlorophyll in the leaves absorbs sunlight, initiating the chemical reactions of photosynthesis.

Also Check – Chapter 10- A Detailed Guide to “Nutrition in Plants” Activities for Class 7 Students

Leaf Structure and Photosynthesis-

  • Leaf Anatomy- Leaves are the primary site of photosynthesis in plants, mainly due to the presence of chlorophyll.
  • Stomata Function- Leaves have small openings called stomata, which allow for the exchange of gases (carbon dioxide in, oxygen out).


Preparing the Leaves-

  • Cut out a small square in the centre of each black paper strip.
  • Cover a part of two leaves with these papers and secure them with paper clips. This creates two sections on each leaf- one exposed to sunlight and one covered.

Sunlight Exposure-

  • Place the plant in sunlight for 2-5 days.
Observation and Analysis-
  • Observe the difference in color between the covered and uncovered portions of the leaf. The color difference indicates whether photosynthesis occurred.

Performing the Iodine Test-

Experiment to Test the Occurrence of Photosynthesis
Iodine Test
  • Conduct an iodine test on the leaf to check for starch presence. The appearance of a blue-black color indicates starch, a product of photosynthesis.

Further Testing with a Second Leaf-

  • Use another leaf from the same plant. Remove the black paper strip and expose this previously covered part to sunlight for 2-3 days.
  • Perform the iodine test again on this leaf.
Observation and Analysis-
  • Before Iodine Test- The color difference on the leaf is first observed, with the expectation that the part exposed to sunlight will be different from the part that was covered.
  • Iodine Test for Starch- The iodine test is used to detect the presence of starch in the leaf. Starch is a product of photosynthesis, so its presence indicates that photosynthesis has occurred.

Interpreting Results-

  • Sunlight Exposure and Starch Production- The uncovered part of the leaf should show a positive result for starch, indicating photosynthesis. The covered part, which received no sunlight, should show a negative result.
  • Further Verification- By uncovering a previously covered part of a leaf and exposing it to sunlight, then observing the starch production through another iodine test, the experiment reinforces the role of sunlight in photosynthesis.

Learning Outcomes

  • Understanding the process of photosynthesis, particularly the role of sunlight.
  • Recognizing the importance of leaves and chlorophyll in this process.
  • Developing skills in experimental observation and understanding the scientific method.

Frequently Asked Questions on Role of Sunlight in Photosynthesis Experiment

Question- Why does the leaf turn blue-black in the iodine test if photosynthesis has occurred?

Answer- The iodine test turns blue-black in the presence of starch. During photosynthesis, plants produce glucose, which can be stored as starch. When a leaf part exposed to sunlight produces starch, it turns blue-black in the iodine test, indicating that photosynthesis has occurred.

Question- What would happen if the entire leaf was covered with black paper?

Answer- If the entire leaf was covered, it would not be exposed to sunlight, and thus, it would not be able to perform photosynthesis. As a result, the iodine test would likely show no presence of starch, indicating no photosynthesis.

Question- Can photosynthesis occur without sunlight?

Answer- No, sunlight is a critical component of photosynthesis. Without sunlight, the plant cannot convert carbon dioxide and water into glucose and oxygen, meaning photosynthesis cannot occur.

Question- Why do we use a potted plant with broad leaves for this experiment?

Answer- Broad leaves provide a larger surface area for the experiment, making it easier to observe the effects of covering parts of the leaf and for conducting the iodine test effectively.

Question- How do plants use the glucose produced in photosynthesis?

Answer- Plants use the glucose produced in photosynthesis as a source of energy for growth and development. They can also convert glucose into starch and store it for later use.

Question- Does the colour of the leaf affect the process of photosynthesis?

Answer- The colour of the leaf itself does not directly affect the process of photosynthesis. The green colour is due to chlorophyll, which is vital for absorbing sunlight, but the efficiency of photosynthesis depends more on the health and condition of the leaf rather than its colour.

Question- Can photosynthesis happen in artificial light?

Answer- Yes, photosynthesis can occur under artificial light if it provides the correct wavelengths of light needed by plants. However, sunlight is the most natural and effective source for photosynthesis.

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