Lab Manual: Light is Necessary for Photosynthesis | Lab Manuals for Class 10 PDF Download

Objective

Theory/Principle

Light is one of the major components for occurrence of photosynthesis reaction.
It provides energy for further reaction. The light energy from the sun is absorbed by the chlorophyll pigment and is converted into chemical energy which is utilised by the plants.

Process of photosynthesis involves:

• Absorption of light energy by chlorophyll pigment. This process involves splitting of water molecules to hydrogen and oxygen.
• Conversion of this energy to chemical energy via breakdown of CO2 to form glucose.

The process of photosynthesis is greatly affected by the absence or presence of sufficient supply of light. Importance of light in photosynthetic reaction is studied by conducting this experiment.

Procedure

• Take a de-starch (without or minimal starch) plant [Amaranthus, Tecoma, etc.) with broad leaves. If a de-starch plant is not available, then take any healthy potted plant and keep it in complete darkness for 2-3 days to de-starch it.
• Choose any healthy leaf from this plant. Cut two uniform pieces from the black paper and cover a portion of the leaf from both sides, holding the paper with paper clips.
  Note Ganong’s light screen can also be used in place of black paper strip.
• Place this set up (Fig. 1) in bright sunlight for about 8-10 hours.
• After the designated time the covered leaf is plucked from the plant and black paper covering the portion of leaf is removed.
• Take about 150 ml of water in a beaker and boil it.
• Add the experimental leaf into it till the leaf becomes soft (Fig. 2). Switch off the burner and allow the water to come down to a temperature of 60°C.
  
• Transfer the leaf to a boiling tube containing alcohol.
• Place this set up again in hot water beaker, with temperature maintained at 60°C. Keep the tube in this set up till the leaf become colourless (decolourisation of leaf) as shown in Fig. 3.
• Take the leaf out and wash it with water [Fig. 4(a)]. In a petri dish, place this leaf and add a few drops of iodine solution to it [Fig. 4(b)], Leave the set up for 5 minutes.
  
• Remove the leaf from iodine solution and wash it with water and observe the colours of exposed and unexposed parts of the leaf.

Observations
After treating the leaf with iodine solution (see Fig. 5) following observations were made

• The exposed or uncovered part of the experimental leaf turns blue black in colour.
• The portion of the leaf that was covered with black strips has no appearance of blue colour, it turns to a pale yellowish brown.

Result
Plants produce food, i.e. carbohydrates in the form of starch as one of its end products. From the above observations, we conclude that

• Starch is present in the uncovered part of leaf because it is getting all the sufficient sources for its synthesis, i.e. water and light. Hence, the uncovered parts gives positive result when tested with iodine solution.
• Starch is absent in the covered part of leaf because the process of photosynthesis did not occur in this part due to absence of light. Hence, when tested with iodine, it gives a negative test. Thus, light is very important for occurrence of photosynthesis.

Precautions

• The experimental leaf should be healthy.
• The plant used in this experiment should be completely de-starched for satisfactory result.
• While clipping the leaf, be gentle, do not tear the leaf.
• Direct heating of alcohol is avoided as it is inflammable and will catch fire. Use a water bath for this purpose.
• Wash the leaf before iodine testing.