Photosynthesis: Process, Requirements, and Factors Affecting Its Rate


Process of Photosynthesis

Photosynthesis is the process by which plants manufacture carbohydrates from raw materials using energy from light.


carbon dioxide + water → glucose + oxygen

in the presence of light and chlorophyll

Chemical Equation:

6CO2 + 6H2O —> C6H12O6 + 6O2

Role of Chlorophyll

What is the function of chlorophyll in photosynthesis?

Chlorophyll transfers light energy into chemical energy in molecules, which is then used for the synthesis of carbohydrates.

Use and Storage of Glucose

Outline the subsequent use and storage of the carbohydrates (glucose) made in photosynthesis.

The glucose made by photosynthesis is used by plants in several ways:

  1. Immediate Energy: Some glucose is used immediately to meet the plant’s energy demands. This energy is released through respiration.
  2. Transport as Sucrose: Glucose is highly reactive, so plants convert it to sucrose, a more stable sugar, for transport to flowers or roots. Sucrose can be easily converted back to glucose when needed.
  3. Storage as Starch: Plants also convert glucose into starch, an insoluble and non-reactive form, for storage as granules in the chloroplasts and cytoplasm.

Investigating Requirements for Photosynthesis

Testing for Starch in Leaves

To investigate the requirements for photosynthesis, we can test for the presence of starch, a product of photosynthesis, in leaves.

  1. Submerge the leaf in boiling water: This breaks down cell walls and destroys membranes, making chlorophyll extraction easier.
  2. Place the leaf in ethanol: This removes chlorophyll from the leaf.
  3. Wash with cold water: This removes the ethanol and softens the leaf for spreading.
  4. Add iodine: Iodine is used to test for starch. A white tile helps visualize the color change. A blue-black color indicates the presence of starch.

Investigating the Necessity of Chlorophyll

  1. Destarch the leaves: Place the leaves in a dark place for 24 hours to ensure any existing starch reserves are used up.
  2. Use a variegated plant: Variegated plants have green (chlorophyll-containing) and non-green parts. Test for starch in these leaves using iodine solution. Only the green areas will turn blue-black, indicating starch production in the presence of chlorophyll.

Investigating the Need for Carbon Dioxide

  1. Use limewater: Limewater turns milky in the presence of carbon dioxide.
  2. Destarch a plant: Place a plant with green leaves in the dark for 24 hours.
  3. Set up the experiment: Seal a destarched leaf inside a flask with limewater at the bottom. The leaf should remain attached to the plant. Set up a control flask without limewater.
  4. Expose to sunlight: Place both flasks in a sunny location for 48 hours, ensuring no air enters.
  5. Test for starch: After 48 hours, test both leaves for starch. The leaf in the flask with limewater will not have starch, while the control leaf will, indicating that carbon dioxide is necessary for photosynthesis.

Investigating the Necessity of Light

  1. Block light with aluminum foil: Aluminum foil prevents light from passing through.
  2. Destarch a plant: Place a plant with green leaves in the dark for 24 hours.
  3. Cover part of a leaf: Use aluminum foil to cover a portion of a leaf, leaving the rest exposed to light.
  4. Expose to sunlight: Place the plant in a sunny place for 48 hours.
  5. Test for starch: Remove the foil and test the leaf for starch. The covered area will not turn blue-black with iodine, indicating that light is necessary for photosynthesis.

Factors Affecting the Rate of Photosynthesis

Light Intensity


  1. Set up an apparatus to measure oxygen production by pondweed in a beaker of water at a specific temperature.
  2. Vary the distance between a lamp (light source) and the beaker (0 cm, 5 cm, 10 cm).
  3. Record the amount of oxygen produced per minute at each distance by counting bubbles or measuring volume.
  4. Repeat at different water temperatures (10°C, 15°C, 20°C).

Result: The rate of photosynthesis increases with light intensity but plateaus at high intensities.

Carbon Dioxide Concentration


  1. Use potassium hydrogen carbonate (KHCO3) as a source of carbon dioxide in the water.
  2. Vary the concentration of KHCO3 to change the carbon dioxide concentration.
  3. Keep other factors constant (light intensity, temperature).

Result: The rate of photosynthesis increases with carbon dioxide concentration but plateaus at high concentrations.



  1. Set up the same apparatus as in the light intensity experiment.
  2. Record the temperature of the water.
  3. Measure the amount of oxygen produced per minute at different temperatures.

Result: The rate of photosynthesis increases with temperature, peaks around 25°C (depending on the plant species), and then decreases. Photosynthesis generally stops around 34°C due to enzyme denaturation.