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10th Class Chapter No 31 - Photosynthesis in Science for ICSE

Photosynthesis: The process by which green plants are able to synthesize food from carbon dioxide and water, in the presence of chlorophyll and light energy.

About Photosynthesis:

  • In photosynthesis, the energy from sunlight is used to convert carbon dioxide into sugar and oxygen is the by-product. It can be represented by the following equation:
    • 6CO2 + 12H2O → C6H12O6 + 6O2 + 6H2O (in the presence of sunlight and chlorophyll)
  • Plants also respire by taking in oxygen and giving out carbon dioxide. In respiration, gaseous exchange occurs. Hence respiration and photosynthesis are opposite processes
  • Photosynthesis not only provides all living beings with food, it also maintains oxygen levels in the atmosphereTherefore, you can say photosynthesis is the process that supports life on earth.


  • Chlorophyll is a photosynthetic substance that is able to trap sunlight and use it to generate sugars (nutrition). It is found in plants, algae, chloroplasts and cyanobacteria (thylakoids). 
  • It is made of four kinds of pigments namely chlorophyll a, chlorophyll b, carotene, and xanthophyll.
  • It also gives the green colour to the chloroplast and to the plant in general. Leaves appear green in colour because chlorophyll reflects green light and absorbs all other wavelengths of light.

Stages in Photosynthesis:

Photosynthesis occurs in the following two stages:

Light Reaction: Light reaction occurs in the thylakoids during daytime in the presence of sunlight. Since it results in the formation of chemical energy from radiant energy, it is called a photochemical reaction and it is divided into 4 stages.

  • Absorption of light energy: Chlorophyll pigments present in Photosynthetic units in the thylakoids absorb a photon of energy. This results in electrons being excited into a higher energy level.
  • Photolysis of water: The excess energy is used to split a molecule of water into H+ and OH ions. 
    • Oxygen is formed as a byproduct and is released into the atmosphere.
  • Reduction of NADP: 2NADP + 4e⁻ + 4H⁺ → NADPH
  • Photophosphorylation: In the presence of sunlight, ADP binds with inorganic phosphate to form ATP. (ADP + Pià ATP)

Dark Reaction: This reaction occurs in the stroma. Since this reaction results in the formation of biomolecules, it is called a biosynthetic phase. There are three stages identified in the Calvin cycle.

  • Carboxylation: CO2 is covalently linked to a 5 carbon sugar (RuBP) and converted to 2 three carbon compounds.
  • Reduction: In the presence of the energy stored in the NADPH₂ and ATP, the two 3-carbon molecules combine to form a six carbon glucose molecule. NADP is got back from NADPH₂ and ADP is obtained from ATP. These are used in the next cycle of the light reaction. (The hydrogen released is used to reduce carbon dioxide into sugar molecule).
  • Regeneration: The CO2 acceptor RuBP reforms at the expense of ATP.
Posted in 10th on July 16 2020 at 11:24 AM


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