What are the Two Phases of Photosynthesis?

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What are the Two Phases of Photosynthesis?


There are two main phases of photosynthesis: Light-dependent and Light-independent Reactions (Calvin Cycle)

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Light-dependent Reactions

  • In the first phase of photosynthesis, known as the light-dependent reactions, the process begins when sunlight is absorbed by chlorophyll, the green pigment in plant cells.
  • This phase takes place in the thylakoid membrane of the chloroplasts, which are like tiny solar power plants within plant cells.

Key Points

  • Location: Thylakoid membrane
  • Initiation: Triggered by sunlight
  • Key Molecule: Chlorophyll
  • Products: ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate)

During these reactions, light energy is converted into chemical energy stored in ATP and NADPH. These energy-rich molecules are crucial for the next phase of photosynthesis.

Light-independent Reactions (Calvin Cycle)

  • Moving on to the second phase, the light-independent reactions (also known as the Calvin Cycle) occur in the stroma of chloroplasts.
  • Unlike the light-dependent reactions, this phase doesn’t directly require sunlight and can occur in the absence of light.

Key Points

  • Location: Stroma of chloroplasts
  • Initiation: Independent of light
  • Key Molecules: ATP and NADPH (from the light-dependent reactions)
  • Products: Glucose and other carbohydrates

In the Calvin Cycle, carbon dioxide from the air is combined with the stored energy in ATP and NADPH to produce glucose and other carbohydrates. These products serve as essential nutrients for the plant and are often used for energy or stored for later use.


In summary, photosynthesis is a two-phase process: light-dependent reactions capture sunlight and convert it into chemical energy, while light-independent reactions use that energy to produce glucose and other carbohydrates. Together, these phases are vital for the survival of plants and play a crucial role in maintaining the balance of oxygen and carbon dioxide in our environment.

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