Process of Photosynthesis: Light and Dark Reaction

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Photosynthesis is a complex but vital process by which plants, algae, and some bacteria convert light energy from the sun into chemical energy in the form of glucose (sugar). This sugar is then used by the organism for food and growth, and also released as oxygen, which is essential for animal life.

Here’s a breakdown of the two main stages of photosynthesis:

Read More: Photosynthesis Formula Equation with Lots of Key Facts

The Light-Dependent Reactions (Light Reaction)


  • In plants, the light-dependent reaction unfolds within the thylakoid membranes of chloroplasts.


  • Photosynthesis initiates with the light reaction, occurring exclusively in the presence of sunlight during the day.
  • The Grana, structures inside the thylakoid, function as membrane-bound sacs that gather light and are known as photosystems.
  • Photosystems contain large complexes of pigment and protein molecules, crucial for the primary role in the light reactions of photosynthesis.
  • Two types of photosystems exist photosystem I and photosystem II.
  • In the light-dependent reactions, light energy transforms into ATP and NADPH, essential for the subsequent phase of photosynthesis.
  • ATP and NADPH are produced during light reactions through two electron-transport chains, utilizing water and generating oxygen.
  • The chemical equation for the light reaction in photosynthesis can be simplified to:
    2H2O + 2NADP+ + 3ADP + 3Pi → O2 + 2NADPH + 3ATP

Read More: Steps Involved in Light Reaction of Photosynthesis in details with Q&A

Dark Reaction of Photosynthesis (or) Light-independent Reaction


  • The reaction takes place in the chloroplast stroma, utilizing NADPH and ATP from the light reaction.


  • Dark reaction, also known as carbon-fixing, operates independently of light.
  • Sugar molecules are formed from water and carbon dioxide in this process.
  • Plants capture carbon dioxide through stomata, initiating the Calvin photosynthesis cycle.
  • CO2 enters the cycle and combines with a 5-carbon sugar (RuBP) using the enzyme RuBisCO.
  • This forms an unstable 6-carbon intermediate, which quickly splits into two 3-carbon molecules (3-phosphoglycerate, 3-PGA).
  • Using energy from ATP and electrons from NADPH, 3-PGA is converted into various organic molecules, including glucose (sugar).
  • Some of the sugar is used by the plant for energy and growth, while some is stored as starch.
  • RuBP is regenerated, allowing the cycle to continue.

The chemical equation for the dark reaction

3CO2 + 6 NADPH + 5H2O + 9ATP → G3P + 2H+ + 6 NADP+ + 9 ADP + 8 Pi.

  • G3P stands for glyceraldehyde-3-phosphate, a significant product of the dark reaction.

Overall Equation

6CO2 + 6H2O + Light Energy → C6H12O6 (glucose) + 6O2

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