CBSE Class 10 Answered

Photosynthesis takes place in two stages:

• Light reaction
• Dark reaction

Light reaction:

• The light reaction of photosynthesis occurs only in the presence of sunlight in the thylakoid membranes of grana and stroma lamellae.
• It mainly involves absorption of sunlight by the pigments of photosystems and the conversion of their energy into usable chemical energy in the form of ATP and NADPH + H⁺.
• Light reaction is also called the photochemical reaction or Hill reaction or photophosphorylation.

Steps in light reaction:

• Absorption of Light
• Excitation of Reaction Centres
• Photosynthetic Electron Transport and Formation of Assimilatory Power
• Photophosphorylation

Chemical equation:

Dark reaction:

• The dark reaction is the second phase of photosynthesis which occurs in the stroma of chloroplasts.
• The reaction is also known as the biosynthetic phase or Blackman’s reaction or carbon dioxide fixation or Calvin cycle.
• It uses ATP and NADPH₂ for the fixation and reduction of CO₂ to form carbohydrates.
• The dark reaction is independent of light and may take place in the light or dark without the use of light energy.
• In the dark reaction, CO₂ is reduced to carbohydrates in the form of sugar.

Steps in dark reaction:

• Carboxylation to form 3-phosphoglyceric acid (PGA).
• Reduction to form carbohydrates using ATP and NADPH
• Regeneration to form ribulose 1,5-biphosphate

Chemical equation: