Photosynthesis: Light and Dark Reactions

Photosynthesis occurs in two main phases: light reactions and dark reactions.

Light Reactions:

  • Location: 

Thylakoid membranes of chloroplasts.

  • Energy Capture: 

Chlorophyll molecules absorb light energy, exciting their electrons.

  • Electron Transport: 

Electrons are passed through an electron transport chain, producing ATP.

  • Photolysis: 

Water molecules are split, releasing oxygen and providing hydrogen ions and electrons.

  • NADPH Reduction: 

Hydrogen ions and electrons are used to reduce NADP+ to NADPH.

Dark Reactions (Calvin Cycle):

  • Location: 

Stroma of chloroplasts.

  • Carbon Fixation: 

CO2 is combined with a 5-carbon compound (ribulose-1,5-bisphosphate, or RuBP) to form a 6-carbon intermediate.

  • Intermediate Breakdown: 

The 6-carbon intermediate is split into two 3-carbon compounds.

  • Reduction: 

The 3-carbon compounds are reduced to 3-carbon carbohydrates using ATP and NADPH.

  • Regeneration: 

The 3-carbon carbohydrates are used to regenerate RuBP, completing the cycle.

Key Points:

  • Light Reactions: 

Convert light energy into chemical energy (ATP and NADPH).

  • Dark Reactions: 

Utilize ATP and NADPH to reduce CO2 into carbohydrates.

  • Z-Scheme: 

The light reactions are often represented as a Z-scheme due to the electron flow pathway.

  • Calvin Cycle: 

The dark reactions are also known as the Calvin cycle.

  • Efficiency: 

Photosynthesis is a highly efficient process, converting sunlight into chemical energy with remarkable efficiency.

In summary, photosynthesis is a complex process that involves the capture of light energy, the reduction of carbon dioxide, and the production of glucose. It's essential for life on Earth, providing the energy source for most ecosystems.