Photosynthesis Process

Overview

• Photosynthesis is divided into two main reactions:
  • Light-Dependent Reaction
  • Light-Independent Reaction (Calvin Cycle)

Light-Dependent Reaction

• Occurs in the thylakoid (granum) of the chloroplast.
• Consists of two processes: non-cyclic photophosphorylation and cyclic photophosphorylation.

Non-Cyclic Photophosphorylation

• Photolysis of Water: Light hits water, breaking it down into oxygen, electrons, and protons.
• Photosystem II: Light excites electrons, which travel to Photosystem I.
  • Powers the electron transport chain (ETC) and causes chemiosmosis.
  • Results in the production of ATP.
• Photosystem I: Light excites electrons, which move to enzymes, driving the production of reduced NADP+ (NADPH).
• Products: ATP, NADPH, and oxygen (as a waste product).

Cyclic Photophosphorylation

• Involves only Photosystem I.
• Light excites electrons, which then return to Photosystem I.
  • Powers the ETC and causes chemiosmosis.
• Product: ATP.

Light-Independent Reaction (Calvin Cycle)

• Occurs in the stroma of the chloroplast.
• Begins with Ribulose-1,5-bisphosphate (RuBP) and carbon dioxide (CO2).
  • Catalyzed by the enzyme Rubisco.
• Formation of 3-Phosphoglycerate (3-PGA): RuBP and CO2 form a six-carbon molecule that breaks down into two three-carbon molecules (3-PGA).
• Reduction Phase: 3-PGA undergoes reduction using ATP and NADPH to form Glyceraldehyde-3-phosphate (G3P).
• Regeneration of RuBP: Some G3P is used to regenerate RuBP, utilizing additional ATP.
• Production of Organic Molecules: Some G3P forms sugars (glucose, fructose) and can combine with nitrogen to form amino acids.

Key Points

• Non-Cyclic Photophosphorylation: Produces ATP, NADPH, and oxygen.
• Cyclic Photophosphorylation: Produces ATP.
• Calvin Cycle: Uses ATP and NADPH to form G3P, which can be used to regenerate RuBP, produce sugars, and form amino acids.
• Enzymes: Rubisco is crucial for carbon fixation in the Calvin Cycle.