Photosynthesis Notes

Light Reactions

• Two photosystems:
  • Photosystem I: Stroma, absorbs light at $700$ nm.
  • Photosystem II: Thylakoids, absorbs light at $680$ nm.
• Photolysis of water occurs, releasing oxygen.
• Proton gradient generated.
• Synthesis of $6$ ATP and $6$ NADPH.

Carbon Fixation Reactions (Calvin Cycle/Dark Reaction)

• Does not require light.
• Three main steps: Fixation, Reduction, and Regeneration.

Fixation

• Rubisco enzyme binds three molecules of carbon dioxide ($CO_2$).
• Ribulose 1,5-bisphosphate is converted to three unstable intermediate compounds.

Reduction

• The three unstable intermediates convert to six molecules of glyceraldehyde-3-phosphate (PGAL).
• $6$ ATP are consumed (hydrolyzed to ADP and inorganic phosphate).
• $6$ NADPH are consumed (converted to NADP+).

Regeneration

• One molecule of PGAL is used to synthesize sugars, starch, amino acids, and fatty acids.
• The remaining five PGAL molecules regenerate ribulose 1,5-bisphosphate, requiring an additional $3$ ATP.

ATP Consumption

• Total of $9$ ATP are consumed in the Calvin cycle ($6 + 3$).