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Understanding Photosynthesis Processes

Apr 21, 2025

Photosynthesis Lecture Notes

Overview of Photosynthesis

  • Photosynthesis captures energy from the sun to produce sugars.
  • Occurs in prokaryotes (e.g., cyanobacteria) and eukaryotic autotrophs.
  • In eukaryotic autotrophs, it takes place in the chloroplast.

Structure of Chloroplast

  • Chloroplast: Double membrane organelle.
    • Thylakoids: Compartmentalized into stacks called grana.
    • Stroma: Fluid region outside thylakoids.
  • Compartmentalization increases surface area, reduces competition, and enhances reaction efficiency.

Two Main Processes of Photosynthesis

  1. Light Dependent Reactions: Occur in the thylakoid.
  2. Calvin Cycle: Takes place in the stroma.

Light Dependent Reactions

  • Location: Thylakoid membrane and thylakoid compartment.
  • Key Components: Photosystem II, proton pump, Photosystem I, and other membrane proteins.

Electron Transport and Oxygen Production

  • Water is split, releasing H+ ions, electrons, and oxygen gas.
  • Electrons enter Photosystem II, where chlorophyll absorbs light energy, making electrons excited.
  • Excited electrons pass through an Electron Transport Chain (ETC), releasing energy.

Proton Pump and Gradient

  • Proton pump uses energy to create a proton gradient (electrochemical gradient) by actively transporting protons from stroma to thylakoid compartment.

ATP Production

  • ATP Synthase: Enzyme creating ATP from ADP and inorganic phosphate via chemiosmosis.
  • Chemiosmosis: Flow of protons down gradient powering ATP synthase.
  • Photophosphorylation: Process powered by light energy.

Role of NADP+

  • High-energy electrons picked up by NADP+ to become NADPH.
  • NADPH carries electrons to the Calvin Cycle.

Calvin Cycle

  • Location: Stroma.
  • Converts CO2 into organic carbohydrates (sugars).
  • Powered by NADPH and ATP from light-dependent reactions.
  • NADPH provides electrons; ATP provides energy.

Summary

  • Light Dependent Reactions:
    • Occur in thylakoids.
    • Absorb light energy to excite electrons and power proton pump.
    • Produce ATP via photophosphorylation.
    • NADPH and ATP proceed to Calvin Cycle.
  • Calvin Cycle:
    • Occurs in stroma.
    • Uses ATP and NADPH to convert CO2 into organic sugars.
    • ADP and NADP+ return to light-dependent reactions.

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