Photosynthesis Lecture Notes

Overview

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

Chloroplast Structure

• Organelle: Double-membraned, compartmentalized.
• Thylakoids: Membranous structures; stacked into grana.
• Stroma: Fluid region outside thylakoids.
• Compartmentalization: Increases surface area, decreases competition, enhances reaction efficiency.

Photosynthesis Processes

1. Light-Dependent Reactions
2. Calvin Cycle

Light-Dependent Reactions

• Location: Thylakoid membranes and compartment.
• Key Proteins & Structures:
  • Photosystem II
  • Photosystem I
  • Proton pump
  • Electron transport chain (ETC) proteins
• Process:
  • Water is split into H+ ions (protons), electrons, and oxygen gas.
  • Electrons enter Photosystem II; chlorophyll absorbs light energy, exciting electrons.
  • Electrons pass through ETC, releasing energy for proton pump.
  • Proton pump creates a proton gradient across the thylakoid membrane.
  • Protons flow back to the stroma via ATP synthase, driving ATP formation through chemiosmosis.

ATP Production

• ATP Synthase: Enzyme that creates ATP (adenosine triphosphate) from ADP (adenosine diphosphate) and inorganic phosphate.
• Photophosphorylation: Process driven by light energy (original source).

Electron Transport & NADPH Formation

• Electrons re-excited in Photosystem I, picked up by NADP to form NADPH.
• NADPH: Electron carrier; transfers electrons to Calvin cycle.

Calvin Cycle

• Location: Stroma
• Function: Converts CO2 into organic carbohydrates (sugars).
• Power: Fueled by NADPH and ATP from light-dependent reactions.
• Recycling: ADP and NADP return to light-dependent reactions.

Summary

• Light-Dependent Reactions:
  • Occur in thylakoids.
  • Light energy excites electrons; power proton pumps to establish proton gradient.
  • ATP synthase produces ATP.
  • Electrons and protons help form NADPH.
• Calvin Cycle:
  • Occurs in the stroma.
  • Uses ATP and NADPH to convert CO2 to sugars.
  • ADP and NADP recycled back to light-dependent reactions.