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 eukaryotes, photosynthesis takes place in chloroplasts.

Structure of Chloroplast

• Double-membraned organelle.
• Contains thylakoids, stacked into grana.
• The stroma is the fluid region outside of the thylakoids.
• Compartmentalization increases surface area and reduces competition, enhancing efficiency.

Main Processes of Photosynthesis

1. Light-Dependent Reactions
2. Calvin Cycle

Light-Dependent Reactions

• Occur in thylakoid membranes and compartments.
• Involve proteins like Photosystem II, a proton pump, and Photosystem I.

Steps:

1. Water Splitting:
   • Produces H+ ions, electrons, and oxygen gas.
2. Photosystem II:
   • Absorbs light, exciting electrons to a higher energy level.
3. Electron Transport Chain (ETC):
   • Electrons pass through a series of reactions, releasing energy.
   • Energy used by proton pump to create a proton gradient.
4. Photosystem I:
   • Re-excites electrons using chlorophyll.
   • Electrons are captured by NADP, becoming NADPH.
5. ATP Synthase & Chemiosmosis:
   • Proton gradient powers ATP synthase.
   • ADP and inorganic phosphate combine to form ATP.

Photophosphorylation

• Photophosphorylation: ATP production powered by light energy.
• ATP and NADPH generated are essential for the Calvin Cycle.

Calvin Cycle

• Occurs in the stroma.
• Converts CO2 into organic carbohydrates.
• Powered by ATP and NADPH from the light-dependent reactions.
• NADPH supplies electrons; ATP provides energy.

Summary

• Light-Dependent Reactions: Occur in thylakoids, produce ATP and NADPH.
• Calvin Cycle: Occurs in the stroma, converts CO2 into sugars using ATP and NADPH.
• ADP and NADP return to be reused in the light-dependent reactions.