Photosynthesis Lecture Notes

Overview

• Photosynthesis: essential for converting sunlight, CO2, and water into glucose and oxygen.
• Developed 450 million years ago, despite being inefficient and complex.

Key Reactions in Photosynthesis

Two Types of Reactions

• Light-dependent reactions
• Light-independent reactions (Calvin Cycle)

Requirements for Photosynthesis

• Water: Absorbed by roots and transported via xylem.
• Carbon Dioxide: Enters through stomata in leaves.
• Sunlight: Absorbed by chlorophyll.

Chloroplast Structure

• Chlorophyll: Found in thylakoids within chloroplasts.
• Thylakoids: Membranous sacs stacked into grana.
  • Inside thylakoid: Lumen
  • Outside thylakoid but within chloroplast: Stroma
• Thylakoid Membranes: Phospholipid bilayers maintaining concentration gradients.

Light-dependent Reactions

• Begin with a photon exciting a chlorophyll electron (Photoexcitation).
• Photosystem II (PSII): Complex of proteins, lipids, and chlorophylls initiating electron transport chain.

Electron Transport Chain (ETC)

• Mobile Electron Carrier: Transfers excited electrons.
• Water Splitting: Replenishes lost electrons, producing hydrogen ions and oxygen.
• Cytochrome Complex: Pumps protons into thylakoid, aiding proton gradient formation.
• ATP Synthase: Generates ATP from ADP using proton gradient.
• Photosystem I (PSI): Re-energizes electrons, producing NADPH.

Light-independent Reactions (Calvin Cycle)

• Occur in daylight but do not require photon energy.
• Stages of Calvin Cycle:
  1. Carbon Fixation: CO2 fixed to RuBP with RuBisCo enzyme.
  2. Reduction: ATP and NADPH convert 3-Phosphoglycerate to G3P.
  3. Regeneration: G3P regenerates RuBP, using ATP.
• Products: G3P, leading to glucose, cellulose, and starch.

Important Enzymes & Molecules

• RuBisCo: Enzyme fixing CO2 to RuBP.
• NADPH and ATP: Energy carriers generated in light-dependent reactions.

Summary

• Photosynthesis is crucial for life, transforming sunlight into chemical energy and producing oxygen.
• The process involves complex biochemical pathways and structures.

Additional Resources

• Review the lecture or selected references for deeper understanding.
• Leave questions in the comments for further clarification.