Photosynthesis Process Overview

Overview

This lecture explains the process of C3 photosynthesis, detailing both the light-dependent and light-independent (Calvin Cycle) reactions, and their roles in converting light energy into glucose and oxygen.

Introduction to Photosynthesis

• Photosynthesis uses solar energy, carbon dioxide (CO₂), and water (H₂O) to produce glucose and oxygen.
• Occurs in chloroplasts, which have inner (thylakoid) and outer (stroma) compartments.

Light-Dependent Reactions

• Take place in thylakoid membranes; require light and water.
• Main proteins: Photosystem II, Plastoquinone, Cytochrome B6F, Plastocyanin, Photosystem I, Ferredoxin, Ferredoxin NADP Reductase, ATP Synthase.
• Photolysis splits water, releasing oxygen (waste), protons (build gradient), and electrons (to electron transport chain).
• Light excites electrons in Photosystem II (p680) and Photosystem I (p700).
• Electron transport chain passes electrons through protein complex, reducing their energy and pumping protons into thylakoids.
• NADP+ is reduced to NADPH, which carries electrons and protons to Calvin Cycle.
• ATP is generated via chemiosmosis as protons move through ATP Synthase; process called phosphorylation.

Light-Independent Reactions (Calvin Cycle)

• Occur in stroma; do not require light directly.
• Use ATP and NADPH from light reactions plus CO₂ and water.
• Begins with RuBP (Ribulose 1-5 BiPhosphate) combining with CO₂ (carbon fixation by Rubisco enzyme) to form unstable 6-carbon intermediates.
• 6-carbon intermediates split into 12 PGA (3-carbons each).
• PGA is converted to PGAL using ATP and NADPH.
• 2 out of 12 PGALs produce glucose; 10 PGALs regenerate RuBP with energy from ATP.
• Calvin Cycle repeats as long as reagents are supplied.

Key Terms & Definitions

• Chloroplast — Organelle where photosynthesis occurs.
• Thylakoid — Membrane disks in chloroplast; site of light-dependent reactions.
• Stroma — Fluid-filled area around thylakoids; site of Calvin Cycle.
• Photosystem — Protein complexes (II and I) that absorb light energy.
• Photolysis — Splitting of water by light to produce O₂, protons, and electrons.
• Electron Transport Chain (ETC) — Series of proteins transferring electrons and pumping protons.
• ATP Synthase — Enzyme that generates ATP from ADP and inorganic phosphate.
• NADPH — Electron and proton carrier molecule.
• Calvin Cycle — Set of reactions synthesizing sugar from CO₂ using ATP and NADPH.
• Rubisco — Enzyme that facilitates carbon fixation in the Calvin Cycle.
• RuBP — 5-carbon molecule starting the Calvin Cycle.

Action Items / Next Steps

• Review lecture diagrams and study guide as provided in the course materials.
• Study the roles of each protein in the electron transport chain.
• Understand the steps and carbon accounting in the Calvin Cycle.